Shipboard Electromagnetics (Artech House Communication and Electronic Defense Library) [2nd ed.] 0890062471, 9780890062470

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The A rtech House A ntenna Library Antennas by L.V. Blake Shipboard Antennas by Preston E. Law, Jr. Shipboard Elec/romagnetics by Preston E. Law, Jr. Antenna Design Using Personal Co m p uters by David M. Pozar Micros/rip Antennas by I.J. Bahl and P. Bhartia lnt elference Suppression Techniques for Microwave Antennas and Trans­ millers by Ernest R. Freeman Principles of Electromagnetic Compatibility by Bernhard E. Keiser Practical Simulation of R adar Antennas and Radomes by Herbert L. Hirsch and Douglas C. Grove

Shipboard Electromagnetics Preston E. Law, Jr.

Artech House Boston· London

Library of Congress Cataloging-in-Publication Data

Law,

Preston

E.

Shipboard electromagnetics Biblography: p. Includes index.

1.

United States.

Navy-Electronic installations.

2. Electronics in military engineering. electronic equipment.

Ships­

1. Title.

YM480. 3. L38 1987 ISBN 0-89006-247-1

Copyright ©

3.

359.8'5'0973

87-19293

1987

ARTECH HOUSE, INC.

685 Canton Street Norwood, MA 02062 All rights reserved. Printed and bound in the United States of America. No part of this book may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording, or by any information storage and retrieval system, without permission in writing from the publisher.

International Standard Book Number: 0-89006-247-1 Library of Congress Catalog Card Number: 87-19293

10 9 8 7 6 5 4 3 2 1

This book is dedicated to the M emOI' of Benjamin Logan Fowler, Sr. and Preston Eugene Law, Sr. Two magnicent fathers: My wife's and mine

"We Make a Living by What We Get but We Make a Life by What We Give" -Winston Churchill

Contents

Preface . . . . Acknowledgements Introduction Chapter 1 HISTORICAL BACKGROUND. Prelude . . . . . . . . . . 1-0 Naval Adoption of Electromagnetics 1-1 Early Attempts to Reduce Wireless Interference 1-2 From RFI to EMI . . . . . . . . 1-3 Warld War II Naval Electronics and RFI 1-3.1 Postwar Efforts . . . . . . . 1-3.2 EMC and the Vietnam War Period. 1-3.3 1-4 The Modern Era . . . . . . Emerging Management Interests 1-4.1 Establishment of TESSAC 1-4.2 1-4.3 Implementation of EMC Management 1-4.4 Rising Interest in EMP. 1-4.5 The CUTent Status . . . . . . . Conclusion. . . . . . . . . . 1-5 Chapter 2 THE SHIPBOARD ELECTROMAGNETIC ENVIRONMENT (EME) . . . . . . . . . . . The Tangible Environment . . . . . 2-0 The Composite RF Energy Environment. 2-1 Effects of the Shipboard EME 2-2 2-3 EME Control Techniques . . . 2-4 Predicting the Shipboard EME . 2-4.1 Derivation of the Projected EME 2-4.2 EME Deinition Guidance. . .

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Xll X 111 XIV

1 1 2 6

7 7 8 9 12 12 13 15 17 17 19 23 23 25 25 27 28 30 31

Chapter 3 SHIPBOARD ELECTROMAGNETIC COMPATIBILITY (EMC)

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33

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3-0

Deining EMC

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33

3-1

Implementing EMC Measures

34

3-1.1

The EMC Program Plan

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34

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3-1.1.1 Frequency Spectrum Management .

35

3-1.1.2 The EMCAB .

36

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3-1.1.3 The EMI Control Plan .

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38

3-1.2

EMC Test and Evaluation.

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39

3-1.3

EMC Coniguration Management

40

3-1.4

EMC Training and Awareness

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41

Chapter 4 SHIPBOARD ELECTROMAGNETIC INTERFERENCE (EM!).

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45

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4-0

The Shipboard EMI Problem.

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45 .

4-1

Sources of Shipboard EMI

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45

4-1.1

Natural Sources of Shipboard EMI.

47

4-1.2

Man-Made Sources of Shipboard EMI

47

4-2

Shipboard EMI Control

57

4-2.1

Shielding Techniques .

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58

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58

4-2.1.2 Shielding Methods and Materials

68

4-2.1.1 Shielding Theory. 4-2.2

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Grounding and Bonding Techniques

4-2.2.1 General Deinitions .

104 104

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4-2.2.2 Bonding Classiications

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105

4-2.2.3 Grounding Requirements .

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108 121

4-2.2.4 RF Bonding Procedures for EMI Control 4-2.3

Nonmetallic Topside Material Techniques

131

4-2.4

EMI Filtering Techniques.

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140

4-2.4.1

Filter Classiication and Characteristics

142

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4-2.4.2 Shipboard Filter Applications 4-2.4.3 Filter Installation Precautions

143 .

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147

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148

4-2.5

EMI Blanking Techniques

4-2.6

Topside Systems Arrangement Techniques

4-2.6.1 Antenna Interference Characteristics .

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150 151

4-2.6.2 Preliminary Antenna Arrangement Considerations

152

4-2.6.3 The Topside Systems Design Team

153

4-2.6.4 HF Antenna System Integration.

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154

4-2.6.5 EMC Considerations

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160

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4-2.6.6 Candidate Antenna Systems Arrangements

162

4-2.6.7 Post-Design Phase .

162

4-2.7

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TEMPEST Electromagnetics .

viii

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163

Chapter 5 SHIPBOARD ELECTROMAGNETIC RADIATION HAZARDS (EMR) .

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167

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5-0

The Radiation Hazards Problem In General.

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167

5-1

Biological Effects of Radiation .

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168

5-2

Shipboard Hazards of Electromagnetic Radiation to Personnel (HERP) .

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5-2.1

Origin of Radiation Exposure Limits .

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l70

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171

5-2.2

Emergence of Modem Radiation Exposure Standards

171

5-2.3

Shipboard Permissible Exposure Criteria.

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172

5-2.4

Shipboard EMR Hazards Protection Techniques

l75

5-2.4.1 Ship Design Criteria to Control EMR Hazards.

l77

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5-2.4.2 EMR Hazards Measurements and Analysis .

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l79

5-3

Hazards of Electromagnetic Radiation to Fuel (HERF)

192

5-3.1

The Nature of HERF Combustion .

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193

5-3.2

Shipboard Fueling Precautions .

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194

5-4

Hazards of Electromagnetic Radiation to Ordnance (HERO)

196

5-4.1

HERO Classiications .

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196

5-4.2

HERO Controls in Port and Territorial Seas

197

5-4.3

Shipboard HERO Controls

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197

5-4.4

Shipboard HERO Surveys

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201

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Chapter 6 SHIPBOARD ELECTROMAGNETIC PULSE (EMP)

6-0

Preparation for an Eventuality .

6-1

EMP Characteristics

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6-1.1

High-Altitude EMP Generation .

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205 205

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206

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207

6-1.2

High-Altitude EMP Electrical Properties.

211

6-2

Shipboard EMP Damage Effects

213

6-3

Shipboard EMP Hardening Techniques

216

6-3.1

EMP Shielding and Grounding .

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217

6-3.1.1

Cable Shielding Requirements .

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218

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6-3.1.2 Waveguides, Pipes, and Metal Tubes Grounding .

221

6-3.2

Circuit Protection Devices

221

6-4

EMP Testing and Modeling

227

6-4.1

EMPRESS Testing .

228

6-4.2

EMP Modeling .

6-4.2. I

EMPAL Analysis Process.

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230 230

6-4.2.2 Scale Modeling Process

232

Chapter 7 SHIPBOARD ELECTROMAGNETIC ASSESSMENT (EMA) 235

7-0

The Need for Predictive Analysis .

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235

7-1

Predictive Analysis Techniques .

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236

7-2

Electromagnetic Assessment Modeling

238

7-2.1

SEMCAC Modeling

240

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7-2.2 7-2.3 7-2.3.1 7-2.3.2 7-2.3.3 7-2}.4

SEMCAM Modeling TDM Performance Assessments TDM Geometry . . . . . . TDM Omnidirectional Antenna Performance Evaluation TDM Directive Antenna Performance Evaluation . Shipboard EM Assessment Summary .

Glossary Bibliography Index . .

240 241 242 244 248 250 255 257 259

.

x

The Author

Preston E. Law, Jr., is currently Head of the Ship Topside Electromagnetics Design Branch of the Naval Sea Systems Command where he is responsible for ensuring Electromagnetic Compatibility (EMC) and control of Electromagnetic Interference (EMI) in Shipboard Combat Systems. His past experience includes employment as a Communications Systems Engineer with the De fense Com­ munications Agency in Thailand; the Voice of America in Washington, D. c.; and the Federal Aviation Agency in Anchorage, Alaska. Mr. Law is an electrical engineering graduate of the University of Maryland and a registered Professional Engineer in the State of Maryland. He is a Senior Member of the Institute of Electrical and Electronics Engineers (IEEE), a member of the American Society of Naval Engineers (ASNE), Armed Forces Communications Electronics As­ sociation (AFCEA), Association of Old Crows (Electronic Defense Association), Association of Scientists and Engineers (ASE) of NA VSEA, and Tau Beta Pi and Eta Kappa Nu Engineering Honor Societies. He is author of Shipboard Antennas (Artech House, Norwood, MA, 1983; Second Edition, 1986).

Preface Since the end of the nineteenth century , when the science of radio com­ munications was irst embraced for sea service, naval ships have never been without electromagnetics. The adoption and evolution of naval shipboard elec­ tromagnetics has resulted in a series of marvels from communications to nav­ igation to radar to weapons control and electronic warfare. Yet each of these marvels has been countered step-by-step along the way by that bane of electro­ magnetic science, interference. From the initial experimental ship-to-shore wireless tests of 1899 to this very moment, electromagnetic interference has posed a serious and perpl exing problem to reliable, effective naval operations. Moreover, after a century of shipboard electromagnetics, electromagnetic interference has changed over the y ears onl y in severity and compl exity. Today , with a myriad of highly sophis­ ticated electromagnetic sy stems on board ships, the problem of electromagnetic interference is intense. It has required the marshaling together of the inest of electromagnetic engineering specialists in an all-out ight to bring the interference under control. Continued ingenuity and dil igence is absol utely necessary to ensure that the integrity of shipboard electromagnetics, so essential to modem­ day el ectronic weapons and warfare, is sustained. Preston E. Law, Jr. 15 May 1 987

xii

Acknowledgel1lents

Not just for their personal encouragement but more for their matchl ess engineering contributions to the United States Navy and our country , I wish to acknowl edge my coworkers, my col l eagues, and associates. Both in govenment and in private industry, they are toil ing dil igentl y in the highl y special ized and pecul iarl y demanding iel d of shipboard el ectromagnetics. As dedicated, con­ scientious experts, they deserve far more recognition than my mere tip-of-the­ hat here. Special thanks are due three who careful l y reviewed portions of the manuscript: Russel l D. Powel l (Grounding and Bonding); Neil T. Baron (Ra­ diation Hazards); and James A. Kudzal (El ectromagnetic Pul se)-and to our Division Director, James F. Ganett, for his l eadership in prodding us to a goal of excel l ence in service to the leet. Al so, in preparing the manuscript for this book, I was unusual l y bl essed to have the professional assistance of a irst-rate technical editor, Ms. Jean E. Kennedy, and the outstanding qual ity of Ms. Jennifer A. Love's spl endid ty­ pography . (One coul d say in al l truth that Jean's and Jennifer's endurance in the preparation of this manuscript vvas a l abor of l ove.) Final l y , I thank my wife sincerel y for her wonderful patience and cheerful interest in my endeavors despite seemingl y being left single and al one for many an evening and weekend over the past year.

xiii

Introduction Electromagnetics, in the special sense that we will be concerned with in this book, is the interaction of electric and magnetic ields associated with

a­

diated energy-i. e. , energy in the form of radiated electromagnetic ields. As such, the principal topic of interest here will be shipboard devices that emit or sense electromagnetic waves in the radio-frequency spectrum from extremely low frequencies through extremely high-frequency microwave. Other portions of the electromagnetic spectrum such as light waves (e. g. , infrared and iber optics) will not be addressed in depth. Nor is any rigorous mathematical treatment of electromagnetic theory in­ cluded herein. Since the days of James Clerk Maxwell's classic treatise, exquisite theoretical works have been published in full measure. A representative sampling is included in the Bibliography. Emission of shipboard electromagnetic energy may be desired, as in the case of communications, navigation, radar, and weapons control systems through associated transmitting antennas. Emission of electromagnetic energy, however, may be (and, unfortunately, too often is) undesired, as in the instances of un­ suppressed intermodulation products, harmonic frequencies, broadband noises, spurious signals, impulse noise bursts, high-level sidelobe energy, parasitic re­ radiation, multipath relections, and radiation hazards. Similarly, the sensing of electromagnetic energy can be categorized into that which is planned (desired reception of communications traic, radar return pulses, navigation position ixes, and weapons control tracking), and that which is clearly unwanted whenever extraneous electromagnetic emissions are picked up or induced as interference. Naval ships, being so generously equipped with electronic systems, must contend with all aspects of electromagnetic radiation and reception. Very-high­ power emitters must coexist and operate simultaneously with ultrasensitive re­ ceptors compatibly in the complex shipboard environment. Interference between these many systems must be eliminated, or suppressed to minimum, in order to

xiv

have each system perform effectively in support of the ship mission. The effot is not easy. This book endeavors to discuss each of the chief concerns of shipboard electromagnetics and to shO\v how the problems of compatibility and interference are resolved. It is hoped that in this manner the book will provide much needed understanding and assistance to naval engineers working in ship systems design and operations.

.\T

Chapter 1 Historical Background

1-0

PRELUDE In the 16 May 1986 Washington Post, and in the 14 July 1986 issue of

the weekly Navy Times, it was revealed to the American public that the captain of HMS Sheield, the British destroyer sunk by an Exocet missile during the Battle of the Falklands, had his ship's radar turned off so that he could use radio communications back to England. It was because the radar was interfering with his operational phone communications that the captain ordered it shut down, and it was during the time the radar was off that the Exocet came in undetected.

1.2

The Navy Times repoter in paticular stressed the point that the problem was one of electromagnetic interference (EM!) between electronic systems, and that the same or similar problems could occur in a US ship. He quotes Navy sources who acknowledge that EMI is not understood as well as it ought to be, and that the Navy has for the last three or four years "been on an electromagnetic compatibility campaign." The Sheield incident dramatically underscores the serious impact that EMI can have in disrupting or degrading ship electronic systems, especially in crises. And most certainly there is an urgent effort ongoing to control the harmful effects of EMI. However, the problem is not new, nor are the strenuous efforts to contain it satisfactorily. There is in the very nature of shipboard electromagnetics an essential need for systems electromagnetic compatibility (EMC) and freedom from EM!. Were it not for this inherent relationship and the continuous struggle it breeds, elec- . tromagnetics aboard ship would be as satisfying an engineering discipline as any of those ashore, where the lUXury of widely separated facilities mitigates many of the compatibility problems. The shipboard situation unquestionably intensiies the issue. Because of this, each of the successes in ship electromagnetics from

1

2

SHIPBOARD ELECTROMAGNElCS

the beg i n n i n g (e . g . , radi o com m u n i c at i o n s, n a v i gation radar, and weap o n s c o n­ trol) has i ntroduced attendant electromagneti c i nc o m p at i b i l i t ies and i n terference pro b lem s . Thus, in effect, the h i story of s h i pboard electrom agnet i c s can be traced by rev iew i n g the efforts m ade fro m the very beg i n n i n g to stem i nterference and to estab l i sh c o m p at i b i l ity .

1-1

NAVAL ADOPTION OF ELEC TROMAGN ETICS It a l l began j u st pri or to the twentieth century w i th that forerun ner of

modern rad i o known as "w ireless . " The new c oncept of w ireless telegraphy, spurned by many skepti c s as of no real val ue, generated c o n s i derable i n teres t i n the U S Nav y . Recognizing that w ireles s m i ght o verc o me the l i m i t a t i o n s o f v i s u a l c o m m u n i c ations a n d n a v i gation a t sea , parti c u l ar l y i n fog , on starless n ights, and i n foul wea ther , the Na vy , on 26 October 1 8 9 9 , i n i t i ated the ir st s h i pboard exper i mental tests . It was the b irth of Amer i c an naval radi o c o m m u n i c ations and of the p henomenon of EM!. As part of the experi ments, the Navy Dep artment had the foresight to i n vesti gate the use of two tran s m i tters operat i n g s i m u lta­ neo u s l y and the meth ods u sed to overcome i n terference . L . S . H o weth i n h i s dein i t i ve History of Communications-Electronics in the United States Navy, wri tes w i th good h u m or: The res u l t s of the i n terference tests were perfect . That i s , the i n terference was perfect . From t i me to t i me the l and station tra n s m i tted s i g n a l s w h i le one s h i p was recei v i n g from the other , w h i c h al ways res u l ted i n utter confu s i on w i th the tape being rendered absolutely u n i ntel l ig i ble . Con­ cern i n g th i s defect it was reported: " When s i g n a l s are bei ng tran s m i tted from one stat i o n to another , as be­ tween the U S S NEW YORK and the H ighlands L i ght, and another vessel comes w i t h i n s i g n al i ng d i stance and attempts c o m m u n i c ation w i th the H i ghlands Li ght, then the s i gn a l s from the two s h ip s bec ome c o nfu sed, and the rece i v i n g station on sh ore is unable to d i s t i n g u i s h between them . " Th i s was very di sappo i n t i ng , but s i nce the three i n stallat i ons were operati n g on about the same frequency the result w a s i ne v i table . I f the same exper­ i ment were to be repeated today w i t h broad band tran s m i tt i n g equ i pment on approx i m ately the same frequenc ies the res u l t would be the same . The i n ab i l i ty to employ tunable eq u ipment at the t i me was unfortun ate, for i m pres s i o n s developed about the i nev i tab i l i ty of i n terference w i th ( w ireless) > equ ipment w h i c h pers i s ted for years . S o here we h ave the Navy ' s i n i t i a l u se of electromagnet i c s and its i m­ medi ate trou bles w i t h EMI, a natural relat i ons h i p sti ll w i th us near l y a hundred years later . It should be noted , too , that EM] was not the o n l y adver s i t y expe­ rien ced . The naval i n spec tors were c areful to record other potent i al problems

3

HISTORICAL BACKGROUND

associated with wireless transmission that s erved as precur sors of the electro­ magnetic h azards with us today: a . "The spark from the sending coi l , or faulty i n s u l ation of the sending wire , wou l d be suficient to ignite an inlammable m ixture of gas or other easily l i ghted matter . ,,4

b . "The shock from the sending coi l may be quite severe and dangerous to a person with a weak heart . ,,4

c . "The sending appar atu s and wire would injuriously affect the compass if pl aced n e ar it . ,,4

The irst of these potentialities we now refer to as hazards of electromagnetic radiation to fue l ( HERF) and to ord nance ( HERO) . The second i s now known as radiation h azards (RAD HAZ) to personnel and RF b un hazards . And the third is another of m ajor signiicance fami l i ar to us as e l ectromagnetic compat­ ibil ity , or more common l y , EMC . It is worthwhile to point out here that , depending on one ' s v i ew , i nter­ ference may not be always b ad . Less than two years after the Navy ' s ir st shipboard tests , i nterference was clever l y exploited for a deliberate gai n . I n the 1 90 1 Intenational Y acht Races three wire serv ices were to report on events: wireless experts Lee De Forest for Publishers Press As sociation , Guglielmo Marcon i for Associ ated Press , and John Pickard for A merican Wireless Tele­ phone and Telegraph . As might be expected , the competition among the three was stron g . Howeth recounts the amusing results: During the contest both the M arconi and De Forest mobile stations noticed their shore units signaling frantically with lags asking''What is the matter? S ignals confu sed . Cannot read . "

De Forest tried to i mprove h i s trans m i s ­

sion s , and , s e e i n g no more signaling , gained t h e i mpression he w a s getting through satisfactor i l y . When h i s tug docked he expected to be overwhelmed with congratul ation s , feel i n g he had made a great showi ng agai nst h i s competitors . However , t h e event h a d produced three losers , (the y acht) S HAMROCK II, Marcon i, and De Fores t . A merican Wirele s s , having no sponsor , h ad nothing to lose and everything to gain by pre venting the reception of their competitors ' tran smiss ions: "There is an account that the true culprit in this iasco was A merican Wireless Telephone and Telegraph Co . , which , upon fai l i ng in its efforts to get the press as sociations to m ake use of their apparatus in the 1 90 1 y acht races , set up a very powerfu l station near the Navesink Highl ands . Throughout the races they sent out so powerful a stream of electric d i s­ turbances that they produced the results previou s l y noted i n the Marconi and De Forest reception . Pickard maintains that ( A m erican Wire l e s s ) did report these races , saying 'And when I say "reported , " I mean reported and not what the M arconi and De Fore st people call reporting; namel y , manufactured news that had no basis of fact whate ver . ' H e stated that

4

SHIPBOARD ELECTROMAGNEICS

( A meri c an W i rel e s s) used a p l a i n aerial, 20-inch Queens c o i l, and a t u l i p i nterrupter m i n u s a l l weights, so t h a t spark freq u e n c y w a s quite h i gh. They put as much c u rren t in the pri mary as their i n terrupter wou l d stand and, i n so doing, rad iated cons iderabl e energy . . . Their rec e i v i n g station was l o c ated at Galilee and used aural reception as did D e Fores t. That, inci­ denta l l y, gave them an advantage over Marconi with h i s coherer and inker. Pickard c l aimed that o n the trip down to the race area a bright idea c am e to h i m as to the modus operandi to be employed to p revent Marc o n i a n d De Forest from rece i v i n g the trans m i s sions. He happened t o have a news­ paper at h and, in which one page had b e e n folded over i n pri n t i n g, s o that a l arge-type h e ad l i n e was s u perimposed over the ine prin t of the text. H e n o t e d that the s m a l l t y p e was almost u nreadab le b u t t h at the head l i ne was undamaged. Thi s gave b i rt h to his idea. W h y not u s e l arge type-n a m e l y l o n g dashe s many s e c o n d s i n duration to s m e ar the smal l-type ordinary dots and dashes of the competitors? Pickard proceeded to work up a c ode, which, he said, 'was s i mp lic ity itself.' As an example, one l o n g d a s h o f 1 0 seconds w o u l d m e a n COLUMBIA w a s ahead; two such d a s h e s wou l d i ndicate S H AMROCK w a s i n t h e l ead; three, t h e y were n e c k a n d neck. Fol l owing the irst seri e s wou l d come other long dashes from one to n i ne, identiied in the code as conveying common actions tak i n g p l ace. T h u s equ ipped, they were able to g e t their signals through and interfere with the others . 'Marconi and D e Forest didn't have a ghost o f a c h a n c e and our c lever rewrite men m ad e up a n i c e l o n g story from our coded simp l e i n structions.' S trange as it m a y seem, they rec e i ved i n s truc t i o n s from G a l i l e e sometime l ater to s p l i t time with Marc o n i, an order con s i dered coward l y b y Pickard. Contac t i n g the MINDORA, the A s s o c i ated Pre s s boat, with the Marconi s o - c a l l e d apparatu s on board as Pickard p u t i t, a liai son was arranged. I n rel ating thi s i ncident, the professor tel l s of his encounter with the p resident of the A s s o c i ated Pre s s, 'When some h u n dred feet away, none other than Me l v i l l e S tone c ame on deck with a megaphone and began to berate u s. For fu l l y 1 0 minutes he c u s sed us, n o t repeating one word twi c e, and wou l d prob ably be c u s s i n g u s yet if I had not gone bel ow, gotten an egg, and b y a lucky th row applied i t to him via h i s megaphone. Inc idental l y, h e stopped c u s s i ng, and a t t h e s am e t i m e the negotiation s topped.' In rel atin g what he called 'The inal incident of the race "report i ng,'"

Pickard s aid, 'When the yachts crossed the i n i s h l i ne,

we held down the key and then continued to hold i t down, b y the s i m p l e method of p u t t i n g a weight o n i t. Thus, radiating waves, far from prac t i c a l l y contin uous, though continuous in o u r s e n s e o f the word, we s a i l e d for our home port, and the b atteries l a s te d for the entire hour and a quarter that we u t i l ized to send the longest dash ever sent b y wire l e s s.' Fol l ow i n g the races, Pick ard retuned home via Nav e s i n k, where the l ig h thousekeeper

HISTORICAL JACKCROL'')

showed him around and said . ·Oh . by the w ay . we had wire l ess te l egraphy here the other day . The Marconi men \\'ere here with a l ittle blaCK box l ike a stock ticker. and paper came out of it with l ong b l ack l ines running down the middle of it. Every few minutes the operator wou l d pick up this tape. look at a few feet of it . swear unholi l y. tear the tape ofr. and jump on it. ' Of this Pickard stated. 'This was the best appreciation of effo1s .. that I ever received. ' " The above incident may be the i rst recorded use of EMI empl oyed for personal advantage. Later. as the wor l d went to war . intentional interference was introduc ed to jam enemy systems. and a \vhol e new science of using inter­ ference as an aid devel oped in what is nm\' known as elec tronic v.:arfare (EW)h Returning to our review of undesired interference associated with \\ire less. by 1902 the sudden proliferation of commercia l stations and amateur hobbyists with homebuilt wirel ess sets began causing such a high degree of interference that naval ofi cia l s urged the Government to regu l ate al l wire l e s s operations. Speciically citing wireless's chie f defect as its vulnerabil ity to interference. and of the opinion that the principal use of wireless would be for seagoing com­ munications for many years to come. the N avy D epaI1ment proposed itsel f as managing agency for a l l Government and private wire less stations on or ncar the coast, in order to prevent mutual interference. A fter considerabl e officia l debate and interservice jousting. agreement was reached. On 29 Ju l y

190t.

President Theodore Roosevel t signed an Executive Order design ating the N avy as control ling bureau for all Government stations (and (/I! stations during war: meantime the Department of Commerce wou l d begin regulating private stations). This act irmly established the Navy as the l eading devel oper of rad io e l e ctronics in our cou ntry during the early years. Before the end of 1904. the N avy had .D ships and 20 shore stations equipped with wire l ess. Despite these regu l atory attempts . however. the annoY'ing prob lem of in­ terference continued to increase among the competing Govenment . private. and amateur stations. Commercial operators . as an example. made unconceal ed at­ tempts to prevent each other from transmitting. And man y amateurs enjoyed the fun f interrupting both Government and commercia l traffic. The resu l t was more requests for crfective Government re gul ation. Unhappi l y in the meanwhile. the N avy was expe riencing its own dificulties with wire less interference during operations at se a.

In

1906. in an effort to

evalu ate the strategic use of \vire less. the Atl antic F l eet conducted large-scale oce an exercises. Because of the l imited ranges of the equipment and the vexing effects of interfe rence . the resul ts were quite disappointing. Such fail ures cau,cd senior na\'al oficia l s to have ,trong reserYations about the reliability of \\ irelcss cOIllIl1unications . s e rio u s l y retarding its development fr na\'al

me.

7

SHIPBOARD ELECTROMAGNEICS

6

1-2

EARLY ATTEMPTS TO REDUCE WIRELES S INTERFERENCE The Nav y, q u i c k to recogn ize that spark-generated wideband-tran s mitted

RF energ y wou l d have a high l i k e l ihood of cau s i n g mu tual i nterference, b e gan immediate l y to evaluate all avai lab le equ ipment prior to dec i d i n g w h i c h to pur­ chase in quan tity . Part i c u lar emphas i s was placed o n good s e l e c t i v i t y c harac­ teri stics i n the equ ipment to reduce mutual i n terferen c e. To fac i l i tate t h i s task, the Radio D i v i s ion of the Nav y's Bureau of Equ ipment was estab l i s h e d in 1 90 3 a n d c harged w i t h the re spo n s i b i l i t y of developing a n d procuring rel iab l e w i re l e s s sets that would operate without interfere nce. By 1 906 , separate l y tuned pri mary and sec ondary c o u p l i n g c i rc u i t s and improved spark-qu e n c h i n g schemes were u s e d to l i m i t the radiati on of u n d e s i re d wideband energ y . A t the same ti me, better operat i n g d i s c i p l i ne and c arefu l assignment o f wire l e s s frequency channe l s were i n c o rporated. 8 In 1 9 1 2 the new name "radio"

d i s placed the o l d e r word "w i re l e s s."

Two

ye ars later, during the 1 9 1 4 occ upation of Verac ruz, the Nav y experienced i t s irst use o f radio communications u nder war conditions. The res u l t s were not entirely sati sfac tory . The spark tran s m i tters of nearby fore ign wars hips generated such heavy interfe rence as to d i srupt c o m m u n ications tota l l y. A t i me- s h aring plan had to be worked out among the part i c i pants, re s u l t i n g i n US operators b e i n g a l l otted a two-h our period for radio tran s m i s s i o n and the other fo ur nati o n s o n e h o u r apie ce . T h u s there was a four-hour i n terval e a c h day w h e n i t w a s n o t p o s s i b l e for m i l itary h eadquaters i n Was h i ngton to be i n c o ntact w i t h its force s in t h e ield . Note that a n operational method ( ti me-sharin g) had to be i m plemented to avoid the con sequences of in te rference-a method proposed (along with fre­ 9 quency manageme nt) as early as 1 9 1 1 , and though better reined in vario u s ways, s t i l l u s e d t o t h i s day . The ne c e s s i ty and val u e of rad io was proven man y times over, at sea and on land, during World War I. Such widespread u sage of course s p u rred devel­ opment of equ i pment i mprovements . In 1 9 1 8 , superheterody n e techniques in re ceiver c i rc u i try were introduced to allow broader R F ampl iication, better s e l e c t i v i ty, and much eas i e r operati on . I t was soon appare nt to the Navy, h ow­ ever, that the s u perhet rec e i v e r was far more s u s c e p t i b l e to i nterferen c e aboard, s h i p where many tra n s m i tters and re c e i vers were operating i n c l o s e prox i m i t y. It took carefu l appl ication of s h i e l d i n g te c h n o l o gy, c irc u i t i s olation, o s c i l lator stab i l ization, and RF preselection to adopt the s uperhet sat i s factori l y to s h i p b oard serv i c e .

10

. It was during th is period, too, that means were sought to e l i m i nate the considerab le harmonic i nterfere nce created by the Navy's arc type of tran s m i tter . H i gh-pass il ters were added as an arc shunt, and c o u p l i n g of the arc to a rej e c tor circ u i t was empl oyed to suppre s s u nwanted e m i s s ions. Nev e rthe l e s s, l eet ex­ erc i s e s of 1 9 2 2 - 2 3 c l early showed that arc and spark transm itters generated so

7

HISTORICAL BACKGROUND

much interference that s i m u ltaneous shipboard reception was vit u a l l y i mpos­ sibl e. With requ irements for the n umber of communications chan ne l s rapidly increas i ng, a solution to the i nterference prob l e m had to be found. Fotun atel y, the answer came i n the early 1920s with the app l i cation of e lectronic v ac u u m tubes. Transmitters using tube c ircuits produced far l e s s RF "trash" than the arc and spark predecessors. A t about the same time an anti­ keyclick device was adopted to eliminate tran sient c l icks being received during transmitter keyi ng, and thereby affording much c l oser frequency c h annel spacing.

1-3

FROM RFI TO EMI By the 1930s there arose an engineerin g art devoted s o l e l y to the study,

measurement, and re solution of radio i nterference . Naval l aboratories and private indu stry alike sought methods to cope with both the production and the reception of shipboard noise interference. The term radio frequency interference, or RFl, began to be used for describing the n ature of undesired electromagnetic e m i s sion phenomena, whether b y radiation, induction , or conduction. Results of Navy testing, and indings, began to appear i n documented journal aticles and reports. A representative samp l i n g from the 1930s incl udes:

I I

a. B u lletin of Engineering Information No. 101, October 19 36: "Tra n s m i tter I mprovement I nterference Elimi nati on" b. Bulleti n No . 103, Apr i l 1 9 3 7: "Spurious Interference Responses i n S u, perhets' c. Bul l et i n N o . 104, J u l y 1 9 3 7: "Interference S urveys" d. B u l letin No. 105, October 1 9 3 7: "Noise E l i m i nation" e. Bulletin No. 106, January 193 8: "Interference and Recei ver S e l ectivity" f. Bul letin No. 109, October 193 8: "S urvey of Radio Noise on USS Yorktown"

1-3.1

World War II Naval Electronics and RFI Fueled by the urgent needs of Wor l d W ar II, the 1940s witnessed a tre­

mendou s s urge in n ew technology and in the n u mber and type of shipboard electronics syste m s . RFl prob lems compou nded dramatic a l l y. Now air search radars, surface search radars, weapon iring radars, radio navigation systems, and e lectronic countermeasures equipment vied with radio communications i n t h e congested shipboard environment for a share o f t h e crowded electromagnetic spectr u m. Because of i mmedi ate operational needs, new equipment h ad been hurriedl y installed without regard to compatibi l i ty or interference. While osten­ sibl y increasing the ship's m i s s ion c apabili ties, the mutual disturbance resulting fro m addi ng so many new e lectronic systems soon l i mited equipment effecti ve-

R

SHIPBOARD ELECTROMAGNEICS

n e s s. Whereas l arge prewar s h i p s might have h e l d ive or s ix rad io tran s m i tters and a half dozen radi o commu n i c ation and navi gation rec e i vers, the l arge World War I I combatant ships c arrie d a s many as 1 2 radi o tran smi tters, 1 8 rec e i v ers, p l u s a c o u p l e of radars. So many h i g h-power rad i at i n g and s e n s i t i v e rec e i v i n g d e v i c e s h a v i n g to operate i n proximity c reated s evere i ntra- a n d i n ters h i p RFl.

12

Now the Navy perc e i v e d that it h ad to fac e a real struggle, and, on 1 4 J u n e 1945, i s sued the irst j o i n t Army-Navy R F I standard, J AN- I - 225, t i t l e d Radio Intelference Measurement. A n a l y s e s of what RFI is and what causes it were promoted, as w e l l as continued i mprovements to prevent it. Better s h i e l d i n g methods were app l i ed to i s o l ate and contain in terference with in the s o urce, w h i l e at the same time to exc lude its e ntry i nto s u sceptible c i rcu its. Fi ltering networks were d e v i s e d to reroute RFI away from c a u s i n g h arm.

1-3.2

Postwar Efforts Ofi c i a l concern about the grow i n g complexity of RFI and the pote n t i a l

c atastrophic fai l ures it might produce was demon strated by the tri -serv i c e c o n tract awarded to the Armour Research Foundation ( now I l l i no i s I n s titute of Tec h­ nology Rese arch) in 1 9 5 3 to determ i ne the magnitude of the RFI pro b l e m and to recommend means for red u c i n g i t. S h ortl y there after the First Tri-Service Conference on Radio Frequency Interfere nce, sponsored b y govern m e n t and 13 i n dustry, was h e l d i n 1 9 54. Three years l ater, on 1 0 October 1 9 5 7 , the I n s titute of Rad i o Engi neers ( n ow Ins titute of E l ectri c a l and E l ectro n i c E n g i ne ers, IEEE) estab l i shed a Rad i o Freque n c y I n terference profess i o n a l soc iety. In June 1 9 5 8, the Navy pu b l i s hed i t s Electronic Intelference Control Manua l for Forces Aloat [ 1 1 ], a handbook to a s s i s t i n prac t i c a l app l i c ation of i nterfere nce detection and reduction i n s h i ps. The manual was a c l a s s i c for its time as a c o mpre h e n s i v e source of s h i pboard R F I descript i o n, cau ses, and re med i e s. Further, Appendix 1 of th i s manual i s a v a l u ab l e b i b l i ography l i s t i n g naval reports, art i c l es, and ield changes from 1 9 36 to 1 9 56 re l at i v e to rad io i nterfere nce. Interest i n contro l l i n g RFI was g a i n i n g rap i d l y as e v i denced b y a quote 14 from the 1 October 1 9 5 8 Fourth Tri-S erv i c e Conference on RFl: Unfortu nate l y there has been a tendency on the part of many of u s in e l e ctron i c s to tre at the prob l e m of i n terference e i ther as a n e c e s s ary e v i l o r o n e wh i c h wou ld g o away if w e i g nored i t . W e poured huge human and ina n c i a l re sources i n to the devel opment of tru l y m arve l o u s e l e c tro n i c equ ipments a n d systems, o n l y to h a v e t h e m re ndered, i n m a n y i n s tances, complete l y i neffective because we have fai l ed to app l y what wou ld have been a rid i c u l o u s l y smal l port i o n of the overa l l e ffort to the probl e m o f i n te rference reducti on. Today, t h e fo l l y of th i s overs ight i s c learl y e v i dent.

9

HISTORICAL BACKGROUND

Interest in undesired electromagnetic radiation c h aracteristics was not l im­ ited to equipment interference effects , however. There was growing anxiety about s afety h azards involved with electromagnetic radiation . To review these concerns and set a course of action , the Department of Defense Electromagnetic Radiation Hazards Worki n g Group conducted its inaugural meeting on 30 Sep­ tember 1 9 58. As an outcome of the meeting , the Navy's Bureau of Ordnance was assigned responsibi l i ty for developing standards of HERO; the Bureau of Aeronauti cs was c h arged with estab l i s h i n g standards for H ERF; and the Bureau of S h i p s was ass igned to develop RF RADHAZ technology as fol l ows: a. Terminology b. Units of measure c. Field intensity measurement techniques d. Instrumentation for measureme nts e. Bibliography of papers publi shed in RF RADHAZ f. Directory of current RF RADHAZ projects.

IS

Just three months l ater , on 9 January 1 9 5 9 , the Navy conducted its irst of a series of power density tests on hi gh-power rad iating equ ipment at a man­ ufacturer's fac i l ity. The measurements , done spec iic a l l y to determine RADHAZ safety zones , were performed on a long-range shipboard UHF air search radar operating at two megawatts peak power with a pulse repetition rate of 300 Hz and pulse width of s ix microseconds . The test results concluded that the 1 0 m i l l iwatts per square centimeter safe expos ure l i m i t for personnel wou ld be 16 exceeded within 1 20 feet of the main beam . These initial contro l l ed fac i l ity tests served as the foundation for building an exte nsive l i brary of equipment radi ati ng hazard levels in terms of power density and di stance. Thus , the period fol l owing the accelerated el ectronics growth of World War II was one of examining the many aspects of RFI and of seeking e ffective ways to contend with it. Admiral Joseph E. Rice , noting the Navy's sponsors hip of n umerous studies , development of new test equ ipment , and experi mental work in grou nding , shielding and bonding , remarked in h i s opening addres s to the Tenth Tri-Serv ice Conference on EMC that the 1 950s could be c h arac terized as 17 a ti me of "learning the phenomena" of RFI .

1-3.3

EMC and the Vietnam War Period The 1 960s ushered in a broade ning scope and hei ghtened aware ness of

electromag netic systems interfere nce . Electronics equipment on typical aircraft cariers , for exampl e , had increased threefold to 3 5 radio transmitters , 56 radio recei vers , 5 radars , 7 navi gational-aid systems, and we l l over 1 00 antennas .

18

The formal use of the term electromagnetic compatibiliy began appearing , when, in January 1 960 , the Navy's Bureau of Ships distri buted its irst Compatibility

10

SHIPBOARD ELECTROMAGNEICS

of Shipboard Electronics Systems Manual, s urement of radi ated RF energy .

19

outl in i n g procedures for the mea ­

Of even more s i g niic ance, i n 1 96 1 the De­

partment of Defense establ i s hed the Elec trom agnet i c Compatib ility A n aly s i s Center ( EC A C) , loc ated a t the Naval E n g i neeri ng Experiment S tatio n i n A n­ napo l i s, Mary l and . The Center was m ade res pon s i b le for apply i n g the newe s t computer m ath model i n g and data proces s i n g analyses for ev a l u at i n g t h e elec­ tro m agneti c envi ronmen t. develop i n g procedu res to i n c rease s ystem compati­ 0 bil i ty . and red u c i n g the causes of in terference and s u s ceptibil i ty . 2 By the m i ddle of the 1 960s, the o l der term "RFI, " w h i c h had been i n use for 3 0 years or so. was gradu ally d isp l aced b y the more comprehen s i ve and descrip tive expression "EM!. " Deined as any u ndesired radiated or c o n d u c ted peturbation which degrades the proper operation of e lectri c a l or electro n i c equ i p­ ment. EMl enco mpasses a broader spec tru m of interest than RFI . As s u c h, the ca uses of EMI were also becoming much more speciic: in addition to atmospher i c noises and man-made noises generated b y elec t rical mac hinery , igniti o n systems, luorescen t lighting. we l ding equipment. and circ u i t breakers and switches, i n­ terference resu l ted from (I) i n termodu lation noise from m ixi ng of s i g n a l s i n non linear transmitter or receiver circui t s t o create new s u m and difference fre­ quencies: (2) intermod ulation noise caused by mixin g of s i g nals i n extenal n o n linear me tallic ju nctions in the ship stru ct ure. r i g g i n g. and appendages­ pri ncipal ly in corroded or oxidized fastenin gs and join ts. i . e .. the so-called' 'ru s ty bolt"

effect: (3) harmo nic and spurious noise products generated i n tr ansmitter

circuits and not properly i l tered out or attenuated: a n d (4) cochan nel and adj acent chan nel in terference presen t when portions of a sig nal from one channel penetrate into another. 21 To con trol the levels of EMI emissions and susceptibi lity in the design and pro d uctio n of elec tronic sys tems. the Navy iss ued MlL-S TD-469 , "Radar En gi neering Design Requirements for Electromag net i c C o m p atib i l i ty," in 1966. and MIL-STO-461 . "Electromagnetic Characteristics Req uirements for Equipmen t." in 1967. Along with more detailed kn owledge of what EMl is came better ideas of how to preve n t

r

suppress it. Meth o d o l o gies were proposed in the des i g n proce s s

for optimizing the topside arrangement o f the many electro magnetic e m i tters a n d sensors to enhance isolation (RF decoupling) and t o reduce degradat ion . That is. a bet ter un dersta nding of the shipboard elec tro magnetic en viron ment w as 22 stron gly encouraged. Moreover. i mprovements in ship des i g n and construction

tech niques were urged to red uce EM!. such as the liberal use of n o n meta l l i c J materials for lifelines and vertica l ladders . 2

HISTORICAL BACKGROUND

1/

Problems of EMI were n oted with much dis m ay during naval combat operations of the Vietnam W ar. Recounting the times, Captain J. S . Oller, Jr . , USN, wrote: By the l ate 1 960s, the magnitude and nu mber of elec tromagnetic prob lems were having appreciable efects on Fleet operation s to the point where Fleet capabilitie s were actual l y constrained by them. Task Force and U nit Commanders were required to take into account the limitations of their electronic s interfaces when ordering actions . In some instances, it was standard practice to shut down certain search rad ars and communic ations tran smitters when missile alert condition s were set in the G u l f of Tonkin. In other instances, aircraft takeoffs and l andings dictated such ac tio ns. It was a re al- life, very constraining enviro nment in whic h U . S. combatants . 24 were operatIng. In recog nition of these c o ncens, the Secretary of Defe n se, on 5 Ju l y 1 96 7, signed a directive to establish an in tegrated Department of Defense program to ensure electromagnetic compatibi lity. Fo l l owi ng this, the Chief of Naval Op­ erations acknowl edged the magnitude and seriousness of EMI prob lems by cre­ ating an ofice of Tactical Electro magnetic Coordin ator on 24 No vember 1 969, and made the fol l owin g statement in his directive: One of the Navy's most urgent problems is the management of the elec­ tromagnetic environment of naval task forces. Elec tromagnetic equipment is essential to every mode of naval warfare. In many instances ship and aircraft systems u sing electronic device s have been developed with in ad­ equ ate regard for comp atibility with the total electromagnetic environment . Electronic pl anning has in many cases been in the nature of a reaction to meet speciic, independent needs. The urgen c y of immediate problems has in many cases dic tated actions wi thout reg ard to the more invol ved con­ sideration of systems integration. This frequently has encouraged random proliferation of electronic programs and has created a m u l titude of budget items in a l l appropriation categories. As a res u l t, optimization of the el ec­ tromagnetic environments for both offense and defe n s e has not been 25 achieved. These high-level moves c l early signalled the Navy's organized scientiic and engin eering management approach to ighting the battle of EMI i n the 1 9 70s and ' 80s.

12

SHIPBOARD ELECTROMAGNEfCS

1-4

THE MODERN ERA

1-4. 1

Emerging Management Interests In 1 97 0, at the request of the C h i e f of Naval Operati o n s, a thorough study

of e l ectromagnetic probl e m s b e i n g experienced i n the leet was i n i t i ated. In Febru ary 1 97 3, t h i s i n v e s t i g ation , known a s the Tac t i c a l Electromagnetic S ys­ tems S tudy , or more s i m p l y the TES S , produced an i mpre s s i v e e leven-v o l u m e report w h i c h identiied over 600 problems. U n fortun ate l y , o n l y l i m ited d i s tri­ bution o f the report was m ade; c o nseque n t ly , n o conc erted action was taken to re solve the prob l e m . Even though adequ ate l y identiied, the k nown pro b l e m s pers i sted , a n d n e w ones were b e i n g i n troduced upon acqu i s ition of n e w s y s t e m s . Quoting again from O l l e r on the s i t u ation exi s t i n g at that t ime: There are a complex set of c irc umstances w h i c h m i l i tated aga i n s t improve­ ments . A l t h o u g h dire c t i v e s c l e arly required e l ectromagnetics c o n s i derat i o n in acqu i s i t i o n s a n d for i n-serv i c e equ ipments. i n rea l l i fe t h i s fe l l through the cracks . Acqu i s i t i o n and Program M anagers ' attentions were on many other m aj o r prob l e m s . and ele ctromagnetic interfere nce prevention s i m p l y w a s lost i n t h e shufl e . I m provements were a l s o h o b b l ed by the in adequate spec iicat i on s . cons trained b y a l ac k of fu nds w i th whic h to pre v e n t or correct prob l e m s . and . lastly . suffered fro m an i n s u fic i e n t fee d-b ack o n 26 e i ther prob l e m s i n exi s ting s y s tems o r t h o s e d e v e l o p i n g i n acqu i s i ti o n s . Coinc idental l y , at the very t i me of the TES S report i n d i n g s, i n Febru ary 1 9 7 3, the Navy also launched i t s amb i t ious S h ipbo ard Electromagnetic Com­ pat i b i l ity I m provement Program ( S EMCIP) . S EM C IP was c h artered to develop standards by which corrective acti ons required to s uppre s s EMl could be effec­ t i v e l y tested. re g u lated, documented, and pro m u l gated to the l e e t . M oreover , S EM C IP was to g i v e broad appl ication to preventive measure s for the reduction 7 of E M!, mainly b y accom p l i s h i n g a threefold task:2 a . The design and proc urement of U S naval s h i p s with e l ectronic s y stems that wou l d be ele ctromagn e t i c a l l y compati ble . b . The identiication and re duction of E M I aboard s h i p s c u rre n t l y i n the operating leet . c . The prov i s ion of training to a l l personnel invol ved in the design , pro­ cu re ment, i n s tal l ation , maintenance , and operation of a naval s h i p and its electron i c systems to ensure that these i n d i v i d u a l s have an understand i n g of t h e requirements a n d proc edures for ach i e v i n g a n d m a i n t a i n i n g s h i pb o ard EMC throughout the l i fe of a s h i p .

13

HISTORICAL BACKGROUND

H ere was the irst i n stance of a h i g h l y organized quick-response engineeri ng approach dedicated solely to resol v i n g EMI problems being reported by the leet . There was continued pressing, too, in the early 1 97 0 s, for the adoption of new ship construction techniques to reduce EM! . I nnovative developments in welding and j o i n i n g processes so as to do away with an exc ess of bolted and riveted j o ints were emphas ized, as were new seal i n g compounds, gasketing materi als, and bonding methods, a l l for i ncreased corrosion resistance and de­ creased likel i hood of "ru sty-bolt " i ntenod u l ation s . Along with these tech n o l ­ o g i e s were additional c a l l s for the replacement of l arge metal l i c topside items such as storage boxes, l ag b ags, stanchions, j ackstaffs, and l adders with non­ metal l i c g l as s-reinforced p l astics; and j udicious separation, routing, and shielding of cables i n order to preclude EMI pickup and reradiation .

28. 29

Likewise, very speciic electron i c c ircu i try methods were b e i n g employed to reduce equ ipment performance degradation caused b y EM! . For exampl e, the 30 fol lowing improvements were c ited for shipboard surveil l ance radars in 1 9 76: a. Prevention of receiver s aturation b. Reduction of fal s e a larm rate c. Enhancement of signal-to-i nterference ratio d. Di scrim i n ation of d irectional interference; e.g . , s i delobe j amming e. S uppression of s tationary ( s l ow-mov i n g) c l utter Figure 1 - 1 i l lustrates the EMI suppre s s i o n methods used to achieve the above-l i s ted improvements .

1-4.2

Establishment of TESSAC A special Tactical Electromagnetic S y stems S tudy Action Cou n c i l, or

TES S AC, was formed in August 1 97 5 "to examine the TES S report and deter­ mine the u n derl y i n g causes for the many unresolved problems, and, i n a l l y, to provide a p l an of action for resolution of existing problems and prevention of ,,31 future problems. By queryi n g naval programs, l aboratory personnel, and systems engi neering directorates to ascert ai n whether the TES S-reported prob­ lems s t i l l exi s ted and what remedies had been appl ied, the TES S A C found that: a . The maj ority of Fleet tactical electrom agnetic problems which prompted the TES S effot s t i l l exi sted. b . I nadequate emphasis was being given to tactical electromagnetic consid­ erations i n the deve l opment and acqui s ition of n ew systems and equ ipment . c . Know n electromagnetic deic iencies i n systems and equ ipment i n serv i c e were not being aggressively corrected . d . Directives were being c ircumvented . e . Existing management of the tactical electromagnetic effort was being ig­ nored or man i p u l ated so that overal l effectiveness was min imal .

14

SHIPBOARD ELECTROMAGNElCS

PERFORMANCE IMPROVEMENTS

z 0 -

W � : :

� : : � � :

Z 0 -

)

INTERFERENCE SUPPRESSION TECHNIQUES

: w > -

� z w > w : ..

w U w :

2

: :

z 0 -

:

w

) 1

: L

1

2

AUTOMATIC NOISE LEVELING (ANL)

1

2 1

2

w u z : I Z w

,

0 � : :

-

1

: z 0 -

� U w : -

0

DICKE-FIX(OF)

1

1

FAST TIME CONSTANT (FTC)

1

1

1

1 1 1

PULSE COMPRESSION (PC)

1

PULSE-TO-PULSE CORRELATION (PPC) 1

1

2 1 1

SIOELOBE CANCELLATION (SLC)

2

VARIABLE REPETITION FREOUENCY (VPRF)

Note

Numeral 1 denotes occasional benefit.

1

1 1

VIDEO INTEGRATION (VI)

WIDE-PULSE BLANKING (WPB)

1

2

SIDELOBE BLANKING (SLB)

WI DE-BAND LIMITING (WBL)

)

2

MOVING TARGET INDICATION (MTI)

SENSITIVITY TIME CONTROL (STC)

Z 0 ) ) w : .. .. �

1

FREOUENCY AGILITY (FA)

NARROW-8AND LIMITING(NBL)

: W � z � 1 0 u � : : z : z 2 0 : 0 � ) : � 0 )

1

BURN-THROUGH (BT)

MANUAL GAIN CONTROL (MGC)

� z w

0 � , � U 1 : � 0 z w J : )

1

AUTOMATIC GAIN CONTROL (AGC)

BEAM-TO-BEAM CORRELATION (BBC)

w u z w : W L : w � Z -

w u z w : w L : w � z

1

2

1

1

primary purpose of the technique, 2 denotes secondary

Figure 1-1 S h i pbo ard Surve i l l ance Radar Interference Suppre s s i on Tec hniques

(1976)30

HISTORICAL BACKGROUND

15

In March 1976, the TESSAC released its recommendations. with particular emphasis on managing the tactical electromagnetic effort, enforcing existing policy, and ensuring the implementation of existing directives. In recogmtion of its continued need, the TESSAC was asked to continue its work and \.vas directed to investigate contemporary electromagnetic effects; to determine the capabilities of naval laboratories and engineering commands to correct electro­ magnetic deiciencies: to determine adequacies of speciications and standards in electromagnetic effects: to develop detailed plans to ensure the consideration of deleterious effects of EMI throughout the acquisition process: and to develop electromagnetic technology research and development programs. The results of this work were summarized in the TESSAC report of Sep­ tember 1977. The report noted that: (1) the current state of technology was viewed as adequate to prevent or reduce most of the Navy's electromagnetic problems; (2) capabilities varied among analysis, testing, prediction and instr�­ mentation, with the depth of manpower insuficient; and (3 ) speciications and standards were unanimously viewed as the weakest area of all. being cited as not satisfying the need for electromagnetic controls in acquisition, as overlapping, as contradictory, as noncurent to technology, and as impractical to implement. As previously, the TESSAC stated its opinion that policy and implementing directives were adequate to provide for necessary inclusion of electromagnetic considerations. The primary recommendation of the Council was that the Navy ensure that policy and directives be complied with, that funding be provided. and that cognizant commands establish and suppOl1 programs adequate to handle electromagnetic problems effectively on a continuing basis.

1-4.3

Implementation of EMC Management Reacting to Chief of Naval Operations policy guidance, and likely antic­

ipating the forthcoming recommendations of the TESSAC. an instruction was issued on 13 January 1977 to implement EMC management procedures at the 32 ship systems command level. The instruction directed that an Electromagnetic Compatibility Program Plan (EMCPP) be prepared upon initiating development of all electronics equipment and systems designs which involve electromagnetic radiation. Furthermore, planning. programming. and contractual documentation must provide ror EMC requirements. analyses, measurements. test and evalu­ ation, and all applicable standards and specifications must be invoked. An EMC Advisory Board (EMCAB) must be instituted for all ship and major systems programs during the design. acquisition. and construction phases for review. advice. and technical consultation on all electromagnetic aspects to identify and

/6

SHIPBOARD ELECTROMAGNEICS

reso l v e potential e l e c tromagnetic prob l e m s . A l l s h i p alterations ( S H IPALTs) , equ ipment ield changes , engi neeri n g c h ange proposals ( EC P s), and requests for waivers must inc lude an EMC impact statement . A l l new e l e c tro nic equ ipment and systems must be s u bj ected to thorough EMC anal y s e s prior to commencement of development to ensure electromagnetic c ompati b i l i t y with the operational environ ment . Further, EMC training and education m u s t be provided for naval and con trac tor personnel . Program man agers m u s t e n s u re that adequate fu n d i n g i s requested t o pe rform required EMC a n a l y s e s and measurements to c o m p l y with t h e requ i rements a n d pro v i s i o n s of the i n struction , a n d to re s o l v e ex i s t i n g a n d a n t i c i pated l eet EMC prob l e ms . This i nstruction made i t clear i n no uncertain terms that henceforth EMC wou l d never be an afterthou g h t in s h i p design or equipment development for the Navy. An e l e c tromagnetic doc tri ne for the modern Navy was irm l y estab l i s hed from t hat po i n t . A s a n a i d t o better under s t a n d i n g the causes and e ffects o f E M L i n J u n e

1977 t h e N a v y pub l i shed t he CommClnding fic er ' s G liide to the Sh ipboard Electromagnetic Em 'ironl11 el1 l . This m ilestone document d is c u s sed t y p i c a l ex­ amples . and sources of EMI and the preve n t i ve and corre c t i ve meas ures taken to m i n i m ize EMI degrada t i o n . A l i t tle over a year l ater , in September 1978 , a second pu b l i ca t i o n followed . e n t i t led Th e Electronic Mmeriul ficer ' s G uide to Ship boord Elecrromo,neric inrerFerence Conrrol , to pro v i de tec h n i cal i n for­ mat ion and management proced ures helpf u l i n the perform ing of EM I c o n tro l f u n c t i ons . Also i n 19 7 8 . as an adjunct to SE M C I P . a new plan of action was i n troduced at the sh ip yard level called the Waterfr o n t Correc t i ve Action Program . or W C A P . The succes sful applicat i o n of E M I soluti ons learned thro u g h S EMCIP wou l d now b e i ns t i t u t i o n alized i n t h e y ard s to ensure t h a t surface s h ips wo u l d be repa ired. o verhauled . and m a i n ta i n e d i n

a

man ner to i m prove E M C . J J Tra i n i n g

a n d aware ness mater i al. stan dard ized procedures . d a t a i les o f known prob l e m s , and i mprove men ts of s pec i fi c at i o n s w o u l d be developed to i m p lement and extend the life span of sh ipboard EM! con trol. Typ i cal W C AP tec h n ical assistance was 14 offered to i n c l u de : , a . Selec t i ve bond i ng and grou n d i n g -such i te m s as i n c l i ned l adders, c l i m ber safet y ra ils , l i fel i nes . stan c h i o n s . metall i c flag s taffs and jackstaffs , expan­ sion jo i n t s . tilt i n g a n ten n a mou n t s . and safety nets . b . Sh ield i ng - s u c h a s mas t-mo u n ted cables agai n s t ma i n beam rad iations fro m radars . c . Bla n k i ng -such as the empl oyment and pro per progra m m i n g of pu lse­ acti vated blankers w i t h radar di rectors and EW rece iver s . d . U se o f glass -re i nforced pla s t i c s or o t her nonmeta l l i c materials a s selec t i v e replacements for life l i nes , lad ders , b o a t span ner wires, preventer stay s , boat gripes , and nag jac k s ta ff s .

HISTORICAL BACKGROUND

17

e. S e l e c t i v e replacement of ferrous hardware topside a n d i n ante n n a near ields with nonmagnetic materials. f. I n s u l ating-such portab l e items as fog nozzles, davits, l ifel i nes, booms and personnel stretchers to prevent metal-to-metal contact.

1-4.4

Rising Interest in EMP

Toward the end of the 1 97 0s, another form of EMI began to raise grow i n g 35 concern for naval shipboard sy stems-that o f e l ectromagnetic p u l s e, o r EMP. Generated by the h i gh-altitude detonation o f a n u c l e ar warhead, t h e extremely h i gh levels of ield intens ity i n EMP could prove c atastrophic to the very sensitive micromini ature sol id-state c ircuit components employed widely in s h ipboard equ ipment. As a consequence, new technologies in s h i e l d i n g and in surge pro­ tection devices were being devel oped and incorporated to h arden s h i p sy stems­ agai nst the pote ntial e ffects of EMP.

1-4.5

The C urrent Status By the 1 9 80s the Navy had become well accustomed to the phenomena

of s h i pboard electromag netic i nterference. More than eighty years of experi ence had made EMI both a fami l i ar and an expec ted challenge . Procedures are now quite well known about how to recognize and measure EMI for what i t i s, and management methods on how to contend wit h it are explicitly stated as mandatory policy throughout the Departmen t of the Navy. Fore most, it i s c u rre n t l y we l l 36 estab l i s hed that control o f EMI has t o begin with the electron i c design engineer: Each desig ner of a component or circuit or new equipment or entire electronic sy stem must b e aware of, and use, a l l available means to con trol EM!. Then, upon completion of the design, the device must be thoroughly s u bj ected to tests for evidence of EMl generation ( or suscepti b i l i ty). Last, the sy stem i ntegration engineer must cons ider the electromagnetic environment i n which the device must operate, and the instal l ing engi neer must conform to exact i n g methods to m i n imize EM!. This process i s essential to affordi ng the equ ipment and systems at l east an opportunity to operate effectively i n performing the i n tended m i s s i on, and it results in much saving of time and money. Making corections after the fac t i s costly. S econdly, there i s now a s trong empha s i s on documented requirements . The operational requirement ( O R) for any sy stem should deine the electromag­ netic environment, fri e n d l y or hostile, in which the system w i l l operate. Furth er, the implementation plan should identi fy sy stem v u l nerabi lity to EMl and means to reduce the risk. The Development Proposal should addres s methods for ob­ tain i n g the speciied levels of EMI control. The Top Level Requirements should

SHIPB OARD ELECTR OMA GNEICS

18

s tate the amount of acceptable EMI degradation . The Test a n d Eval uation M a s ter Plan ( TEMP ) s h o u l d specify the appropriate te s t i n g to e n s ure that requ ired op­ erational c h aracteri s t i c s are met . S i m i l arl y , the Request for Propos a l s must i n ­ c l ude t h e a n t i c ipated e lectro m agnetic e n v iro n m ent, t h e performance requ irements i n that e n v iro n m ent, and the e l e c tromagnetic test, e v a l u ations, anal y s e s, s i m u ­ lations, a n d data to control EMl . Final l y, the Electromagnetic C o mpati b i l i t y Program P l an i s the top- l e v e l management d o c u m e n t for EMC during t h e d e s i g n a n d acqu i sition . T h i s P l a n i s u s e d pri mari l y by t h e d e s i g n and proc uring activity to e n sure that a l l pert i n e n t EMC c o n s i derati o n s are i m p l emented throughout the acqu i s it i o n program, i n ­ c l u d i n g the m e a n s for EMI contro l , fro m start thro u g h i n a l d e s i g n a n d pro d u c t i o n a n d throughout the operat ional l i fe of the equipment . I n the area of prac t i c a l app l i c ations, the 1 9 8 0 s have seen several n e w i m­ prove ments . For one thi ng, h ardw are solutions that have proven s u c c e s s fu l for spec iic prob l e m s and are seen as appl icable to c o m m o n l y experienced troub l e s have been devel oped i n t o generic standard ized modu l ar u n i t s . T h e s e add-on interfere nce s uppre s s i o n modu les are used to correct s h i p board EMI d e i c i e nc i e s . They i n c l u de s u c h items as t i me and frequency b l ankers, notch i l ters, s i gn a l process ors. broadband i n terference c ancel lers . s e l f-interference c a n c e l l ers, and a chemical bond i n g agent to red uce i n termodu l ation by neu tral izing n o n l i ne ar corroded j u nctions . 3 7 Another i mportant innovation for re d u c i n g the effects of EM I i s the ren e w e d in terest i n u se of radar absorb i n g m ateri al ( RA M ) . T h e u n ique abi l ity of R A M to abs orb RF e n e r g y make s i t p at i c u l arly usefu l for the decoupl i n g of c l o s e l y located electro magnetic s y s te ms and for the reduction o f re lected ( mu l t ipath ) e l ectromagne t i c energy from s h i p structure s . Because of these meri tori o u s fe a­ tures , RAM is becoming an i n d i spe n s a b l e e n g i n eering technique for control of shipboard e l e c tro magneti c degradation . 38 The 1 9 80s have w i tnes sed. too.

a

re m ark able s u rge in the application of

computer mode l i n g as an aid i n enhanc i n g shipboard E M C . Col or-graphic i l - ' l u strati ons are rap i d l y generated to d i splay prospective performance and deg­ radation as a fu nction of s y s tem i n tegrat ion . The d e s i gner is able to di scern i m mediate l y the advantages, or pitfal l s , in vary i n g arran gements of e l e c tromag­ n e t i c s y s tems, and then to present the rationale for re comme nded options v i s u al l y . . S o many years of naval experience w i t h the cau ses, e ffects, and re s o l u t i o n s of s h i pboard E M I have res u l ted i n t h e accumu lation of a n en ormous amount of data . To fac il itate eficient use of this data, the Navy has implemented a com­ puterized data man agement program for EMC de s i g n feedback and anal y s i s . 3 9 Th i s autom ated data base prov ides a u n iied s y s te m of col lecti n g, c o n s o l idating, reporti ng, and anal yzing EMI prob lems . The data is then s tored for feeding i n fo rm ation back into the s h i p system design and procure m e n t proc e s s . In this manner i t is hoped that the res u l t i ng lessons learned will s y s temati c a l l y pre c l u d e recurre nce o f t h e proble m .

I)

HISTORICA L BA CKGRO UND

1-5

CONCLUSION Nearly a century ago the Navy eagerly became the i rst user o f radiated

electromagnetic energy in America . I t was a wise decision-remarkably astute . for that original need of w ireless communic ations aboard ships has proven ab­ solutely essential ever since . From our twentieth century perspective . shipboard communication is accepted as an inherent part of naval ship design and opera­ tions . Moreover. it appears destined to be so as long as there is a Navy. Yet, at the very instant of accepting wireles s electromagnetics as an op­ erational need , the N avy unwittingly accepted the unwanted phenomenon of EMI . Thus, these two oppo s i ng natures . electromagnetic s as an asset and elec­ tromagneti c s as an interference. have evolved together from the simplistic days of wireless radio to the present sophistication of a vi rtually electronic Navy . Doubtless, the naval scientists and ofi c ials who sub scribed to wireless for ships at the dawn of electromagnetics would be utterly astonished. if not petriied . to see today what man and nature have conspired to create together . And just where are we today, after so long and complex an electromagnetics evolution-opposed at every stage of development by insidious modes of in­ terference '? Navy ships today could not function without electronic s. Electronic s pro­ vide communications. command and control . navigation. radar surve illance and tracking, weapons controL and data proces sing . With so many systems competing for s carce portions Of the i nite frequency spectrum as well as for the limited space aboard ship, [ there are ] serious problems in trying to make the systems work well together . 40 But work together they must � And as we have seen in this historical overview. the N avy has had to develop an enti re doctrinal policy to see that shipboard electromagnetic systems do indeed work together effectively . It has been a long. hard battle, and the tide has tun ed in our favor: The Surface Navy is on the verge of having its electromagnetic s act together on Navy ships . For the irst time the necessary as sets are coming in place: Org anization Funds •

•

Authority-Workable policy is in place at all levels . C N O S upport--The Chief of Naval Operations h as personally approved EM program progress .

•

Fleet Recognition and Support-The Fleet has taken on training and self- h elp responsibilities and is actively imp l ementin g E M c ontro l .

H o w e v e . the job is not done . nor are we e ven past the bow tinued activ e d e fen s e and

u se

wave .

Co n ­

o f t h e s e a s s ets is require d . T h e poten t i a l is

SHIPBOARD ELECTROMAGNEICS

20

clearly there to produce and modernize ships which fully utilize their

electromagnetic systems and have maximum combat capability.41

To assure electromagnetic compatibility among all the sensitive electronic equipment installed on naval platforms will require careful attention to potential EMI problems by the entire shipbuilding community: the designer, the builder, and the operator. The ability to establish workable compatibility is in place and . 2 common practIce.4 becommg ·

REFERENCES 1.

"Captain's Phone Call Jammed Ship's Defense in Attack," The Wash­ ington Post, Washington, DC, 16 May 1986.

2.

F. Elliott, "Navy Learns From Brit's Error in Turning Off Radar," Navy Times, Springield, VA, 14 July 1986.

3.

L. S. Howeth, History of Communications-Electronics in the United States Navv, U.S. Government Printing Ofice, Washington, DC, 1963, p. 32.

4.

Ibid., p. 34.

5.

Ibid., pp. 38-39.

6.

P. E. Law, Jr., "Electronic Warfare," ShipboardAntennas, Artech House, Dedham, MA, 1983, pp. 413-417.

7.

Howeth, op. cit., pp. 69-83; 117-124; 521.

8.

L. A. Gebhard, Evolution of Naval Radio-Electronics and Contributions of the Naval Research Laboratory, Report 8300, Naval Research Labo­ ratory, Washington, DC, 1979, pp. 5-6.

9.

D. W. Todd, "The Navy's Coast Signal Service," Jounal of theAmerican Society of Naval Engineers, Vol. 23, No. 4, 1911, pp. 1101-1102.

10.

Gebhard, op. cit., p. 61.

11.

"Electronic Interference Control Manual for Forces Aloat,"

International

Electronics Engineering Inc (IEEI) Report 8031, June 1958, Appendix I, p. 9. 12.

K. D. Wilson, "Communications, A Limiting Factor In Naval Warfare,"

Naval Engineers Jounal, February 1963, p. 52. 13.

S. 1. Cohn, "Electromagnetic Compatibility," American Society of Naval Engineers Jounal, November 1961, pp. 763-764.

14.

L. W. Thomas, Sr., "EMC Twenty-Five Years Ago," IEEE Transactions on Electromagnetic Compatibility, Vol. 25, No. 3, August 1983, pp. 133-134.

15.

G. J. Marks, "Minutes of the First Meeting of the Electromagnetic Ra­ diation Hazards DOD Working Group,"

Department of Defense Armed

Forces Supply Support Center, Washington, DC, 16 October 1958.

21

HISTORICAL BACKGROUND

16.

"RF Radiation Hazards Measurements on Long Range Air Search Radar," Department of the Navy Bureau of Ships Task 2-1-59, IEMC Report 9008,

Washington, DC, 9 January 1959, pp. i, 1-2. 17.

ADM J. E. Rice, USN, "Compatibility at the Turning Point," Keynote Address, Proceedings .f the Tenth Tri-Service Conference on EMC, No­ vember 1964.

IS.

Wilson, op. cit., p. 53.

19.

"Instrumentation Techniques for Radiation Hazards Measurements," Compatibility of Shipboard Electronic S.vstems, RCA Report 311-60, De­

partment of the Navy Bureau of Ships, January 1960. 20.

Cohn, op. cit., pp. 764-765.

21.

H. M. DeJarnette and H. J. DeMattia, "Electromagnetic Interference Con­ siderations for Shipboard Electronic Systems,"

Naval Engineers Jounal,

June 1966, pp. 459-464. 22.

CAPT M. Eckhart, Jr. , USN, "Topside Design for Electromagnetic Ef­ fectiveness," Naval Engineers Jounal, June 1969, pp. 57-65.

23.

CDR J. V. Jolliff, USN, "Research and Development: The Key To The Electromagnetic Interference Problem," Naval Engineers Jounal, De­ cember 1969, pp. 52-57.

24.

CAPT 1. S. Oller, Jr. , USN (RET), "The Navy's Tactical Electromagnetic Program-Problem or Panacea," Naval Enginccrs Jounal, October 1975, p. 51.

25.

Ibid., p. 52.

26.

Ibid., p. 53.

27.

J. C. P. McEachen and H. K. Mills, "The Shipboard Electromagnetic Compatibility Improvement Program (SEMCIP)-A Program for the Op­ erating Fleet," Naval Engineers Jounal, October 1976, p. 65.

2S.

"Electromagnetic Inluence on Shipbuilding Performance Speciications and Practices," Naval Ship Engineering Center, Hyattsville, MD, IlTRI Report E61S l -F, December 1970.

29.

CDR J. V. Jolliff, USN, ., Improvements in Ship Construction Techniques to Reduce Electromagnetic Interference," Naval Engineers Journal, April 1974, pp. 13-27.

30.

R. J. Biondi, Handbook for Shipboard Surveillance Radar, NAVSEA 0967-LP-5S7-0010, Naval Sea Systems Command, Washington, DC, Jan­ uary 1976, pp. 41-42.

31.

Oller, op. cit., pp. 53-54.

32.

NAVSEA Instruction 2410.2, Department of the Navy, Naval Sea Systems

Command, Washington, DC, 13 January 1977, pp. 1-5. 33.

D. B. Winter and R. V. Carstensen, "Waterfront Corrective Action Pro­ gram (WCAP) Pilot Shipyard Project: Resolving EMI Problems in Con-

SHIPB OARD ELECTROMA GNEICS

22

j unction with the Ship Overhaul Process , " Naval Engineers Joumal , February 1 98 2 , p . 4 1 . 34 .

LCDR J . N . Lay l , U S N , and C D R G . D . El l i s , U S N ( RET) , " A Fleet Oriented Electromagnetic I n terferen c e Contro l Program , " Naval Engin eers Jou n a l , Febru ary 1 9 8 2 , p . 4 8 .

35 .

R . V . Carstensen , " A n Approach to EMP H arden i n g for N av a l S h i p s , " Naval Engineers Jounal , A pri l 1 9 7 9 , pp . 1 4 1 - 1 5 4 .

36.

J . F . G arre tt , R . L . H ard ie , and P . A . Rogers , " Let ' s Design O u t E M I , " Na val Engin eers Jouna l . Febru ary 1 9 8 2 , pp . 3 7 -40 .

37 .

J . F . G arre tt and R . J . H ai s l m ai e r . " EM C For Forc es A l o at , " 1 983 IEEE

Illlemationo l Symposium on Electromognetic Compa libility . 2 3 - 25 A u g u s t 1 9 8 3 , pp . I 1 3 - 1 1 7 . 38.

R . T . Ford . " Enhanc i n g E l e ctromagnetic Compati b i l ity On Naval S h i p s W i th Radar Absorben t Materi al , "

1 983 1EEE Illlenariol1ol Symposium 011

Electromagnetic Compatibility , 23 - 2 5 August 1 9 8 3 . pp . 1 3 0- 1 3 3 . 39.

J . F . G arrett . A . Jackso n . J . Kelleher, a n d A . L i g h t ,

" EMC D e s i g n

Fee dback and A n alys is Data Man agement Program . " 1 983 IEEE In ler­ national Symposium

011

Elecll'Omagnelic CUl11palibility . 23-25 August 1 98 3 ,

pp . 1 1 8 - 1 2 2 .

40 .

G arrett a n d H a islmaier . up . cil . . p . 1 1 3 .

41 .

J . C . P . McEachen , " On Getti n g Our EM Act Together a n d Put t i n g It on a Navy Ship . "

42 .

Na m l Engineers jOllnw l . August 1 9 8 2 , p. 5 7 .

G arett . H ardie . Rogers . op . eil . p . 3 7 . .

Chapter 2 The Shipboard Electromagnetic Environment (EME) 2-0

THE TANGIBLE ENVIRONMENT The topsides of modem naval surface ships have been aptly described as

environments of multiple electromagnetic scattering obstacles. To anyone inti­ mately familiar with the concept of EMC, who has spent any length of time above deck on a Navy ship, that description is visually deinitive. There have been other, less elegant, illustrations offered, ranging from "electromagnetic jungle" to "electromagnetic nightmare." Certainly all would agree that it is a most unfriendly environment for the well-being and good operation of electronic systems. First, there is simply all that passive metal. A host of inert metallic pro­ jections greets the eye: exterior bulkheads, inclined ladders, stanchions and booms, mast legs and yardarms, chocks and bits, stacks, cranes, boat 'davits, storage racks and lockers, handrails and lifelines, lag staffs, cable rigging, upright hatch covers, gun mounts, weapons launchers, and, of course, a multitude

of antennas of every sort. (See Figure 2-1.)

These objects, arrayed in an extraordinary mixture of shapes and sizes, act in every conceivable manner to block, intercept, conduct, relect, scatter, diffract, and reradiate electromagnetic energy-and sometimes to create new electromagnetic products in the form of intermodulation interference. There is no escape. A single electromagnetic emitter or sensor might be placed at the very top of the highest mast. A couple of others might be stacked vertically a short distance below in an around-the-mast circular fashion. But all others must suffer from the detrimental effects of the mass of passive metallic objects­ those multiple electromagnetic scattering obstacles.

23

SHIPBOARD ELECTROMAGNEICS

24

Figure 2-1 Topside of Modem Warship (Numbers Indicate Individual Antennas)

Then there is all that electrically active metal; i.e., machinery devices being powered by motors and generators to operate tools, cranes, and booms; to point weapons systems; and to rotate antennas. These electrical entities not only augment the family of metallic obstacles, they also contribute mightily to the onboard ambient electromagnetic interference. Finally, there is the matter of the natural marine environment. Exposed to the atmospheric elements and to battering seas, the topside of a ship is subjected to near continuous coatings of salt spray. Such moisture, particularly when mixed with stack gas contaminants, promotes early corrosion and rapid physical de­ terioration. Thus combined, so much metal, so congested and conined, in so harsh a nature, can result only in a clearly hostile environment for topside electronics systems. We have not yet even mentioned the deleterious effects of invisible contributors-the wildly varying electromagnetic radiating ields adding to the overall environment. No wonder, then, that it takes a corps of highly trained EMC specialists to cope with shipboard electromagnetic design and integration.

THE SHIPBOARD ELECTROMAGNEIC ENVIRONMENT

2- 1

25

THE COMPOSITE RF ENERGY ENVIRONMENT

For the specialist , there i s much more to the shipboard e lectromagnetic environment than meets the eye. The unseen. too , must be grasped and dealt w i th in all its many forms. It is the i n v i s i b l e RF medium that makes the problem so much more dificu l t. The shipboard RF environment i s a complex m i xture of radiated electro­ magnetic energy created from multiple sourc es. The chief contri butors are on­ board emitters , compri sing: ( 1 ) HF communications transmi tters , (2) VHF communications trans m itters , (3) UHF communications transmitters , (4) sate l l i te communications trans m itters , (5) air search radars , (6) surface surv e i l l ance ra­ dars , (7) s urface navi gation radars , (8) air control radars , ( 9) weapons directing radars , (10) e lectronic w arfare jammers , (11) identi ication , friend or foe (IFF) transponders , and (12) tactical air navi gation (T ACAN) homing beacons. B ear i n m i n d that the ship tran smitters c i ted above are a l l onboard inten­ tional, des ired radi ators of RF energy. A l s o present i n the s h i pboard environment are intentional incoming RF transmissions (e.g. , communications and navigation data) from friendly e xternal sources , and , i n most c irc umstances , many forms of unintentional extraneous RF e m i ssions from ne arby friendly sources (e.g. , ships of the leet operat i n g i n pro x i mity). Add to these the pote ntial for undes ired del iberate RF transmissions from unfriendly s ources (e.g. , enemy surve i l l ance and jam m i ng ) . Finally we must incl ude the natural RF i nterferences (lightn i n g , gal acti c , and atmospheric noise) and man-made i nterference emanat i n g from electrical mac h inery and components. The compos ite total of this transparent RF medi um, i t can be appreci ated, i s very comp l e x i ndeed. Into this e n v ironment we i mmerse sophisticated and sensitive electronic systems , demanding that they perform effectively.

2-2

EFFECTS OF THE SHIPBOARD EME

B ecause of the n ature of the s h ipboard e l ectromagnetic e nvironment , no major naval ship is complete l y free of its adverse effects. Some degradation , even if m i ld , w i l l always be evident. It i s the task of EMC design and integration (di scu ssed in the next chapter) to e n s ure th at each electronic system operates effecti v e l y despite the degradation experienced within the intended shipboard EME . It must be stressed , moreover, that severe form s of electromagnetic systems degradation do occur frequently. Therefore , steps must be taken to s uppress and

26

SHIPBOARD ELECTROMAGNEICS

control such problems lest the degrading effects result i n serious d i s ruptions, performance errors, or system shutdown . In general there are two principal causes of electromagnetic degradati o n . The most basic i s undesired strains of RF energy recei ved openl y through an­ tennas and transmission l i nes to gain entry i nto receiving equipment and s y s tem s . The second is unintended penetration of EMI into v icti m equipment v i a unsus­ pected ports . The eas ier of the two problems to correct i s the irst, by proper design and "h ardening" of the recei ver entrance circui try . The second type o f problem i s, however, l ikel y to be quite dificult to correct, a s it usua l l y requires extreme care to detect and suppress . It would be well to point out here, i n simpli stic terms, that for EMI to be

experienced, there must be: (1) an i nterference signal -generating source, (2) a coupling path from interference source to victim equipment, and (3) a system that is s usceptible to the interfering signal and its degrading effects . Depending on the equipment, su sceptib i lity characteri stics such as ampl i tude, frequency, and response time v ary widel y . For ex ample, the victim in question may be n aro w l y frequency selective or it might be a type receptive to broadband un­ focu sed noise . Some v ictims may have microsecond response time to peak burst s of energy, while others w i l l react s l o w l y t o average signal leve l s a n d heatin g . Thus the s usceptibi l i ty ch aracteri stics, along with the selection of components and suppression tech niques such as i l teri n g and shielding, must all be carefu l l y con s i dered when analyzing the unfavorable effects of the shipboard EME . Typical ex amples of ship system performance degradation resulting from the EME include: a . False Targets-Experienced on radar display scopes due to HF trans m i s ­ sions coupled from antenna to cables a n d wavegu ides . A l s o from multipath microwave relected energy received by radar antennas . b . False Alarms-Causing sensitive automatic contro l sy stems of ship pro­ pulsion systems to shut down . Due to HF transmissions coupled into cables to below-deck compartments, and due to EMI generated by below-deck machi nery . c . False Bearings-Generated in TACAN beacon navigation information . Caused by energy relections fro m nearby mast structures and by HF tran smissions v i a eq uipment cabling . d . False Tuning-Undesired and erratic tuning of antenna couplers, caused by close-prox imity energy coupled from l ike equipment located nearby . e . Distortion of Communications-High data error rate and noisy aud i o com­ mun ication, cau sed by h u l l - generated "rusty bolt" intermodul ation i nter­ ference and by antenna-to-cable coupling of HF- to UHF-receiving equipment . f. Distortion

01

DispLay Scopes-Spoking and picture eradication on radar

screens, cau sed by antenna-to-antenn a coup l i n g of navigation radar energy to air-control radar recei vers, and by HF transmission coupled into radar cabl ing .

THE SHIPBOARD ELECTROMAGNEIC ENVIRONMENT

27

g . Radiation Patten B lockage- Experienced chiely i n omnidirectional sys­ tems such as HF, VHF, and UHF communication, and in rotation of directional systems such as radars, EW, TACAN, and sate l l i te commu­ nication (S ATCOM) . Caused by multiple obstructions in the rad i ation ield . Results i n loss of coverage and range i n the direction of blockage . h . Radiation Hazards-D angerous levels of electromagnetic ield exposure to personnel, fuel, and ordnance due to high po wer concentrations of RF energy i n the topside environment . The cumul ative effects of these types of performance and equipment prob­ lems have been known to result in serious mission delays and aborted exerci ses and to gain the i m mediate attention of headquarters personnel .

2-3

EME CONTROL TECHNIQUES

Proper contro l of sh ipboard electromagnetic environmental interference is essential to ensure effective performance of ship electronic system s . The topic is so l arge and important as to warant detailed discuss ion in Chapter 4. S ufi ce

it to say here that good control is irst accomplished b y : (1) recognizing the

problem as interference degradation, (2) identifying the i nterference source and means of coupling, and (3) taking the neces s ary action to correct the problem . Over the years sh ipboard experience i n dea l i ng with EMl i n its many, often subtle, forms has resulted in general i zed methods to mitigate and control it. These include: a . Decou pling-Decreasing the offending energy level by use of physical distance . For example, providing wide separation between high power emitters and broadband sensors . b. Frequency Managemen t-Careful selection and assignment of operating frequencies to avoid mutual -se interference among on board and task force -

intership electromagnetic systems . c . Shie lding-The prevention of interference energy emanations and the re­ duction of interference susceptibil ity . d . Grounding and B onding-The precl usion of conduction of unwanted elec­ tromagnetic energy into susceptible equipment, and the neutra l ization of electrical potential differences between metal l ic surfaces and j oints . e . Filtering- B l ocking the passage of undesired energy and pas s i ng only desired signal s . f. B lanking

-

B locking the reception of direct energy radiation by u se of

electronic pulsed switching c ircuitry. g . Element A rrangemet- Optimum p lacement of electromagnetic systems in the ship topside to minimize radi ation pattern blockage, RF energy relection and reradi ation, and radiation hazards to personnel, fuel, and ordnance .

28

SHIPBOARD ELECTROMAGNEfCS

h. A n tenna R eduction-Use of multi couplers and multifunction arrays to lessen the number of onboard antennas .

1.

Power Redu ct i o n - Operating at lower e m itter power l e v e l s to lessen sen­

J.

Metallic R eduction - Use of nonmeta l l i c materials throughout the s h i p

s i t i v ity degradation of sensor performance. tops ide t o l e s s e n t h e number of energy sc attering obstac les. k. AM-Employing RAM to prevent energy multipath relection and re­ radiation by absorbi n g or h i g h l y atten u at i n g undes ired RF e m i s s i on s. Appl i c ation of these techniques to the shipboard e n v ironment for contro l ­ l i n g E M I w i l l be examined ful ly i n Chapter 4 .

2-4

PREDICTING THE SHIPBOARD EME

It frequently h appens that plans are formu l ated to install newly developed or i mpro ved e lectronic systems into an e x i s t i n g s h i p e n vironment. Somet i m e s t h e i n stal l ation i s a n upgraded replacement , a n d at other time s i t i s an addi t ion. In e i ther eve nt, the electromagnetic c h aracteristics of the new system are. fairly weII know n , along w i th the s h i p EME into which the equi p ment is to be i nte­ grated. Thus, an evaluation of the i mpact of the i n tegration can be made be­ forehand and veriied by actual testing and analysis ater the work is accompl ished. Accord i n g l y, additions or deletions of e q u i pment , relocation of antennas , or modiications to the s h i p structure res u l t i n the need for continuous updating of the active s h i pboard EME characteristics.

I

The more d i ficult problem, however. i s to predict and deine a projected s h ipboard EME; th at i s , for the case of a totaIIy new ship design and combat systems i n tegration program. We do know that to achieve system compat ib i lity with the e n v ironment we must deine the EME w e l l i n to the future so as to cover

the entire l i fe span of the proposed equipment. 2 Th i s must i n c lude both t � e e q u ipment and systems p arameters and the operational employmen t , sufic iently

described to afford dein ition of the ant i c i p ated EME, as w e l l as the res u lting impact of i n tegration of systems into the EME. A threat analy s i s of the fri e n d l y a n d h o s t i l e EME expected t o be encountered by t h e s h i p must also be performed. The task i s complex and requ ires the assi stance o f known and predicted EM data such as that found in MIL-HDBK-23S , Electromagn e tic (Radiated) En vironment Considerations for D esign and Procuremen t of Electrical and Elec­ tron ic Equipment, Subsystems, and Systems, samples of which are depicted in

Figures 2-2 and 2-3. Here the EM env ironment levels are presented i n terms of peak and average power density and iel d strength; i t should be noted , however, that there are many other EM-rel ated factors that w i l l i nluence systems perfor­ mance. These include antenna ch aracteri stics such as aperture, p o l arization,

29

THE SHIPBOARD ELECTROMAGNEIC ENVIRONMENT

APPROXHIA TE FREQ

LOCATION

-

Table IV

Hangar Deck

(CV's and CVN's)

RANGE

2 PO\Jer Density (m�,1 cm )

(�IHz)

Peak

2000

-

Table Veal

Flight Deck

of Aircraft Carriers (Cv's and CVN's) Table V(b)

- \,eather Decks,

(CG, CGN, DOG, FFG

Non

-

& FF's)

- \,eather Decks,

Table V(e)

u r e m ent o bj e c t i v e > . t e > t e q u i p m e n t C < l I1 -

f i g u ra t i ( )n> . t e > t p o inh . d e ta i h ( ) f ll1 e a > ur e lllent p ro c e d u re > . and d a w re c o rd i n g

frm a t . I t i > r e q u i re d . t o o . t h a t the l e s t p r o c e d u re s b e d e s c r i b e d in > u i c i ent d e t a i l t o ena b l e t h e N a v y p ro j e c t l e a d e r t o h a v e any o f t h e t e > l ing d u p l i c a t e d fr fu rth e r ana l y s i > .

To be c o m p l e te l y v i a b l e . o pe r at i o na l t e s t ing o f t h e ne w l y d e v e l o p e d > y s t e ll1 s h o u l d be c ond u c te d i n the most rea l i s t i c s h i p b o a rd

the e q u i pment

r

EME p o s s i b l e . That i s . i f

s y s t e m i s to be p l aced w h e re i t w i l l be s u bj e c ted to h i gh l e v e l s

o f e l ectro m a g n e t i c e n e rgy . s u c h a s i n a s h i p tops i d e . t h en t e s t s s h o u l d be p e r ­ formed to veri fy sat i s factory ope rat i o n in the i n tended env i ronmen t . Th i s w o u l d i n c l u de normal s i m u l taneou s ope rat i o n o f a l l s h i pbo ard e m i tt e rs a n d s e n s o r s . and mak i n g u s e of data acq u i red fro m pre v i o u s e l e c t ro m agnet i c e n v i ro n m e n t a l pred i c t i o n s a n d operat i o n a l e xperience . F i n a l l y . req u e s t s for appro v a l o f s e rv i c e u s e of t h e n e w e q u i p m e n t o r s y s t e m m u s t i nc l ude cert i i c a t i o n that t h e req u i s i te EMC ( se l f and p l atform ) has b e e n ac h i eved .

3- 1 . 3

EMC C onigu ration M a n a gement

It w o u l d be hope l e s s to e x pe c t that a s a t i s factory l e v e l o f s h i pboard E M C c o u l d be pre served u n l e s s c o m p l ete c o n trol of t h e s h i pbo ard c o n i gurat i o n i t s e l f w e re s t ri c t l y m a i n t a i n e d . H e n c e . e ffe c t u a l c o n i g u ra t i o n m a n age m e n t i s a re­ q u i re m e n t in a l l n a v a l EMC progra m s . whether fo r new s h i p d e s i g n or for m aj or mod i i c a t i o n s and a l terati o n s . E v e n so . a c t u a l e x pe r i e n c e has s h o w n that fre ­ q u e n t l y there are w i de variat i o n s i n the same s y s t e m i n s t a l l ed i n the s a m e c l a s s or type of s h i p . D e s p i te a l l the e fforts e x pended o n a n a l y t i c a l and mode l i n g tec h n i q u e s d u r i n g the d e s i g n phase to e s t ab l i s h the appropri ate b a se l i ne c o n i g ­ urat i o n . i n prac t i c e ch an ges s ti l l too oft e n h a v e b e e n approved a n d i n c orporated w i th o u t proper e v a l u at i o n o f the e ffe c t o f EMC . U n fort u n ate l y . these vari a t i o n s freq u e n t l y re s u l t i n degraded performance o f t h e i n s tal l ed s y s t e m and , therefore . x of the s h i p m i s s i o n .

A case i n po i n t i s i n the engi n e e r i n g d e s i g n of s h i p b o ard t o p s i d e arrange­ m e n t s , where the pri m ary obj e c t i v e is to pro v i de opti m u m cov erage a n d per­ f011a nce of guns . missile launchers , weapon direc tors . radars . and communication s y s t e m s to fu l i l l the sh i p ' s w ari g h t i n g m i s s i on . Th i s obj e c t i v e i s v e ry d i fi c u l t t o att a i n d u r i n g n e w s h i p d e s i g n . a n d i t i s e v e n more d i fi c u l t t o p re s e rve thro u g h ­ o u t the a c t i v e l i fe t i m e of t h e s h i p b eca us e of the c o n t i n u al proce s s of m od i i c at i o n s a n d a lterat i on s . C o n s e q u e n t l y . t o m a i n t a i n good EMC . i t i s c ru c i a l t h a t a n y proposed changes t o t h e s h i p c o n i g u rati o n be c are fu l l y e v a l u ated t o a s c e rt a i n t h e e x t e n t of EMC effect . S uch e v a l u a t i o n s h a v e to be c o m p l eted i n s u fi c i e n t t i m e t o decide whether t h e c h ange shou l d i n fac t be a l l o w e d . o r j u st \vhat corre c t i o n s are needed in order not to d i s t u rb the EMC .

SHIPBOARD ELECTROMAGNEIC COMPAIBILITY

41

One effective means of mainta i n i n g shipboard coniguration management i s to requ i re that changes be made only through formal approval of an ECP or a S HIPALT . In this process it i s incu mbent upon the proj ect manager to make sure that appropriate EMC analyses are conducted and that an EMC Impact S tatement be i n c luded in the ECP or S H I PAL T . I n formation in the Statement should incl ude any proposed c h anges i n the physical location of equipment ; changes i n the emission characteri stics ( e . g . . frequency , modu l ation , power output , and antenna type ) ; changes in sensor characteri stics ( e . g . , bandwidth , sensiti vity , selecti vity . i ltering , frequenc y . and antenna type ) ; or changes to the ship h u l l structure which could affect shielding , bondi n g , and grounding i nteg­ rity . Moreover, the state ment must contain supporting rationale for the ori g i ­ nator ' s proposed change s . I n many i nstances a s i mple review o f i l e case studies w i l l aid in predicting whether s i m i l ar system changes i n s i m i l ar situations h ave caused any problems . I n such events EMC troubles can be anticipated and prevented by app l y i n g known solution s . B u t i n other cases the system i n tegration problems re sulting from alterations can be so complex , and the effects expected s o detri mental , that an in-depth EMC eval uation is q u i te necessary . In such cases the proj ect manager must assess the ri sks invol ved as well as the results of not adopting corrective measures . Here agai n . the EMCAB i s heav i l y re l i ed upon to prov ide adv ice of critical i mportance to preserv i n g the w e l l -being of the program .

3- 1 . 4

EMC Training and Awareness

We turn our atte ntion now to yet another facet of sh ipboard EMC by no means of least impotance . The subj ect is tra i n i n g . For no matter how earnestly the engineering designer and program man ager have w orked together to produce a unit of equipment or a system or a new w arsh ip having an optimum i n itial level of EM C , the product user must be fu l l y aw are of the need for constant EMC upkeep . Left unattended , shipboard e l ectromagnetic systems and compat­ i b i l ity w i l l degenerate inexorab l y due to the natural consequences of time and change . Th u s , an appreciation of shipboard EMC and the deleterious conse­ quences of i ncompatibil ity must be made known to the operators and users . It behooves the proj ect manager. therefore , to prepare an EMC training plan when­ ever newly devel oped equi pment i s to be introduced , or a sy stem mod iied , or a new ship de l ivered . The p l an should address in part i c u l ar the procedures requ i red for preserv i n g the total pl atform EMC . Part of the proble m , even among ship personnel trained i n the operation and maintenance of e l ectronic equipment , i s that there may l ik e l y be a general lack of knowledge of the many causes of EM! . Y Ship operators q u ite often may be unaware of the el ectromagnetic subtleties which work i n s i d i o u s l y to degrade

SHIPBOARD ELECTROMAGNEICS

42

system performance . Even routine mai ntenance proc edures req u i re d for a con­ ti nued high degree of individual system performance can be damag i n g to the total ship EM C . Th i s i s espec i al l y true i n the case of oste n s i b l e i mprovements which involve modify i n g or alteri n g a topside syste m . S uc h p aroc h i a l changes can cause serious overal l system degradation . Therefore, i t is very i mportant that each new i n stall ation or mod i i c ation be thoroug h l y assessed and tested for ful l compati b i l ity in the s h ipboard EME . S h i p personnel should be aware also that many EMI probl e m s can be avoided by such everyday practical tec h n i q u e s as proper t u n i n g and a l i g n i n g of e lectro n i c equipment; c arefu l bond i n g, groun d i n g, and stowage of topside items ; operating tran s m itters w i t h i n pre scribed power l imits and w i th adequate fre­ quency spac i n g ; and selection of al ternati v e antennas for c o m m u n ications c ir­ cuits . Likewi se, operators should be conscious of the many electromagnetic susceptib i l ity mechani s m s that contribute to upsett i n g the d e l i c ate b a l ance of EMC . Futhermore, ship tec h n i c i an s and operators should be trained to identify sourc e s of performance degradation and taught how to employ EMI reduction methods to restore good performanc e . Recogni z i n g t h i s v i tal need for E M C awareness, the N a v y n o w requ i res each ship to have an EMI control oficer assigned the respon s i b i lity for main­ tai n i n g the ship ' s EMC i ntegrity . A s such, i n implementing an e ffecti v e s h i p w i de EMC awareness program, the EMI control ofi c er m u st :

10

a . Develop and i mplement a tra i n i n g program to e nsure that a l l crew m embers are kept i n formed of the need for shipbo ard EMC, and of w h at each i n d i v i dual is e x pected to do tow ard maintai n i n g EMC . b . Develop m anagement and i nspection procedures to e n sure that a l l s h i p ' s force efforts are coord i n ated and scheduled t o ach i ev e, restore, a n d m a i n ­ tain EMC . c . Ensure that proper correct i v e maintenance i s performed on e q u i p me n t or systems causing EM! . d . Procure and maintain test e q u i p ment needed for EMC testing, and e n sure that all e q u i pment is properly c a l i brate d . e . Ensure that thorough and compreh e n s i v e i nspectin g, test i n g, grounding, and bonding tec h n i q ue s are practiced to detect and suppress EM! . In summary, the ach ievement of satisfactory s h ipboard EMC and its pre s ­ ervation throughout t h e l i fetime of t h e s h i p i s an arduous task . It requ ires a continual effort of awareness and tra i n ing, as w e l l as alet response o n the p art of the s h i p operations and m a i n tenance personnel . T i m e l y identii c ation of EMC probl e m s and aggre s s i v e corrective action are essential to the proper functioni n g o f t h e total s h i p e lectromagnetic sy stem i n effective ful i llment o f m i s s io n ob­ jective s .

SHIPBOARD ELECTR OMA GNEIC COMPA IBILITY

13

REFEREN C ES I.

Electromagnetic Compa tibility (EMC) lvith in the Na a l Sea Systems Com ­ ma nd, N A V S EAINST 24 1 0 . 2 , Department o f the Navy . 1 3 J an u ary 1 9 7 7 .

E n c l o s u re ( 1 ) , Dei n i t i on s . 2.

Procedures for Conducting a Shipboard Electrom agnetic Interference Sur­ vey , M I L-STD- 1 60S ( S H I P S ) , Department o f Defense , 20 A p ri l

1 97 3 .

p. I . 3.

B . E . K e i s e r , Prin ciples of Electromagn etic Compa tibility , Artech H o u s e . Dedham , M A , 1 979 , p . 1 .

4.

Electromagnetic Compatibility Program with in th e D epartment of the Nm 'y ,

O P N A V I N S T 24 1 0 . 3 I D , O fice o f the C h i e f o f Naval Operation s . 6 A u ­ g u s t 1 9 84 , p . 3 . S.

" Engi neeri ng Req u i rements for E lectromagnetic Compatib i l i ty . "

NAV­

S EA N OTE 24 1 0 , Department o f t h e Navy , 8 M arch 1 9 8 3 . Enc los ure ( I ) . p. 3.

6.

Electromagnetic Compa tibility Manag eme11l G u ide for Pla forms , Sys tem s , a n d Equipment, M I L - H D B K - 2 3 7 A , Department of Defense , 2 Febru ary

1 9 8 1 , pp . 4- 1 2 . 7.

Ibid . , pp . 7 7 - 7 8 .

8.

Ibid . , pp . 3 9-40 .

9.

" S h i pboard EMC M an ageme nt , " The Electronics Material Ofice r ' s G u ide to Sh ipboard Electromagnetic Intelierell ce Contro l , STD-407 -S 2 8 7 S S 6 .

Departme n t of the Navy , A u g u s t 1 9 8 0 . p . 3 - 1 . 1 0.

Ibid . , p . 3 - 3 .

Chapter 4 Shipboard Electromagnetic Interference (EMI) 4-0

THE SHIPBOARD EMI PROBLEM

W ithout question the crux of shipboard EMC engi neeri ng technology i s t h e prevention a n d control o f E M I . It h a s been so, a s w e di scovered i n Chapter 1, s i nce the ori g i n of shi pboard e lectronics. Yet , despite the most d i l igent man­ agement techn ique s duri ng sy stems de sign and production; despite expert know l­ edge , experience , and appreciation of the shipboard E M E; and despite the best of efforts in trai ning and awareness , EMI is ever present aboard naval ships. Its presence is due to the mere nature of the shipboard environme n t , the density of complex , highly soph isticated e lectron ic systems, and the extraordi n ary require­ ments of critical ship missions. As a con sequence , each ship must be tested , evaluated , and treated for E M I on a case-by-case basis. Therefore we w i l l now examine in deta i l the engi neeri ng practices for effective control of onboard EM!. 4-1

SOURCES OF SHIPBOARD EMI

E M I i s deined as any electromagnetic di sturbance which i nterrupt s , ob­ structs , or otherw ise degrades or l imits the effective performance of el ectronic and e lectrical equipment. 1 As conined to and contained within the boundaries of a s h i p , this deinition encompasses an astonishing number of pos sible source s , by far the most of which are quite un intentional. Occuri ng through both con­ duction and radiation paths , shipboard EMI generally is c ategorized as being either natural or man-made . Figure 4-1 i l lustrate s the rel ative ampl i tude and spectrum of these EMl sources.

45

SHIPBOARD ELECTROMAGNEICS

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-

I

;

I

I

T IM E ! 1 III I

, ,

I

I

I

D AY

I

I

I:

I

i

I

;.,,

I

I

I I

I I

'\'

I

I :

I

I

I I I

I

i

�

j

t

,

I

BAN

I�-i!

,

I

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I J� �Etl l i ll --.. I

> � I Z W 1.0 � Z

t

,

, ,

I II !

10

i

i

...

I I

I

"

I

11111

:_��.:.. I ... I " I t ��

I!' ..

I

1

I'

I

00

10.00

Figure 4-1 Source s of Electromagnetic I n terfere nce ( from U S A F EMC Hand­

book D H 1-4)

SHIPBOARD ELECTROMA GNEIC INTERFERENCE

4-1 .1

47

Natural Sources of Shipboard EMI The world e n vironment is replete with natural l y occu rring electrom agnetic

disturbances . These distu rbances are created both within our earth's atmosphere and from a variety of points out in the u niverse . As a signiic ant contributor to shipboard ambient noise levels, n atural interference is a mixture of random disc rete impulses and a steady broadband hiss . Fortunate l y , the characte ristic s of n atural interference are well known and are rec koned with at the equipment design phase s . a . A tmosphe ric Noic e

-

Generated primarily from lightning disch arge s, at­

mospheric noise prod uces inte rmittent high-inte n sity b u rsts of interfere nce d u ring loc al electric a l storms and a contin uous low-le v e l rattling and crackling disturbance from n u merous storms in the distance . This electro­ static intefere nce is strongest in tropica l are as of high thunderstorm activity . As a n atural phe nome non, atmosphe ric noise is present from v e ry low freque ncies to about 100 M H z . It is the predomin ant noise source, however, below 30 M H z, and, v arying somewhat according to the season

8

and whether it is night or day , it disturbs most strongly in the HF region, peaking at approximately

MHz.

b . Cosmic Noise -EMI originating i n nature beyond the earth's atmo sphere (i . e . , in outer space) is clas siied as cosmic noise . This type of EMI is a combination of g a l actic , the rmal , and inte rste llar noise e missions . 2 The amplitude of the composite cosmic noise is lowe r than that of atmo sphe ric noise below 10 M H z . H owever, above 50 M H z , cosmic noise levels are notably higher than atmospheric noise . Moreover, cosmic noise is wide­ band, being bothersome in the V H F range, and an noying even out to E H F (well over 3 0 G H z) . There are times, too , d u ring c yclic al s u n spot activity , whe n cosmic noise bu rsts last several minute s and e xceed atmospheric noise le vels in the H F band . Two principal sources of noise interfe re nce within our solar syste m

are: ( l ) nonthe rm a l electron activity in J upiter's magnetic ield, and (2)

therm a l e missions from the moon caused by s o l ar heating of the l u nar surface . Outside the solar sy stem the most inte nse n oise source is the 3 supernova star Cas siopeia A .

4-1 . 2

Man-Made Sources of Shipboard EMI M a n , in co ntriving electrical apparatus to lighte n his burdens and increase

human comfort s, has u nwitting l y allied with nature to produc e even more sources of EM I . From rotating e lectric a l machinery to electrica l lig hting to e l ectromag ­ netic transmission of information , the byproducts of the se beneits to mankind

SHIPBOARD

4$

£L£

TROMAG

ETiCS

have been i ncreased l e v e l s a n d varied t y pe s o f e l ectro m a g n e t i c noi e . The more man-made n o i se i s generated , the l e ss re l i a b l e a n d l e s efic i e n t become the 4 electrical and e l e c t ro n i c s y s tem s . The pro b l e m is g reat l y e xacerbated a board s h i p, where very many e l e c t rom agnet ic d e v i ce s and s y ste m s are requ i re d to operate s i m u l tane o u s l y i n a confined v o l u me . a . Shipboard Transmitler System EMf- I n cary i ng out its rou t i ne operati onal fu nction s, a naval s h ip has a n u m be r of RF e m i t te rs in con c u rre nt serv ice . These i n c l ude severa l H F , VHF , UHF, S H F , and EHF com m u n ications tran s m i tte rs; a i r searc h , s u rface

earch, and nav i gati o n rad ars; T ACAN

and IFF tran sponde rs; weapo n s detec t i on, acq u i s ition, and trac k i n g d i rec­ tors; meteorol o g i c a l and te l e m etry data tra n s m itters; a n d , at t i mes, e l e c ­ tro n i c warfare c o u nterm e a s u re e m itters . M a n y o f t hese rad iate very h i g h power , and some tran s m it o m n i d i recti o n a l l y or rotate 3600 conti n u o u s l y . A s a con seque nc e, the s y ste m s are c apa b l e of m utual l y i nterfe ri n g with eac h other . Furthermore , the o n board as soc i ated sen sors are pro ne to i n­ te rcept u n de s i re d e m i s s i o n s e ithe r thro u g h d i rect coupl i n g or by m u ltipath re l ecti o n s . E v e n if the e m itte rs are c are fu lly d e s i g ned so as to rad iate i nte ntiona l l y o n l y a spec i i c frequency or b and, i n actu a l u se u nwanted RF energy esc apes at a l arge n u m ber o f spu rious frequ e n cies to c a u se pote nti a l EMI prob l e m s . Li kewi se, t h o s e e m itters t h at employ h i g h l y d i rectio n a l antennas e m a n ate u ndes i red energy i n t h e s i de l o b e s and bac k l obe pot i o n s of t h e rad iat i o n pattern . A s s u m i n g that the tra n s m itte r s y ste m s des i g ners h ave i ncoporated adequ ate i nte rfe rence s uppre s s i o n i n the d e s i g n and prod uction of s h i pboard eq u ipment, the fo llow i n g tra n s m i tte r-related EMI spu r i o u s e m i s s i o n prob­ l e m s are frequent l y experienced aboard s h ip as a re s u l t of i mproper i n­ stallation, operation, a n d m a i nte n ance: 1. Harmonic Fre que ncy

P roducts

-

G e n e rated by n on l i n earities in trans­

m i tte r power output stages, h a rm o n i c products are i nteg ral m u lt iples of the des i red fundamental rad i at i o n frequen c y . Even tho u g h equ ipment m a n u fact u rers are requ i red to d e s i g n tran s m i tters with secon d-order h a rmo n i c s s uppres sed to 60 dB below the fu n d a mental , and h i gher order harm o n i c s 80 dB below , i mproper tu n i n g and operati o n of a tra n s mitte r w i l l l i k e l y re s u l t i n prod uct i o n of harmon i c interference by the forc i ng o f nonli near e x c itati o n .

2. Cross-Modulation a nd fnte rmodulation-EMI i n these i n stances is caused . by i nteracti o n o f two or more s i g n a l s present at the same time in a n on l i near c i rc u i t . lntermodu l ation re s u lts fro m the mix i n g ( heterod y n­ i n g) o f s i g n als to prod uce new freque n c y c omponents, while c ro s s­ mod ulation occurs upo n the transfer o f mod u l at i o n energy from one RF c arri e r to a n oth e r . These problems o ften happe n whe n anten n a s a re

SHIPBOARD ELECTROMAGNEIC INTERFERENCE

49

in stalled so closely to one another that e nergy is directly coupled be­ tween them and thereby fed across to the trans mitter output stages to mix with the desired radiated signals . 3.

Parasitic

O scillations- P arasitic EMI re sults from self-e x citation of

tra n smitter circ uitry, cau sing oscillatory radiatio n of u nde sired e nergy .

This problem u s u ally occurs whe n incorrect alignment procedures are u sed, or upon physically disturbing the origin al circuitry (moving of wires and components) during trouble shooting and repair . It is e xtre mely important to e x ercise care in replacing electronic parts with e x act type s to restore the precise coniguration as origin ally installed .

4. Sideband Splatter- Spuriou s sideband components produced outside the intended modulated RF bandwidth result in an EMI k n own as splat­ ter. Again, it u s u ally res ults from faulty transmitter operation, either through overmodulation or through poor tuning practice s such as over­ driving the intermediate and inal output stage s by overzealou s atte mpts to eke out the peak radiated power .

5.

Broadband Arcing Noise-High

power transmitters produce very high

RF currents and voltages along the transmission syste m . If the trans­ mitters are not properly m atched and loaded into the ante n n a (maxim u m power transfer), standing waves along the transmission line can cause arcing and corona discharge . Similarly, RF energy induced in nearby rigging and structural appendages may exhibit arcing and sparking . The result is broadband noise . 6. Waveguide and Coaxial Cable Leakage- W hen RF energy escapes from poorly designed, in stalled, or maintained tra n s mission lines such as waveguide and coaxial cable connectors and j oints, unde sired EMI is evidenced. This problem is particularly apparent in shipboard radar and microwave systems . b.

Shipboard Receiver System EMf-Although certainly n ot contributing s uch high levels of EMI as trans mitters do, interference generated within a shipboard receiver may still have as pronounced an effect and re sult in serious performance degradation . Sources of internal receiver EMI (i . e . ,

EMI origin ating within the receiver) include: (I) image frequency inter­ ference created by ordin ary loc al oscillator heterodyne mixing but escaping u n atte n u ated in a well-de signed and iltered receiver because of faulty alignment and tu ning; (2) e xtraneous interference signals produced in the receiver by intrusion of strong e xternal signals coupled from nearby high power transmitters; (3) intermodulatio n and spuriou s interference products resulting from u nintention al sign al mixing in nonlinear receiver circ uitry; and (4) cro ss-modulation when signals are u ninte ntionally tra nsferred from an u nde sired RF carrier to the inte nded receiver c arrier .

50

SHIPBOARD ELECTROMAGNEICS

c.

Shipboard Electrical Apparatus EMI- A ship is i n a sense a small, se l f­ contained community . That is, i n addi tion to prov i d i n g a workplace and job for each onboard re sident, it a l so supp l i e s many ame n i t i e s for comfot and entertai nment: bethi ng, food, med ical attention, sani tary fac i l ities, and choices of l e i s ure act i v i t ie s . A v i s i tor on a guided tour aboard a moden naval w arship might be surprised to see a barber shop, laundry , post ofi ce, variety store , l ibrary, pharmacy, c l i n i c , c arpenter shop, mac h i ne shop, gym , radio, TV, n i g h t l y mov ies, and even a bri g . What i s not apparent to the casual observer i s that the s mooth op­ eration of each of these fac i l ities , i n addi tion to the primary mission func­ tions of the sh ip, i s dependent upon a great n umber and variety of e lectrical apparatuse s . These devices range from the smallest hand- h e l d hair dryers to c irc u i t breakers , s witche s , re lays , massive prop u l s ion system generators, l arge welding mac h i nes , and assoted l i gh t i n g req ui rements throughout the ship . Each e lectrical apparatus i s a potential source of unde s i red noise emi ss ions add i n g to the ambient E M I l e ve l . 1.

Motors and G enerators-B roadband noise produced b y s h ipboard mo­ tors and generators i s a common but serious source of E M I . I t is es­ pec i a l l y as soc iated with arc i n g at the bru s h contacts of commutators and s l ip-rings . It a l so re sults from the i nstantaneous b u i ldup and col­ lapsing (c urent reversal s ) of e lectric i e l d s and from fri c t ional static discharges i n belts , gears , and beari n g s . Additional l y , harmonic com­ pone nts are generated in armature magnetic i e l d non l i neari t ie s .

2. Circuit Breakers. Switches. and Relays-The sudden ope n i n g and c los­ i n g (so-called making and break i n g ) of e lectrical contac ts res u l t s in both radiated and conducted w ideband EM! . The usual occ u rrence is a voltage i m pu l se transient as the c irc u i t current is abrupt l y ch anged, c a u s i n g an arc as the dielectric breakdown strength is e xceeded between the meta l l i c contacts . T h e noise spectru m for contact E M I ranges from VLF through U H F (about 10 kHz to 400 M H z ) . 3. Engine Ignition Noise Igni tion systems are commonplace aboard s h i p for u s e i n s u c h i t e m s as portab le i reighting p u m p s a n d for stating t h e e n g i n e s o f helos a n d aircraft . These d e v i c e s are perhaps the s trongest source of man- made noise i nterference i n the H F to VHF range ( 1 0 M H z to 100 M H z ) . 4 . Lighting- l uorescent l ighting is employed throughout the i ntenal spaces of a ship and is a notoriou s source of noise . EMI is c reated w it h i n lamps upon e lectrical breakdow n . It i s a l so conducted through the power c ir­ cui try and, most signi icantly, radiated from the power source connec­ 5 tion l i nes . This type of i nterfere nce i s trou blesome from approx i mate l y 100 k H z t o 3 M H z . -

SHIPBOARD ELECTROMAGNEIC INTERFERENCE

51

In addition to luore scent l ights , man y ships use sodium vapor and mercury arc lamps for lighting the tops ide areas. S i m i l arly, these l ights generate electrical noise from 1 00 kHz to about 1 M H z. 5. Miscellaneous Electrical Items-There are a great many sources of shipboard man-made EMI other than the major contributors li sted above. These i nclude such seemingly i nnocuous e lectrical apparatuses as heat­ ers, power supplies, dc rectiiers, solenoids , rheostats , tran sformers, buzzers , PA systems, walkie-talkie radios , tape recorders , computers, data processing equipment, and microwave ovens. Each is a potential contributor to the overall shipboard noise. d.

Hull-Generated Intermodulation-Noise interference resu lting from h u l l ­ related intermodulation is o n e o f t h e more pronounced and w idespread o f shipboard E M I problems. It i s man- made in sofar a s man provides the mechan ism for its genesis. Yet the effect is natural ; that i s , it is evidence of nature taking its course. H u l l - generated EMI , therefore , may be thought of as a hybrid interference. That it is promoted by the complex metallic structure of the ship and the harsh maritime operating environment cannot be denied. As a shipboard e lectromagnetic phenomenon , h u l l -generated inter­ modulation is a direct consequence of: (I) the quantity of onboard trans­ mitters and the ir radiated power leve l s ; (2) the quantity of on board rece ivers and their sensitivity leve l s ; (3) the q uantity of onboard anten nas and the ir constricted placement; (4) the quantity of onboard operational frequencies in a congested spectrum ; and ( 5 ) the quantity of poss ible non l i near elements and j u nct ions in the structural makeup of the sh ip. Hull-generated intermodulation is ot::. times referred to as the "usty­ bolt effect. " It originate s at many of the nonl i near components or j u nctions that abound in naval surface shi ps. Indeed , there have been esti mates that perhaps thousands of often obscure nonl i near e lements e x i st in the tops ide of any given ship. Moreover, it should be poi nted out that stee l itse l f is intrinsical l y nonl inear. Nevertheless, the maj ority of non l i neari ties act ing to create shipboard noise intermodu lation is due simply to metallic junct ions exposed to the sea environment. 6 Table 4- 1 l i sts a variety of metis by their standing in what is known as the galvanic series. Note that materials commonly u sed in ship con stuct ion , such as aluminum and steel , are near the top of the l i st. As a con sequence , they are metals that are more eas i l y coroded and are classed as be ing least noble. If two metals are joined together, the farther they are apat in the gal van ic series , the greater the like l i hood of chem ical reaction prod ucing corrosion. So long as the two meta ls are clean , dry , and held tightly In contact, the impedance between them is virtual l y zero .

52

SHIPBOARD ELECTROMAGNEICS

Table 4-1. Galvanic Series of Metals

Corroded End (anodic or less noble) Magnesium Magnesium A l loys Zinc A l u m i n u m 1100 Cadmium A l u m i n u m 2017 Steel or Iron Cast Iron Chromium Iron ( acti ve) Ni-Resi st. Irons 18-8 Chrom i u m - n ickel - i ron (acti ve) 18-8-3 Cr-Ni -Mo-Fe (active) Lead-Tin S olders Lead Tin Nickel (acti ve) lncone l (acti ve) Haste l loy C (active) B rasses Copper B ronzes Copper Nickel A l loys Monel S i l ver Solder Nickel ( pass i ve) I nconel ( passi ve) Chromium Iron ( passi ve) Titan i u m 18-8 Chromi um-n icke l - i ron ( pass i ve) 18-8-3 Cr-Ni-Mo-Fe ( passive) Haste lloy C ( passi ve) S i l ver Graphite Gold Platinum Protected End ( cathodic, or more noble)

SHIPBOARD ELECTROMAGNEIC INTERFERENCE

53

U pon exposure to moisture, however, unprotected joints begin im­ mediate deterioration due to oxidation and corrosion. The j unction i mped­ ance then increases, and a semiconductor device is formed. When an electrolyte is present , such as sea w ater, a si mple battery cell evolves , electrolytic action develops , and a n accelerated rate of corrosion occurs at the less noble ( anode) metal. The result is termed a nonl i near j u nction. If RF energy from on board transmitters impinges upon, or is induced across, the j u nction ( wh ich acts as a dc rectiier) , i ntermodulation signals are produced to emanate as EM! . Ideally , of course, only the same or adj acent metal s in the galvanic series should be mated together. U se of such dis­ similar metals as steel bolts through brass l anges or aluminum clamps across copper piping should be strictly avoided . The problem aboard naval ships i s that there are numerous instances of aluminum in contact with steel. Therefore , even though these two metal s are close i n the galvanic series, corrosion w ill develop rapidly. There are types of corrosion other than galvanic that create nonlinear j u nction s : 1 . Fatigue Corrosion- Result s from repeated vibrations and bending , whereupon the outer protective ilm of a metallic surface i s broken and the corrosion process begins. 2. Crevice Corrosion-Occurs when shipboard contami nants and moisture combine to penetrate and collect in seams and crevices for a suficient period of time to start corrodi ng. 3. Stress Corrosion Occurs when a metal is stressed to the point that m i n iscule cracking allows moisture to enter and initiate corrosion. 4. Welding Corrosion- Results when the intense heat of welding causes changes i n the molecular structure of one of the simi l ar metals being joined so that it becomes, i n effect , a dissimi lar metal, and, i n the presence of moisture, begins to corrode. -

e.

Intermodulation Theoy and the Ship Hull Environment-lust as when the local oscillator output of a recei ver i s heterodyned w i th a selected i ncoming signal at the nonlinear mixing stage to produce, by inte rmodulation, the .new intermediate frequency ( plus several di scarded sum and difference frequencies), so are i ntermodulation products created by the mixing of extraneous e lectromagnetic energy in certain nonli near elements of the ship hull. The problem with the hull-generated intermodu lation, however, is that these frequency products are always unintentional and very sure l y unwanted shipboard EM!. Assume that an RF signal from an onboard transmitter rad i ating frequency F 1 is by chance applied across an e lectrically non l inear element i n the ship hull structure. Intermodulation action results, and the frequency spectru m generated by th is nonl inear mixing w ill contain the ori g i n al fun­ damental F, plus several other frequencies harmonically related to Fl' That

SHIPBOARD ELE TROMAG ETICS

54

is , there will be a second harmonic 2F1, third harmonic 3F"

fourth har­

monic 4F1, and so on. If two such RF signals of nonharmonic a l l y re l ated frequencies F I and F2 from separate trans mitters sim ul taneou s l y e x cite a non linear el ement , the output spectrum wil l contain not on l y the direct harmonic frequencies 2F1, 3F" 4F1,

.

•

•

, and 2F2, 3F2, 4F2, . . . , but

a l so many new frequencie s re lated to the two fu ndamental s;

viz:

FI

+

F2, known as second-order intermod u l ation products

2FI

+

2F I

F2, known as third-order intermodul ation products

+

2F2, fou rth-order products

3F,

+

2F2, i fth-order , and so on

I n such a manner an enormous nu mber of intermod u lation products are un wittingly generated in the s h i p en vironment. 7 by :

The basic equat ions i n intermod u lation theory for this event are given

R = MTI + NT2

and

Q= jMj where T1, T2 =

and TI < T2·

M,N R

Q

jNj

+

tran smitter RF carrier frequencies e x pressed in l ike unit s ,

i ntegers; i.e., zero, pos iti ve, or negative the re s u l tant intermod u l at ion product i nterfe rence frequency (in the same u n i ts as TI and T2) the order of i n termodul ation product

Therefore , in the case of 3F1

±

2F2 above,

jM j

=

3,

jN j

=

2, and the intermodu l at ion product i s i fth -order. For both 2F I + F 2 and F I ±

2F2, the y i e l d is a third-order product. Like w i se , i f a t h i rd s h ipboard

trans mitter paticipates in e x c itation of the same non linear e l e men t, then third-order products cou ld result from FI

±

F2

±

F3, and so on, sum­

marized as fol low s :

FREQUENCY F

PRODUCT ORDER

FREQUENCY

2

l

±

F

l

±

F

F l

±

2F 2

3

3F

±

2F 2

4

2F

2F

2F

l

2 2

3

.

3F l

±

F

F 1

±

3F

1

±

2F

l

±

3F 2

Figure 4·2 Intemod u l ation Product Orders

PRODUCT ORDER

2

4

2

4

2

5 5

55

SHIPBOARD ELECTROMAGNETIC INTERFERENCE

Figure 4-3 ill u strate s how rapid l y the n u m ber of i ntermod u l ation products i ncrease with an i ncreasing n u m ber of R F exc iters . It can be see n that 10 transmitters s i m u ltaneo u s l y rad iat i n g d i screte fre ­ que n c i e s theoretical l y could prod uce 670 th ird - order i nte rmod u l ation prod ­ ucts and over 20 , 000 , 000 13th-order prod ucts! It shou l d be poi nted out that i nte rmod u l ation prod ucts as h i g h as the 60th order have bee n actu a l l y 8 recorded during sh ipboard EMl tests . Figure 4-4 depicts the dramatic e ffect of add i n g j u st one more tra n s m itte r . Here T i s the n u mber of trans­ m itte rs i n serv ice , Q i s the i ntermod u l ation prod uct order , and Pa i s the n u m ber of products generated as a fu nction of i nc reas i n g the n u m ber of rad i ated exc iters . For e x amp l e , if 1 2 tra n s m itte rs are rad i at i n g energy that exc ites a non l i near e l e ment , and a 13th transm itter i s added , the re s u lt wou l d be appro x i mate l y 25 new second-order products , 300 th ird-o rde r products , 2500 fou rth-order , and about 1 2, 000 ifth-orde r .

NO. OF N� MIS 1 2

3 4 5 6

7

8 9

10

NUMBER OF OO-OROER PRODUCTS 3 1

6

19 4 5

5 1 10

51

10

01

9 416

1.75

1 18

13

6 4.5

1 2

23

4

10.15

1 6

9 24 19.5

17.718

1O.n5

474,215

16.44

614.0

l.66,2O

8361

42

7.79

74313

32.073

U7145

6.9.625

14.2

1980

1.0.0

5.18.0

20.18.0

1.182 2.471

670

1 14

13

5.4 18

16 231

44 9

11

9

1

4.712

14.07

6.0

14910

Figure 4-3 N u mber of Poss i ble Odd-Order Products Ve rsus Tra n s m itte rs Op­ erati ng S i multaneously The pote nti a l for h u ll-gene rated i ntermod u l ation i nterference aboard naval sh ips can not be take n l i ghtl y . Degradation fro m such E Ml, e spec i a l l y to sh ipboard communications, can be severe . Tab le 4-2 is a l i st of some of the repre sentative ite m s that act as non l i near devices i n the sh ipboard e n v i ro n m e nt . Methods of dealing with the prob l e m of h u l l-generated rusty­ bolt-type i ntermod u l ation wi l l be taken up i n late r sections of this c h apter .

56

SHIPBOARD ELECTROMAGNEICS

10

.

. ) .. J > c o : . c l

V

W ..

< :

V I I 1/ V

w

Z

w

)

w

..

w

: J ) C

� Y

V

I V

­

O : o

� > z

�

10

1

(

/

5

/

10

�

�/

_II

w

< z o .. c c
C

,OK

..

1 00


0 Z 0 U I > ..

"

.

1

..

>

;

10

I S P E R rA L L O Y

>

l

: ..

� (

u > 0 0 : -

E

l

" w Z � u

,

1 02

.:

i

0 j n

:

..

i �

HYPE R NICK

-�

,

,

,

.

COPPE R

ALUMIN� 0.5

ZINC

BRAS

TIN

1D

1

1 D- 1

Figure 4-8 Shield Thickness

Finally, i t should be noted that shielding effectiveness i s a function of physical parameters, too ; that i s, the manner i n wh ich the shield i s s haped and fastened i n place . No shielding is absol utely perfect. Some energy w i l l i n e v i tably penetrate the barrier through seams, edges, cable entrances and access ope nings, and fastener hole s . Idea l l y, the shield should be a spherical shroud enveloping the source or victim and bonded to ground by soldering or welding . More practical l y, the shield i s coni gured as a rectangular box, cyl inder, or sheet barrie r . It may be sol id, screen, braid, metal foi l, metal l ic tape, impre gnated plastic, or even be coated with a conductive paint or spray . It is usual l y i n stal led

67

SHIPBOARD ELECTROMAGNEIC INTERFERENCE

with rivets or screws atop an RF gasketing material . The important thing to keep in mind is that to achieve optimum shielding for both E-ield and H - i e l d i nter­ ference , the max imum degree of continuous conductivity to ground must be ensured . Table 4-7. K Correction Factors for Iron and Copper in dB

Frequency Shield Thickness (Mils) 60 Hz ]00 Hz ] Hz ]0 Hz Magnetic ields copper ( L = 1 , a= l)

Electic ields and plane waves copper (a = l , L = l )

Magnetic i e lds i ron ( L = 1 000 , a = 0 . 1 7)

Electric ields and pl ane waves Iron ( L = 1 000 , a = 0. 1 7)

1 5 10 20 30 50 1 00 200 300

- 22 . 22 - 2 1 . 30 - 1 9 . 23 - 1 5 . 85 - 1 2 . 55 - 8 . 88 - 4 . 24 - 0 . 76

1 5 10 20 30 50 1 00 200 300

- 41 .52 - 27 . 64 - 2 1 . 75 - 1 5 . 99 - 1 2 . 73 - 8.81 - 4 . 08 - 0 . 62

- 24 . 3 1 - 22 . 07 - 1 8 . 59 - 1 3 . 77 - 1 0 . 76 - 7 . 07 - 2 . 74

+ 0 . 05 + 0.32 + 0.53 - 39. 3 1 - 26 . 46 - 19.61 - 1 3 . 92 - 1 0 . 73 - 6 . 96 - 2.61

+ 0. 1 4 + 0.4 1 + 0.58

1 5 10 20 30 50

+ + + +

1 5 10 20 30 50

- 1 9 . 53 - 6 . 90 - 2 . 56 + 0. 1 6 + 0.58 + 0. 1 3

0 . 95 0 . 93 0.78 0 . 35 + 0 . 06 -

+ + + + -

- 28 . 23 - 1 5 . 83 - 1 0 . 37 - 5.41 - 2 . 94 - 0.58

] MHz -

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

+ 0 . 50

- 29 . 3 8 - 1 5 . 82 - 1 0 . 33 - 5 . 37 - 2 . 90 - 0 . 55

- 19.61 - 6 . 96 - 2.61 + 0. 14 + 0.58 + 0. 14

- 1 0 . 33 - 2 . 6 1 - 0.55 + 0. 14 + 0.57 - 0. 1 0 -

-

-

-

-

-

+ 0.5 1

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

- 1 7 . 4 1 - 8 . 35 - 5 . 1 7 + 0 . 20 - 1 . 3 1 + 0 . 36 + 0 . 54 + 0 . 42 -

kHz

- 1 9 . 6 1 - 1 0 . 34 2.61 - 6 . 98 0.55 + 0. 14 - 2 . 62 + 0 . 5 7 + 0. 1 3 - 0. 1 0 + 0.58

1 . 23 - 1 . 60 0 . 89 - 0 . 5 9 0 . 4 8 + 0 . 06 0 . 08 0 . 06 -

]00

1 . 83

1 .3 1

68

SHIPBOARD ELECTROMAGNEICS

I

300 mils

I I --

2i : 0 0 �/�

o

�

-5

I

-

...

.

20

-15

10

/"

�

- 20

..

�

',I

-25

10 H z

1 00 H z

V

V II

1

/

�

V

/

mi l

.

V

/

/

/

/

t;'

/

I I '� 5� / // 3� /

iO K FA C T O R

- 30

1 1 1111

1l

V

�

I

V

I

'

, T V/ I

I

kHz FREOUENCY

,

T:

10 kHz

I

,

} r I

/'

"

I

I

I ,

,�

/

I

I

,

, , I ER I AL THICK NESS IN M I LS

M AT

I I I I IIII

.

I I

I I I I III

100 k H z

I

MHz

Figure 4-9 K Correction Factor for Copper Magnetic Field

4 -2 . 1 . 2 a.

Shielding Methods and Materials

Multiple Layer Metallic Shields-Low frequency H-ield E M I shie lding freq uently req u ires specia l i zed materia l s and techn i q ue s . While high permeabil ity al loys such as M u metal do prov ide good absorption loss attenuation for the weaker low frequency interference s , strong magnetc energy sources necessitate a combi nation of relection and absoption losses to achieve sufic ient shie lding . S i nce s i n g l e - w a l l relection losses are q uite smal l at low fre q uencie s , the solution is to present multiple barriers to the interfering si gnal, thereby promoting a succession of relection losses re­ sulting from each boundary ( along w i th accu m u l ative absorption losse s ) . The composite partition thus offers a superior shie lding e ffectiveness option to the single-wall shie ld of greater overall thickness . Moreover , s i nce ferromagnetic metal s tend to saturate at a max i m u m lu x leve l , the inter­ spersing of ferromagnetic and nonferromagnetic layers offers better atten­ uation characteri stics over an individual saturated metal . G eneral l y , metal s such as copper, iron , Mumetal , and CoNetic are c l ad � gether ( but separated "

�

by air space or solid d i electric material ) to form a highly effective electromagnetic shield affording 1 00 d B or more of i nterference reduction .

SHIPBOARD ELECTROMAGNEIC INTERFERENCE

69

Low freq uency H - i e l d i nterference is produced aboard ship by such items as motors , transformers , solenoids , and coil i nductors . One practical method used to verify the source l ocation and suppress thi s EMI at its ori g i n i s to shroud a suspected interference generator with an eas i l y formed metall i c shiel d . Various types of l e x ible sheet and foi l materials are avail­ able which can be cut , shaped , and taped i n p lace to determ i ne experi­ mental l y the thickness req uired , the number of layers , and the best metal 10 or alloy needed for effective shielding . b . Perforated Metallic Shieldin g B ecause o f req uirements such as ve nti­ lation and visual monitori ng , there are some i nstances aboard ship where solid metal EMI shields cannot be used ; for example , where equipment cooling call s for continuous air c ircu lation through partition s , and where i nstrument meters and display scopes must be viewed through transpare nt covers . Satisfactory shie lding , though never as good as that of solid bar­ riers , can be achieved using wire mesh screens across ve ntilation openings . Like w i se , ine wove n mesh may be implanted i n , or lami nated w ith , g lass to provide a transparent medium across viewing aperture s . Normal ly the shielding effectiveness o f a metal screen decreases as frequency increase s , and w i th increas ing ho le size ( w ire spac i ng ) . Magnetic energy , however, i s an exception : the shielding effectiYeness increases with increasing frequency for H - i e lds ( i . e . , the attenuation is sma l l at low frequenc ies ) , and , of course , increases with the permeabi l ity of the metal in use . In either event , calculating shielding effectiveness for metal screens i s so tedious that it i s more practical to select material read i l y avai lable from vendors proposed for meeting the attenuation requirements of a par­ ticular situation . The n , after applying the scree n , the resultant ield strength is measured and compared to the EMI stre ngth prior to screen installation . In this manner the shielding adequacy of the metal screen can be eas i l y veri ied . In some situations venti lation and cooling req uirements necessitate an uni mpede d , high air low with screen holes so large that adequate shie lding effectiveness cannot be met with w ire mesh . For these cases metal honeycomb shields are recommended . Honeycomb barriers make use of waveguide transmi ssion l i ne theory to determine hole size and depth so that the shield openings act to atten uate greatly a w ide band of potential EM I below the wavegu ide cutoff frequency . The effectiveness of the honeycomb is a function of the hole size , depth , spac ing (i . e . , hole number) , and type metal . Honeycomb panels are heavier and more expens ive than wire mesh screens ; they offer better shielding effectiveness , however, and greater structural strength . The y are particu larl y effective at the higher freq uencies . For example , at 10 G H z , -

SHIPBOA RD ELECTROMA G E TIC

70

a Y4 -inch diameter ho neycomb tube I inch in de pth ( thickne

) provide

102 d B attenu ation, and, at the same frequency, a Y2 -inch diameter tube

(highe r air circulation) 2 Y4 inche in depth still give 100 d B attenu ation . Moreove r, eve n at the lower frequencie , ho neycomb panels have fairly good shielding e ffectiveness, as

een in Table 4 - 8 for a teel ho neycomb

screen with he x agonal ope nings Y8-inch wide and Y2 -inch dee p . Table 4-8 . Shielding Effective ness of Steel Hexagonal Honeycomb Yg - I nch

Ope ning s Y2 - I nch Thick

Frequency (MHz) 0. 1

45

50

51

J OO

c.

Sh ielding Efectiveness (dB )

57

400

56

2, 200

47

Metallized Surfaces Sh ielding-An interesting change has occurred in the packaging of electronic circ uitry during the i nal q uarter of the twentieth century . The change, so widespread and rapid as to be perhaps un noticed by the casual observer, is the virtual worldwide u se of molded plastic enclosure s . In the past it was common prac tice , patic ularly for high-quality military eq uipme nt , to assemble el ec tronic compone nts on a metal chassis and the n hou se the completed package within a heavy metal cabinet . The metal case afforded a good degree of electromagnetic shie lding as w e l l as excellent structural ruggedne ss . Recently , however , lightweight molded plastics have been universa l l y adopted for e ncasin g the new generations of solid - state microe lectronic devices . This tre nd is readily apparent in the proliferation of such modern ofice items as word processors , desktop computers, printers, and various peripheral eq uipment . To a lesser but signiicant e x tent, even some contemporary large shipboard e lectromag­ netic systems are now being enc losed in plastic cabinets . I n addition to being very light in weight , plastic cases are re lative l y ine xpensive , tough, a n d attrac tively contoured a n d colored . Y e t they are notably dei cient in one important aspect: they are transpare nt to RF energy .

SHIPBOARD ELECTROMAGNEIC INTERFERENCE

71

That i s , unless carefully prepared, they offer no E M I shielding . There fore , special measures must be taken to con�-the_ plastic she l l into a contin­ uously conductive envelope so as to protect otherw i se vulnerable intenal electronic circuitry from extenal electromagnetic di sturbances . Like w i se , intenal l y generated interference must be prevented from escaping into the envIronment . N u merous engi neering techn iques have been developed to transform plastic enclosures into effective EMI shields . It must be assumed that one or more of the se techniques would be i ncorporated in equipment spec iied , designed , and procured for naval shipboard use . Nevertheless , there has been at l east one i mportant inc ident where an electronic warfare system housed in a plastic enclos ure emitted broadband noise from the rear of the case sufic ient to cause severe EMI to other tops ide combat systems . Retroit shielding was neces sary to que l l the i n terference . S i m ilarly , cor­ rective mainte nance and repairs to other shipboard equipment is req uired from time to ti me . One method popularl y employed to provide good enclosure shielding i s to mix metal i bers such as aluminum , stai nle ss steel , carbon, or graphite in w i th the liquid plastic during the molding process to create an electrically conductive composite material . Because the ir high length-to-diameter ratio makes them more eficient conductors , metallic i bers are preferred ove r other metal i l lers such as powder, gran ules, or lakes . I f the plastic has no embedded metal for shield ing, several types of metallic coating techniques are avai lable for plastic surfaces , including vacuum metall i zing , wire-arc and lame metal spraying , electroless metal plating , and the use of metal-fo i l linings , metal-coated fabri c s , and con­ ductive paints . The most commonly used metal s in these applications are aluminum , copper, s i lver, zinc , nicke l , iron , carbon , carbon steel , stainless steel , nickel stee l , and graphite .

1 . Metal Foils and Metallized Tapes-V arious types and thi cknesses of metal foi l are commercially available and are eas i l y faned and applied to fun i sh satisfactory shie lding . The most effect ive foil shie lds are lexible laminates , in which such foi l s as copper or alum inum are sand­ wiched between re i n forcing plastics or paper ilms . Accordingly, the foi l s provide good EMI attenuation , and the dielectric ilm substrates act as e lectric insulating materials . For small items , or for quick stop-gap repairs , metallized foil tape with pressure-sensitive adhesive may be handily used , as it easily con­ fons to nearl y any shape of object i n need of shielding .

72

SHIPBOARD ELECTROMAGNEICS

2.

Fiber-Coated Fabrics-An alternative to foi l , also l ightweight , lexible ,

and simply applied , is metal l i zed fabric . Fibrous material such as rayon , cotton , polyester, polyacry late , polyamide , polyurethane , or even glass or carbon is used as a base and plated with a microthin metal membrane such as gold , si lver, copper, nicke l , cobalt, or chrome . 1 1 By proper combination of iber, metal coating , and coating thickness , the desired �.-.0 EMI protection is achieved , with a �i n �_eff�9iy�'� dB over a frequency range from 1 00 kHz to I GHz . ' -The selectei' fabri� ' ca�n be pu��h���ith an adhesive backing and quickly cut to conform to any practicable size or shape . Moreover , since EM I fabrics are lightweight , breathable , and washable , they may be employed as screen draperies and wall coverings and even may be fashioned into outerwear for protection against EM radiation hazards . 3 . Vacuum Metallizing-As the name suggests , this method of coating i s achieved through evaporation in a high vacuum t o deposit a unifom metallic i l m on a nonconductive surface . 1 2 Before metal lizing, the object to be shielded is treated with a chemical base coat and baked to allow good adhesion of the metal substance . The n , with aluminum as the most commonl y used evaporant , the vacuum process is carried out to coat the enclosure surface . Close l y rel ated to vacuum metall izing is a technique called sput­ tering . It, too , i s accomplished in a high vacuu m , but , rather than using the evaporation process , a metal target i s bombarded by electrically exc ited argon ions that dislodge and sputter the metal atoms to deposi t them o n the desired nonconductive surface . 1 3 General ly a chrome i l m base is applied initially to the bare plastic surface , fol lowed by a copper alloy for high conductivity , and , i n al l y , by another coating of chrome for corrosion resistance . In this way a combined medium of good conductivity with oxidation protection pro­ vides highly e ffective EMI shieldi ng . 4 . Wire-A rc and Flame Spray Shielding-In the wire-arc spray method , wires ( zinc w i res are commonly used) are kept electrical l y isolated and continuously fed into an operating gun so that only the w i re ends main­ tai n contact . At a precise point the ends are melted i nstantaneous l y by an intense arc , and the molten , atomized metal is directed to the target by a high pressure jet of air. 1 4 Upon contacting the plastic casing , the molten zinc quickly solidiies to form a dense , conductive , metal l i c coating . A somewhat related technique known as l ame spray ing differs in that it utilizes an acetylene lame rather than an electric arc . The _.

-

SHIPBOARD ELECTROMA GNETIC INTERFERENCE

73

lame method has the d i sadv antage of heating up the pia tic

u rface .

and e xtre me caut ion mu t be e x e rc i sed to prevent d i s tort ion and warp i n g .

5 . Electroless Plating- A c hem ical _rQ s s ofte n used to coat noncon­ ductive s u fac e s i s c a l led electroles- p l a t i n g . Because the ,Uicie to be coated is d ipped i n an aqueous so l ution th i s method offe rs the i m ­ mediate adv antage of de positing a meta l l i c i l m on both the i n ner and the outer s u rfaces s i m u ltaneously in a s i n g l e appl ication . The s h i e l d i n g e ffectiveness i s thereby m u c h i mproved over sing le-s h i e l ded su rfaces . E l ectro l e s s p l at i n g d i ffe rs rad i c a l l y from e l ectrop lat i n g , w h e re an e x ­ te ma. elec tricl

S O ULce

( e . g . , a d c rectiier supply) i u e d t o p l ate metal

upon a conductive su rface . Electropiug cannot be used on noncon­ ducti ve material£ . . Furthernore , e l ectro l e s s p l ating i s not to be confu sed with so-c a l led i m mersion pl ati ng , w h i c h i n v o l v e s re p lacement of one meta l of h igher e l ectromotive pote ntial by another of lo wer e l ectro ­ motive pote ntial .

15

I n e l ectro l e s s plating the aqueous sol uti ons are a mi xture o f re ­ ducing agents and a reducible metal , and the chemical reaction occ urs only at the formerly nonconductive surface . The item to be coated i s irst pre pared by a n etc hing process a n d i s neutra l i zed . T h e n a cata l y st is e m p l oyed to activ ate and de posit an e lectro l e s s metal such as copper or nic k e l a l l oy . The thickness of the e l ectro l e s s coating is determ i ned by the le ngth of time the object re m a i n

i m mersed in the p l at i ng bath .

As a re s u lt of its s i m p l i c ity , e lectro l e s s p l ating w i l l coat nearl y any type of e n c l o s ure coniguration , no matter the com p l e x ity , w ith a u niform meta l l ic fil m . I f add itional s h i e l d i n g i s d e s i red , e l ectro p l at i n g may b e app l ied on top of t h e e l ectro l e s s metal il m . For exam p l e . a l ayer of e l ectro l e s s copper can g i v e the needed h i g h conduct i v ity w h i l e an overcoat of e l ectropl ated n i ckel al loy al lows good corosion re s i s ­ tance , s o that t h e mu lti l ayer meta l l ic combination affords e x c e l lent overa l l s h i e l ding e ffectiveness . 6 . Condu ctive Paint C oa tin g s - A nother tech n i q u e for meta l l i z i n g plastic

enclosures i s the use of sprays conta i n i n g conduc t i v e meta l pat i c l e - . I n thi procedure the metal coat is norm a l ly app l i ed to the i nterior w a l l of the cabi net . Several conduct i ve substances are avai lab le 1 6 for u e i n s u c h spray

as acry l ic l acquer for fa t-dry i ng good adh e rence to most

thermop l a st ic , or in urethane p a i nt w h e re greate r coating hardne

s

i

req uired . ( a)

G raph it e

-

M i xed i n pai nt so l utions uch a acry l i c . po l y urethane .

epoxy re i n , or le x i b l e rubber compound , graph ite y i e l d � h i e l d i ng e ffectivenes t

of 30 to 40 d B

0

a

er a range of 20 M H z

1 G H z , and 40 to 60 d B for frequencie abo

e

I GHz .

74

SHIPBOARD ELECTROMAGNEICS

(b)

(c)

Copper-Copper has h i g h conductivity , nearly a s good a s si lver, but it oxidizes too easily unless spec i a l l y treated . It has exce l lent shielding characteristics at the lower frequencie s : 87 dB at 30 M H z , 84 dB at 300 M H z , and 36 dB at about 1 G H z . Nickel-Where ox idation and corosion must be avoided , nickel is frequentl y selected . B ecause of their high permeab i l ity , and , therefore , high H - ield absorption , nicke l compounds are curently the predomi nant choice of spray pamt coatings . Shielding effec­ tivene S: n ickel sprays is 40 dB or greater from 20 M H z to 1 GHz. Silver-S i l ver, o f course , has the disadvantage o f being relativel y expensive a s a conductive ingredient . It offers , however, two distinctive merits: it is highly conductive and it resists ox idation . Consequently it i s an exce l lent shielding e lement . Recent devel­ opments have reduced the cost of using s i l ver by taking advantage of RF skin-effect phenomena; that i s , in real izing that h igher frequency currents are concentrated on the outer surface of metal conductors . Accordingly , microspheres of hol l ow glass or ceramic are transformed i nto highly eficient electromagnetic conductors by a thin coat of s i l ver. 1 7 With a diameter of 50 microns and silver coat thickness of one microinch , the m i n iature spheres m i x eas i l y in the paint solutions and fac i l i tate spraying w i th conven­ tional equipment . S i lver-coated spheres are highly effective e lectromagnetic shields . EMI energy penetrating a sphere is dissipated by destruc­ tive i nterference of the multiple internal relections . And overal l surface conductivity of the plastic enclosure results from the m i ­ crosphere s ' b e i n g in natural contact t o form conductive chain network s . I f a denser coati ng is prefered for optimum shielding effectiveness , the hollow spheres may be i l led with a magnetic material to ensure tight packing by magnetic attraction . S i l ver paints , with an effectiveness of over 60 dB across the frequency spectru m , are often selected for m i litary equ i pment as offering the best shielding quality . -- -

(d)

---

�

-

For quick ield repairs and e xperimental test puposes aerosol spray conductive paints are also avai l able . But a signiicant disadvantage of spray paint coatings is that , compared to the other forms di scussed above , they are eas i l y scratched and mared to disrupt shielding i nteg­ rity . d . RF Gasket Shielding-Figure 4- 1 0 is a popularly u sed i l l u stration showing several apertures that are l ikel y to cause shielding degradation of an en-

SHIPBOARD ELECTROMAG

EiC INTERFERE

CE

c l o s u re . I f the rec tan g u l ar out l i n e i s v i e wed a

an eq u i pm e n t cab i n e t or a

l a borat ory scree n room or, u l t i m at e l y , a s h i p h u l l . i t i

appare n t th at , n o

matter h o w h i g h the degree of w a l l s h i e l d i n g , a n u mber of pot e n t i a l weak po i n t s e x i st . Depe n d i n g u po n t h e e n c l o s u re ' s p u po e, there w i l l be pow ­ erl i ne and cable pe netrat i on s , ante n n a connections . ven t i l ation open i n g � . i n st rument v i e w - po rts , and entryways . Each acce ss repre e n t s

a

rat ion of s h i e l d i n g i n tegri t y . V i e wport s and ven t i l at i o n ope n i n g

deterio­ may be

s h i e l ded by mesh or honeycomb scree n s ; contro l s h aft s can u se w a ve g u i de be l o w cutoff atten uators as d i s c u s sed i n Paragraph b of t h i s

e c t i o n : and

cab l e s and powerl i ne s use s h i e l d i n g and i l te r i n g methods . to be addressed l ater i n t h i s chapte r . Of i n terest to u s now are the means w h ereby edge , . cracks , and seams are s h i e l ded by use of RF gasket i n g . • I H EH N .

. -. .j . --.. -._ SHIE L D E D LED-IN

S E A­ M E TLLI C GSKE T

1-

ANTENN.

,:: ����� ,

1� j

UNSHI E L LE A D - I N

LINES U N S H I E L DE D =

SEAM R F G.S K E T

HOL E C R E E N ING

L IN E S

I ..P RO P E R LY F I L T E R ED

INSLATED C O N T R OL S H . F T

LI N E S SHIELDED

F US E

PHONE

J. C t

ME T E R JACK

1� e FI L�TERED T

1

PR

LY

P I LT L . .. P

PANE L 'ETER "OL E

Figure 4-1 0 Typical S h ie l ded E n c l o s u re D i scont i n u i t ie s I n some i n tances seams c a n b e protected fro m E M I pe netrat ion w i t h o u t e m p l oy i n g R F'g asket by proper mat i n g o f the metal s u rfaces . S pec i a l care mu t be taken i n such cases to esta b l i h a good bond b t w een the m e t a l su rface s . Th i s m u , t b e ac h i eved by avoid i n g pe rmanent contact of d i s � i m ­ i l ar metal ( to prec l ude g a l v a n i c corro . ion ) : b y e n s u r i n g t h at t h e bare m e t a l urface t o b e con nected are thoro u g h I u ffi c i e n t fa te n i n g pre . ure w i t h

c l ean a n d dry : and b y m a i n t a i n i n g

cre w . ri et .

o l d e ri n g . or co n t i n u o u �

w e l d . A pre fe rred tec h n i q u e i depicted i n F i g u re 4 - 1 1 : e . g . . fo l d i n g a n d verlapp i n g t h e edges of t h i n w a l led e nc l o, u r s . fo l l o w ed by so lderi n g o r welding .

I

T h e re u l tant . e a r n t he n h a s a , h i e l d i n g t h i c k ne ,

the enc l o ure w a l l .

�

t r i p l e t h at o f

76

SHIPBOARD ELECTROMAGNETICS

For aion f Pernent ve rp eam Noe: olderig o r We ldig t . e s i rable for Mximum Proec tin

Figure 4- 1 1 Formation of Permanent Overlap Seam

The most effective method for en suring that seams and entrance cracks do not unacceptab ly reduce shie l d i ng effec tiveness of enclosures is to use R F gaskets between the connected surfaces . Figure 4- 1 2 shows representative coni gurations of gasketed surfaces common l y encountered . The two types of R F gasket materials most often used are knitted w i re mesh and con­ duct i ve rubber stock . I ..n i tted wire mesh is avai lable in phosphor bronze , Monel ( a nickel -copper alloy ) , alumi num , tinned copper- iron al loy , and s i l ver-plated bras s . The conductive e lastomers use s i l ver, nickel , or carbon particles as i l lers in the s i l icone rubbers . V arious cross- sections of w i re mesh or rubber elastomer gaskets may be selected , including round , el­ l i ptical , rectangu lar, P-s hape . U -channe l , and tubular. Whene ver there i s the sl ightest chance that the enclosure might be e x posed to dampness , such as in shi pboard tops ides , it is imperative that the RF gasket also be a moisture-t ight seal . Electrolytic conditions must be pre­ vented at the l anges or seams . I n this case the conductive e l astomer gaskets are better sealing materi als than the wire mesh type s , al though combination so l i d rubber and w i re mesh gaskets can be very effective for some con­ iguration s . For the extreme salt spray shipboard environments , dual or combination gaskets shou ld be used . Care must be taken in the selection of gasket metal type in these environment s . For example , tin-plated , cop­ per-c lad stee l mesh shou ld be avoided , because i f the copper becomes exposed due to abrasion , the seam w i l l rapidly oxidize . Tin-plated , copper­ clad stee l i s , however, an excellent gasket for attenuation of high H-ields where salt spray conditions are not present . The best asket material .. for -

_

4_

-_

_

.

SHIPBOARD ELECTROMAGNEIC INTERFERENCE

77

Figure 4-12 Gasketed Surfaces ( from U S AF EMC Design Handbook DH 1 -4 )

�ipJ9ard corrosion r�s�s�!tC� �� _a.!.� lill� - ange§_ is .�_s i l Y�J:J�ted alu­ = � l ie (r e E is tomer. . � au se a luminum is so widely used in ship systems design , i t merits spec ial note . When installed between aluminum surfaces , v i rtually all types of EMI gaskets have a high potential for creating galvanic corosion at the 20 langes and seams . Gaskets made only of aluminum are usually avoided because aluminum generates an impenetrabl y hard , nonconductive oxide . Yet , in most other choices where the gasket i s made of metals dissimi l ar to aluminum (e . g . , Mone l , tin , carbon , s i l ver, or bery l l i u m copper) , the dissimilarity readily promotes galvanic cell corrosion . The result is in­ creased re si stance between the shielding gasket and the enclosure surface and decreased shielding effectiveness . For this reason s i lver-plated alu­ minum conductive rubber gaskets are recommended for aluminum seams . It is critical for good enclosure gasketing that adequate and even 21 pressure be appl ied between the shielded surfaces . This i s accompl i shed by providing a rigid surface at the contact poi n t , using several evenly spaced fasteners , and incorporating self-retaining grooves to hold the gasket i n place . In some d i ficult situations the use of adhesives or spot welding

�

�

__

78

SHIPBOARD ELECTROMAGNEICS

. 1 i s recommended .

The type of gasket cross-section selected is also a function of proper compressio n . For pressures of 20 psi or less , k n i tted w ire mesh of round cross-section , knitted w i re mesh sleeves over sponge e lastomers , or solid conductive e lastomers i n tubular form are best . When the seam compression must be h igh , rectangul ar k nitted wire mesh and solid P­ shape , U - shape , and rectang u l ar e lastomers are e mployed . If over-compres­ sion is an anticipated problem , gaskets with built-in metal stops of brass or steel are available . Where frequent access through an enclosure ope n i n g is required , lexible spri ng i nger stock should be i nstal led . Genera l l y avai l able i n bery l l i u m copper o r other h i g h l y corosion-re s i s tant alloys , i nger gaskets are designed to be exceptionally durable to w i thstand thousands of oper­ ation s . They are fastened by adhe s i v e , epox y , solder , ri vet s , or c l ips . Table 4-9 ( from [ 1 8 ] ) l i sts the major advantages and deiciencies of some commonly used RF gaskets . Figure

4- 1 3 i l l u strates typical shipboard i nstal l ations of conductive RF

gasketing used for shielding of l ange connections .

J) Wire and Cable Shielding-The l arge quantity of complex , sensitive elec­

tronic sy stem s i n stalled aboard naval w arships gives rise to an e xtraord i n ary number of cable and wiring i ntercon nections . To c arry out its necessary function s , shipboard electronic equipment req u i re s cabling and wiring for transmission of e lectrical i n formation such as audio , video , pulse , and

control signal s ; operating power; and RF radiation and receive energy . To do so , cableways must traverse v i rtually every below -deck compatment ( see Figure

4- 1 4 ) and , in many case s , must pe netrate the main deck to run

from stem to stem , to and from a variety of tops ide items and spaces , reaching even to the very tips of masts ( see Figure

4- 1 5 ) , where they may

pe exposed to enormously intense e lectromagnetic ields .

CU n fortunate l y ,

i n being so Ubiqu itou s , cables and signal w ires are also

e x pedient bearers of bad news . That i s , because of their o w n e lectromag­ netic nature , they act as convenient antennas and tran s m i ssion l i ne s to i ntercept and transport radiated and conducted EMI straight into the weakest spots of e l ectronic equipme n t . This tendency to aid and abet system deg­ radation results in cables and wire s be i ng given spec ial attention . I n ship­ board syste m s engi neeri ng design and in�tal l ation , cable shielding and

louting practices are of e x treme impotance . J 1 . Cable Types and Ter inati n � ost below-deck i nterference i s genm

o

s

era ted by shipboard power systems and other frequency sources below

l

1 00 kHz, w h i l e i n the topside , radiated EMI occurs ge neral ly at H F a n d above . Therefore , because of the high probab i l ity of encountering

l

f

79

SHIPBOARD ELECTROMAGNEIC INTERFERENCE

several fons of EMI ranging i n frequency from powerl ine to micro­ wave , and because of their natural suscept i b i l ity , shielded cables are i nvariably used . The principal types avai l able are shie lded single w i re , shielded multiconductors , shielded t w i sted pair, and coaxial . Several vari ations of these are used to protect agai nst both magnetic ields and stray RF. Coaxial , even though meant primaril y for high frequency transmi ssion , is an excel lent low-frequency shie l ded cable and is fre­ quently selected because of its adaptab i l ity and comparatively low cost . If subj ec ted to very strong levels of EMI , however , even coaxial m ight not afford adequate shielding . I n such case s , more complex cable de­ signs are employed ; e . g . , twinaxial , triax i a l , and quadrax ial .

Table 4·9 . Characteri stics of Conducti ve Gasketing Materi als

Material

Chief Advantages

Chief Limitations

Knitted w i re mesh

Most res i l ient all-metal gasket ( low l ange pressure requ i red) . . Most points of contact . Available i n variety of thicknesses and re s i l iencies , and i n combination w ith neoprene and s i licone .

Not avai l able i n sheet (certain i ntricate shapes d i fi c u l t to make ) . Must be 0 . 040 inch or thicker. S ubject to compression set .

Brass or bery l l i u m copper w ith punctured holes Oriented w ires in rubber s i l icone

Best of corrosion protection i l m s .

Not tru l y res i l ient or genera l l y reusable .

Combines lexibil ity and RF seal . Can be effective against corrosion i l m s .

Might require w ider or thicker s i ze gasket for same effecti veness . Effecti veness decreases w ith mechanical use .

80

SHIPBOARD ELECTROMAGNEICS

Table 4 . 9 (con t ' d )

Material A l u m i n u m screen

Chief Advantages Combines l e x i b i l ity

Chief Limitations Very low res i l iency

impregnated

and conducti ve seal .

( h i gh l ange pressure

w ith neoprene

Thinnest gasket . Can

requ i red) .

be cut to intricate shape s . Soft metal s

Least expensive i n s m a l l s i ze s .

Metal foi l over rubber

Has advantage of the res i l iency of rubber.

C o l d l ow s , l o w res i l ie nc y . Foi l cracks or shits position . Generall y low absorption loss yielding poor R F propertie s .

Conductive ru bber ( c arbon i l le d ) Conductive rubber ( s i lver i l led)

Combines l e x i b i l ity a n d conducti ve seal . Combines l e x i b i l ity

Provides moderate absorption loss . Not as effective as

and RF seal .

metal i n magnetic

Exce l lent re s i l ience

i e l d s . May req u i re

w ith low compression set .

salt spray env ironmental

Reusable . Available

protectio n .

in any shape or cross section . Contact i ngers

Best su ited for s l i d i n g contact .

Eas i l y damaged . Few poi nts of contac t .

DECK

P O W E R C A B L E D E C K P E N E T R A T I ON

Figure 4·1 3 ( a ) Shipboard I n stallat ion of R F Gasketing at Flanges

OECK SLEEVE

CON D U I T

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ALL GSK E T EDGE.

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CONf i G U R A T I O N S T O M A T C H T H ROUGHOUT T H E C L A S S .

SH I P S I N T H E CL ASS. O N O T R EWORK E X I ST I N G

5.

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E X I S T I N G C O N f i G U R A T IONS "AY Di f f E R A M O N G I N DI V I D U A L

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W H E N O E C K S L E E V E IS U S E D I N S T A L L CONDU C T I V E

C O . .E R C I A L

SPEC i f i CA T I ON

L I S T OF M AT E R I A L

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Figure 4- 1 3( b ) S h i pboard I n s t a l l a t i o n of RF G a s k e t i n g at F l a nges

WAV E G U I D E

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SHIPBOARD ELECTROMAGNEIC INTERFERENCE

(a)

(b)

Figure 4-1 4 Cables Travers ing Below-Deck Compartments

83

84

SHIPBOARD ELEC TROMAGNEICS

(a)

(b)

Figure 4-1 5 Cables Entering a n d Running U p S h i p Mast

SHIPBOARD ELECTROMAGNEIC INTERFERENCE

85

J�riaXi � , ! §

si mply a coax ial cable with an added outer copper­ braid sleeve that encloses the inner coaxial conductors to allow increased shielding . Ihe_.9_uter sJ��::e _ i s _grq���ed so as to shunt coupled noise ' energy and ground loop currents away from the inner si gnal-carry ing conductors . As a result , the signal-to-noise ratio is sub stant iall y im­ proved over that of ordinary coax i al . Twinaxial is a double -conductor , twi sted , balanced wire cable also having an outer metallic braid to shield the enveloped w i re s . Be­ cause the twist conig uration causes cancellation of induced noise e n ­ ergy , good protection against enetrating H-ield inteference is provided . Unfortunatel y , high frequency transmiss ion losses limit the use o f twin­ axial to below about 1 5 M H z . Wbere severe low-freq uency EMI is antici pated , another shielded cable option known as qu adraxial is effective in many case s . Quadraxial is a coniguration i n which twinaxial wiring is enclosed w ithi n a second outer shield . I n this mode , the outermost braid is connected to the " earth ground , " and the i nner shield is connected to the electronic system ground to yield overall high protection f�om i nterference . The effectiveness of shielded cable cannot be fully reali zed , how- ever, unless the cable is carefully and correctly term i nated . Sloppy dressing of the conductors , poor grou nding of the braided shield , and otherw ise improper termi nation of the cable can degrade the shield ing effectiveness by as much as 30 dB . Indeed , unsuspected causes of shielding degradation can oten be traced directly to incorrect RF grounding procedure s . In an otherwise sati sfactorily shielded syste m , RF curre nts are conducted along the cable shields and coupled directly into the system equipment from i nadequ atel y terminated connectors . This is espec ially evident at high frequenc ies\where it i s imperat ive that multiple grounding be employed to minimize RF currents along the shield . To term i nate a cable properly , the entire periphery of the braided shield must be grounded to a low-i mpedance reference . This will ensure re­ duction of RF currents to a minimum at the term i nation surface . Sol­ deri ng of the braid is discouraged in favor of using crimping rings or tapered cone compression methods as shown in Figures 4- 1 6 and 4- 1 7 . B ackshells are commonly used for added protection of the termi nat ion, but it is impotant that the metal compos ition of the backshell match that of the cable connector and conductors to preclude galvanic corrosion and shielding deterioration due to mati ng of dissimilar metal s .

21 Cable Identiication and Sp a c in g

Second only to proper selection , termi nation , and grounding of shielded cable is the importance of careful spac ing and routing of shipboard cables . In times past it may have been that the bundling and routing of w i res and cables throughout a ship was -

86

SHIPBOARD ELECTROMAGNEICS

Figure 4- 1 6 Proper Dre s s o f M e ta l l i c S h i e l d Over Tapered Compre s s i o n Cone

Figure 4- 1 7 Care fu l Gro u n d i n g and S e a l i n g of Cable at Deck E n t ry Po i n t s

SHIPB OARD ELECTROMAGNEIC INTERFERENCE

87

s i m p l y a matter o f re ac h i ng o n e po i n t from another b y t h e shonest . least compl icated route . Not so i n our modem wars h i ps . C a b l e s are appreci ated now as an i n tegral part of the s h i p ' s e l ectro n i c su bsys te m s . J u s t l y so , t h e practice of s h i pboard cab le rou t i n g i n v o l v e s i d e n t i i c at i o n , separat ion , and proper p l aceme n t . The best way to m i n i m i ze cou p l i n g be tween cab l e s i s to se parate t h e l i k e l y rad i ators a s w i d e l y a s pos s i b l e from t h e pot e n t i a l su sceptors and , ideal l y , to ori e n t t h e m at 90° to each other. I n i t i a l l y , s h i pboard cables m u st be segregated . ide n t i i e d , and tagged as to type and fu nction . The Navy has categorized cables by whether they repre s e n t probab l e rad i ators of i n te rfe re nce ( R - t y pe ) , or are pote n t i a l s u sce ptors of E M I ( S -types ) . The c a b l e s are then de s i g n ated accord i n g to u se as spec i i ed in Tab l e s 4- 1 0 and 4 - 1 1 . Th i s i n format ion i s subseq u e n t l y used t o determ i ne the necess ary cab l e - to-cab l e spac i n g t o pre c l ude E M I cou p l i n g ( i n accordance w i t h t h e methods ou t l i ned i n detai l i n [2 2]) . Actual naval s h i pboard i n c i dences have been doc u me n ted . for examp l e , w h e re cables carry i n g H F fre q u e n c i e s have coupled R F e nergy i n to power w i ri n g , and cab l e s t ran s m i t t i n g radar mod u l ator p u l se s have i nduced i nt e rfe ri n g s i g n a l s in rad i o control l i n es to degrade com m u n i ­ cations s i g n i icant l y . Adeq uate cab l e - to-cable spac i n g had bee n ne­ g l ected .

3 ) Cable Conduit Shielding- N aval

e n g i neeri n g prac t i c e i n general i s to

avoid the use of a n y e x tern a l s h i e l d i ng devices if at a l l pos s i b l e . Pre f­ e rab l y , s u fi c i e n t s h i e l d i n g bet w e e n R - and S - ty pe cables s h o u l d be ac h i e ved by j ud i c i o u s rou t i n g and spac i n g so that the need for add i t i o n a l s h i e l d i n g i s m i n i m i zed . Nevethe l e s s , there are i n stances w h e re sat i s factory s h i e l d i n g pro­ tec t i o n cannot be obtai ned by e i ther the cable ' s i n here n t s h i e l d i n g ma­ teri a l or by separation bet w e e n cab l e s . Th i s i s pan i c u l ar l y the case where cables are e x posed to i n te n se RF ields and w here an i ncrease i n s k i n depth conduction a t l o w frequenc i e s nece s s i tates t h e use o f t h i c ker s h ie l d i n g . For such events there i s no a l tern a t i v e e x cept to e n s h roud the cables w i t h i n metal condu i t s , a last re sort because condu i t s add much weight and cost . ( N ote t h at the use of so-c a l led armored cab l e , i . e . . cable j acketed by a l e x i b l e , hard , braided metal outer s he l l , has bee n d i sco n t i n ued i n s h i p board i n t a l l at i o n s bec ause i t i s a severe source of i n te rrnod u l a t i o n i nterfere nce and broadband ( R F arc i ng ) n o i se . ) The two types of p i pe cond u i t pec i i ed for h i pboard appl ication are rigid and l e x i b l e . As seen i n Figure 4 - 1 8 . these condu i t s are h i g h l y e ffective s h i e lds e v e n a t power freq u e n c ie s . e s pec i a l l y above 1 0 k H z .

88

SHIPBOARD ELECTROMAGNEICS

Table 4- 1 0 . R -Type Cable Categorie s

Cable Category

Category Description

Rl

S h i pboard cables that carry 60- H z power .

R2

S h i pboard cables that c arry 400 - H z power.

R3

A l l transmitting systems operating below 1 00 k H z .

R4

Tran smitting systems and tri ggeri ng c i rc u i ts operati n g above

R5

1 00 kHz and u s i n g RG-type coax ial cable s . Cables used to c arry audio signals whose max i m u m values ex­

R6

Cab l e s that carry 60- H z synchro signal s , 60- H z control s i g n a l s

ceed O . 1 vol t . Typical components are announc i n g c irc u i t s , ac recorders , loudspeakers , c a l l bel l s , and al arm bel l s . up t o 0 . 5 amp , a n d 60- H z indicator signal s .

NOTE : A n y 60- H z control signal over 0 . 5 amp must be c l a s s i ­ R7

ied i n t h e R 1 c ategory . Cab l e s that carry 400 - H z synchro signals , 400- H z control s i g ­ nal s up to 0 . 5 amp , a n d 400-Hz indicator signal s . NOTE : A n y 400- H z control s ignal over 0 . 5 amp m u st b e c l as­ s i i ed i n the R 2 category .

R8

Cables used to c arry d i g ital data .

R9

Cables that c ay dc .

Table 4-11. S -Type Cable Categorie s

Cable Category

Category Description

Sl

Rece i v i n g systems operati n g in the frequency band 1 0 k H z to 1 00 k H z .

S2 S3 S4

S ame a s S 1 except d i fferent type of cable . Rece i v i n g and v ideo systems operati n g above 1 00 k H z . Rece i v i n g systems operating below 1 0 k H z .

89

SHIPBOARD ELECTROMAGNEIC INTERFERENCE

FOR

+ 1 20

R I G I D M E TA L C O N D U I T A B O V E 5 k H z ,

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Figure 4· 1 8 Condu it Shielding Effectiveness

Navy spec i ications req uire that rigid conduit used for EM l shield­ ing be seamless stee l pipe i n accordance with M l L-T-20 1 5 7 and have 23 a w a l l thickness of not less than 0 . 1 20 inch . The conduit must be made shock- and v ibration-re s i stant by the use of rubber padded pipe hanger cushions and be grounded by means of bond straps . To absorb further shock and vibration , rigid conduit should be terminated with about 30 inches of lexible conduit at the entry to equ ipment enclosures , bulkhead stufing tube s , and h u l l ittings . Flexible conduits are used at frequenc ies below 1 00 k H z , where low­ level signal cables must be well shielded from strong magnetic ields . Type- I nonj acketed lexible conduit i s recommended where the cable be ing shie lded is not susceptible to EMl from currents lowing along the pipe , and where stray curre nts are min imal . But for extreme ly low­ level , low-frequency signal cables which would likely be susceptible to interfere nce induced from currents lowing on the condu i t , type - 2 rubber-j acketed lexible conduit i s preferred . The rubber jacket reduces the l i ke l i hood of curent low by i n s u l ating the conduit from inc idental grounding contacts . An example of lexible condu it prope rly in stalled i n a ship tops ide i s shown in Figure 4- 1 9 .

90

SHIPBOA RD ELECTROMA GNEICS

Figure 4- 1 9( a ) Fle x i b l e Conduit Shielding Topside Cable R u n s

Figure 4- 1 9( b ) Flexible Conduit S h ie l d i ng Topside C a b l e R u n s

SHIPBOARD ELECTROMA GNEIC INTERFERENCE

91

T o determ i n e t h e correct cond u i t s i ze , a general ru l e of t h u m b i s t h at i f a cable ' s outer d i ameter approac h e s 90 percent of a cond u i t ' s i n ner d i amete r , the next l arger s i ze cond u i t shou ld be selected . For s h i pboard app l i cation , nom i n a l s i ze s of cond u i t i n ner d i ameters range from '/4 _ i n c h up to 3 i nches . Rectang u l ar metal troughs or tru n k s are freque n t l y u sed i n l i eu of several i n d i v i dual cond u i t s to provide EM I protec t i o n for l arge groups of nested cables i n ship tops ide s . A s shown before ( a ) and after ( b ) i n Figure 4 - 20 , w h e n cables ru n n i ng u p a mast cannot be p l aced i n s i de a mast l e g or center pole , they are e n c l o sed w i t h i n an E M I tru n k .

Figure 4-20(a) Enc l o s i n g M ast Cable R u n s W i t h i n E M I Tru nk

92

SHIPB OA RD ELECTROMA GNEICS

Figure 4-20(b) Enclosing Mast Cable Runs Within EMI Trunk

In summary, the best methods to obtain suficient shipboard cable shield­ ing are choosing the correct cable from the requirements of MIL-C24640 and - 2464 3 , eliminating common mode grounds, and carefully

spacing between cable runs. EMC between the cables and suppression of EM! in the cables should be achieved by close adherence to estab­ lished naval installation procedures such as those illustrated in Figure 4 - 2 1 . Only when these practices cannot be followed adequately should

the addition of shielding conduit be considered.

£. RAM Shielding-The problem of relected energy was included in the

earlier discussion of shipboard EM! sources. It was pointed out that, due

to the complexity of a ship topside, the likelihood that radiated power will be unintentionally relected from one or more metallic surfaces or deck objects is high. Depending upon the radiation frequency, relections may be picked up as interference by such onboard receivers as navigation radars, search radars, missile tracking radars, weapons iring radars, TACAN, direction inder sets, and EW systems. The receptions appear as false targets or erroneous indications (i.e., bearings) which prompt inappropriate system reactions. Similarly, because of the congested topside environment and horizon-to-zenith 3 600 omnidirectional mission requirements, a high prob­

ability exists for direct path mainbeam or sidelobe RF coupling between shipboard emitters and sensors, resulting in severe interference and system degradation.

93

SHIPBOARD ELECTROMA GNEIC INTERFERENCE

To a large extent the EMI potential for specularly relected (and

multiply scattered) energy and direct coupled RF can be mitigated by

optimum placement of electromagnetic systems in the ship topside. (The engineering nature of topside design will be taken up later in this chapter.) Nevertheless, EMI due to relected and coupled radiation does occur,

oftentimes unpredicted and unexpected, during leet operations. In answer

to such occurrences, SEMCIP teams are dispatched to assist the ships in

relieving the problems. In many cases the best solution is to employ RAM for EMI shielding.

Radar energy absorbers are specially devised materials which, due

to their carefully contrived electromagnetic properties, have the ability to

radically attenuate RF radiation. These materials have been designed for

a wide variety of applications from 30 MHz through 1 00 GHz, but are

most practical and effective at microwave frequencies. The concept is not

new; experiments with RAM originated a half-century ago. Moreover, for

the past 30 years RAM has been used routinely by the British navy to

reduce false echoes from radar relections off ship masts and superstructure

by as much as 30 dB. Only in the last few years, however, has RAM

begun to gain widespread application by the US military for suppression of EMI and for radar cross-section (platform image) reduction.

There are two principal types of electromagnetic energy absorbers

used as RAM: narrowband resonant (tuned) attenuators and broader band

graded dielectric attenuators. Resonant-type RAM is preferred in general

for shipboard use because of its superior durability at sea and its compar­

ative thinness. Resonant absorbers have developed over the years from the early Salisbury screen, a simple free space spaced precisely one-quarter wavelength

(3R--ohm, thin, resistive sheet 1rom a c�nductive plane. Wave

t

energy impinging upon the Salisbury s reen is p rtially relected at the

screen surface and partially transmitted to he con uctive rear plane. Upon "'.

meeting the conductive plane, the transmitte

portion undergoes a series

-of multiple relections. Part of each relected vector is retransmitted outward, parallel to but 1 800 out of phase with the original surface relected

wave. As a result the vector sum of the multiple retransmissions is, in theory, equal to the original surface relected energy, but, being 1 800 out of phase, effectively cancels it so that the total relection is zero. Therefore,

the Salisbury screen, at a very narrow frequency where the spacing is exactly a quarter wavelength, seemingly absorbs all the resonant incident energy; hence the term absorbing material. In practice, complete cancel­

lation of the incident RF energy is never realized; nonetheless, attenuation of up to 30 d B (99.9% suppression) is achievable.

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3 2

"S-3IB8B-24A

LRA (P/�) P6 l P8

ONLY

LCTN fR 3' (PIS)

4-2I�t;k,CfcJ

LUG

I

C·

SEE NOTE I r �I � )16

IIIG !IIIiIlH,

I 4

S.

4.

).

I. JACKU

SIiAll III

klMO VIO IIV[-[IGI I T S INCII Ok AS

4

I ? )

I I'

.r'MMII« I At M I I-I-/IU

CIIMMI �C I At

NOlE

I ,I,) ,4 1,2. )

Mil -C-q l >/11

S P E CifiCATION

MATERIAL

CI[�

)16

(WI Itl PUNCtt£O

& UIIIS C O . , R A R I I A N , N J :

lI6

� � I -ijI l 6

(IILM« 10NGUI)

III

T O NGUl) SHALL

�IL IC I l l )

B[ INSPCCT[O AFHR COMPL (lIO N

10 PROPIRt'

U [ S I I R U N K O N L Y AfJ[R

1I1[ IUBING bONO SIRAP HAS B[[ N W [ L O ( O

l UliNG SI I A I L U[ AUO[O AS SIIOWN.

I N L i NGIt!.

U O N O S T kAPS I N S T ALL IO ON C L iMULR SArr T Y

L UW[R rn .

IAIL � M A Y AlSO kl{)UI R ( A St l G11i ItlCI(ASI

NilS AI[ �AIS[n O R

10 P R [ V l NI

S U C H AS ON L l r[ N(lS, bONO SIR APS TO U[ >rIGIlIIY L ONGlR JlI[ BONO SIRAP WlilN III[

R [QUII[U Uk[AKING

M A Y Ul

IN [[RIAIN C A S E S ,

W[ AIIIIR S[ AL .

I I I P L A C E S I N C [ H[AI r R O M W[L DING WOUL D L OOS[ N T H [ TUIIIIIG

SHAll

H[AI-SHRINKMLC

fOR A GAP II( l W[[N TH[ C AB L [ J A C K [ I AND WII[R[ wrAIIHRS(A L I IlG I� NOT A P P A R [ NI,

SL IGI I i L Y II [ NOING TH[ U O N O S I R AP AT [ACtI HRMI N A L LUG A N O L OOKING L UG SIlIOUO.

BY

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CR IMP IH[

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S I [ ll Ok AlUMINUM,

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MAiCll

L I IG MA I II I A L ,

( U L AI"

SSA-BJ l b

JI6

SSA-AliS (PUIICII[U 101lGUr)

(Il�

SST-j)tS (I'tIIHII [ D I O NGUl)

& Ol[ WURKS, C Ak S O N , C A.

L UG, ALUMINUM

L UL,

) 16

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III III [

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(UI AIlK 1011LlIl) lUG, ALUMINUM ill-JO�HJ-1J4 12-IUII III AU 11L4l 01 [, I Ol S 1l[ L lUG Iql- ',4/5 I

IUL,

IHOMA�

I Nt; PAk I IWMIlI RS, OR [QUA L :

TOU L S SIiAll

[UNUUCIORS ARC IlR l l,1I1 [UI'PC�.

l UG Mi l l lOIlO STRAP A �SIMIIL Y

[ N � I I R l 1111

IIiC JAUCHI) P O R I I O N o r

A P PkOI'� I AII

o A l lOW 1111 lIPOSLO C O P I'[k C O IIUUCIOR S A N D 1111 CAli [ 1 0 PIN(lkAII 1 0 IHC lUl l Illpitl 01 IlIl l k R[SPICIIVC CIIAMlIlR� I N Jl I [ lUG.

CAIiLI

I kXl -II�

OF

Itf AI- �II� I NKAlil I

Lllr., A IIJMI NUM

7

Wit 0I NG

CAlli I

NO II �:

liST PART

I

ITO. .0

o J

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�

J

�

:l

J : ]

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5

a

J

? � � ]

� l

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SHIPBOARD ELECTROMAGNETICS

lOX

b. Type II-A strap identical to type I except that one lug has a drilled or punched hole to accommodate

a

threaded stud or bolt. Type II straps are

used to bond equipment or devices that cannot be permanently ixed in place, so that only one end of the strap is welded, and the other is bolted down. Use of type II bonding must be kept to a minimum. See Figure 429 for details.

c. Type III-A lat, solid copper strap for use in topside areas or below decks for bonding such items as antenna tuners and couplers, equipment enclo­

sures, and cabinets. These straps are normally available in 3 - , 6-, 9-, and 12-inch lengths. See Figure 4-30 for details.

d. Type IV-A lat, braided copper strap for bonding sound-isolated mounts and for bonding electromagnetic shielding conduit aboard submarines.

Normally available in 3 - , 6-, 9-, and 12-inch lengths. See Figure 4 - 3 0 for

details.

Of the four types, the lat straps offer the l.��§�

Rfjlp����.�!.!

the braided

straps have the highest lexibility, and the wire cable straps are the least expen­ sive. However, the type of bond strap to be employed is selected more in

accordance with the particular situation or circumstance than by such factors as cost or lexibility.

4-2.2.3

Grounding Requirements

On a metal-hull ship the designated ground reference is the hull. All equipment racks, foundations, structures, and other large metal items are welded, brazed, or c1ass-C-bonded by a low-resistance connection to the hull to become, by extension, the same ground reference potential. The basic criterion for electrical protection and

�MI reduction aboard ship

is that all electrical and electronic equipment and workbenches must be grounded. Equipment installed on resilient mounts must be grounded by a third conductor in the power supply cable or bonded to ground as shown in Figure 4 - 3 1 . Equip­ ment not installed on resilient mounts is considered properly bonded by metal­ to-metal contact and installation bolts (i.e., c1ass-8-bonded). Slide-mounted or roller-mounted equipment must be grounded by a conductor within the equipment cable harness. If a ground conductor has not bee� prVided by the manufacturers

or installers, a lexible ground conductor must be in�taJ!eE ,betwe_

frame or chassls"'eencl Osureframe-at

dr.awer

ground potential. The ground con' uCtO -' slz e-n "e u al to '�i gr��ter _ than the size of o �e of the ac-pow er' ' ' on uctors sippiying power to the drawer equfp ment �

I

L

.

Figure 4-30 Type

I" MIN.

.,

WIDTH

-'

r

III

12" MAX. LENGTH

I I I BOND STRAP

TYPE

I v B ON D STRAP

MOUNT ING HOLES orA. TO SU I T

TYPE

MOUNT I NG HOLES orA. TO SU I T

and Type IV Bond Strap Details

>
HTELD GRUNDI%

l

S�[C"'CAT,o .

I ((,/ ,( IAl

I

OT( �

NUIII

I.

I'll �IIHOO OF IIILI IHillO ',�OU'OI'G 'PPLI[I I, ,I' "1 RlIRUF\! INIIOLlOIIO., •• 0 OPPLlII UNU

10 (Oll[1 .il •• ,

O'[RiLl "I[lO.

/.

PNIOR

10 OOlPI[1 IN\"lLOIiON. Ol l I'�[OO[O PHIl 1F

AND 1'1 ARIA WHII[ 1.[ OOAPI[I

1 Y [ T . A I 1 " 1 _ 1 � I I . L L H ( C " � O M I � I C S , I N C " r I l l M U I l I PI G S U R f A c r s C . " � )N , C A , " r O L A ; I I I f I " , OW 1 0 1 J A L l O R 1 1 1 1 G A ', n l ', I I A I L I I I C L f Mi l O 1 0 R R I G II I fI[ I A L A N D C O A I [ O W i l l i A N I I ' � ' 1 1 l C OM P UU II O O f M I L - I - n ) 6 1 P � I O R 10 I N S I Al L I " G GA > ' [ I , SPL I I

SU l I [

S[ � 1 [ 5 ,

I N ; I A l l . I I O Il S I I (J W N

" A 'I > [ A

I S

AS

O[ TA I L l O

IN

[ I M8

O'J b l - U I I � - O I I O ,

Figure 4-35 Waveguide Grounding

transmitter spaces for connection o f each radio transmitter cabinet enclosure. Similarly, antenna tuners and couplers are grounded to the keel ground plates, or, to keep cable lengths short, to the transmitter enclosures , For all other equipment and items, a size 1 A WG cable, connected to the ground plates or transmitter enclosures, is used as the main ground cable to which size 7 A W G branch ground cables are attached, as illustrated in Figure 4-3 6 . By use o f the

1 19

SHIPBOARD ELECTROMAGNEIC INTERFERENCE

ANTENNA TUNER O R COUPL E R

- - - - -

,

: - C� � : � I

V

A D R

I

MA S T

NONME TALL I C

S E E NOTE 3 ( T Y P I CAL )

METAL RACK SEE DE TA I L S HE E T 2

�

GROUND PLATE

LIST

I1(M NO

OF

MATERIAL

,ART

P L A TE

corPER

GPOI 1l0 N .

2

C ML[

OP(R

,IP A N O

)

CABL E

COPPER

NO .

1 1 0 AWG

NO . W

1

RE

NO .

0-

S Y M SOL

NOl [

S P [ Cl f l C A T ION

1

4

GROUND P L A T E

OO- C - 1 7 6

1

M I L - C - 2' 6' l

? )

M I L - ( - 2'60

2

AWG

COPPfR � T RMmf n 1 0 AWG

M I L - C - 2' b' l

l ) 4

L EGEND :

0-

.

S TPANOEO

) 4

�

E L E ( T R ON I (

E QU I r M E N T

E L E ( T R I C AL

E Q U I PM E N T O R M E T A L

I T E MS

NO I C S ·

I .

GROUND P L A T E S SIIALL

C O L O - R OL L E D , O X Y G E N - f R E E I NCH T H I C K A N D SHAL L

APPROX IM Af E L Y ONE - E I G HT H

PROV I DE

APPROX I MA T E L Y

AR E A E AC H S I D E

2.

B E L I GHT ,

(OPPER ,

ALL AS

1 6 SQUARE

BRANCH GROUND CABL E S TO S

I

fEET

or

TOTAL

SURfACE

or TH E K E E L .

IE S H A L L BE

NO .

NOT

S P E C i f i CAL L Y

IDENT I f i ED

7 AWG S T RA N D E D (OPPE R

l.

I N A C T UAL OI RE(TL Y

' .

THE

I NS I AL L A T I O N S ,

BRANCH

T O E A ( I I E QU I P M E N T

CABLES

CABL E

S I ZES

O E T A I L [ O HE R E I N A R E

f U L L - S I ZE

S H I PS

SUCH AS AN MSO OR

f OR

SMAL L E R SH I PS SIIAL L

CAB L E .

Figure 4-36(a) G round Syste m , Nonmetal l i c H u l l S h i ps

BE

MA y C O N N E C T

G R O U I I O E O N N E C T I ON

I [ PM I NA L

S P E C i f i E D fOR

AN MC M .

AS A P P R O I' R I A T E

S I Z I NG

120

SHIPBOARD ELECTROMAGNEICS

GR O U N O C A B L E

GRO U � I O P L A T E

GRO U N D P L AT E

( N OT E S 1 . 2 . AND

BOL T D E T A I L S

GRO U N D C A B L E

3)

G R O UND C A B L E

ME THOD O F PA S S I N G GROU N D B U S T H RO U GH

WAT E R T I H T BUL K H E A O S OR D E C KS ( NO T E S 2 A N D 3 )

IT[" NO.

L I ST OF MAT E R I A L PART

I

GOL T ,

2

,'I U T

JA �

(O��E P

1

!:iuT

HE:

CQPP[P

4

COPPEo

•A S H E P

CQPDE o

\

/A'HfP

P111�n' p

6

STU�

SPECIfiCATION

NOTE

I

2

tl O a s : I .

S I IE Of

THE

S U D S H ,L L

(O P P E R

G R GIJ'I O I IIG P L A T E T H P O U G H - W L l A r i D T H R O U G " ­ � T L E A S T E O U . L T H E S I Z E Of T H E A S S O C I � [ l

(·cL �

)

'J A S H E P , L O U ( O P P E R

�

T E P M l ii A L

'-.

�l [ � J £ , tIO'I ME T A L l l :

l tJG

H U D O f T H E G P O U 'I C l tt S P L A T [ : ,« 0 IJ G H - 6 0 L T S H A L L B E o P A I E O T O T H E ( O P P E R G R O J II O I ' C P L A T E .

,

(OPPER

? P O E ( T I O 'l S H A L L ( O P R O S I 'I [

1

OE

oE

P o O 'i I D E D

rOR

[ f f E C l S O f D A M P .000 ,

O f A IIor;�E , L L I ( S L E E 'I E ,

Figure 4-36(b) Ground Syste m , Nonmetallic Hull Ships

THE TH I S

S T U D AGA I " S T

THE PROTEC T I ON SHALL

SHIPB OARD ELECTROMAGNEIC INTERFERENCE

121

branch cable s , a l l equipment u sing e l ectric al power and a l l fue l tanks , water tanks , engines , engine control apparatu s , metal scree n s , ducts , and metal lic deck ite ms such as standing rigging , cranes , king posts , liferail s , and l adders are connected to the ship ground syste m .

4-2.2.4

R F Bonding Procedures for EMf Control

To precl ude the formation of non linear j unctions ( and thereby reduce the potential for h u l l -generated intermodu l ation interference ) , ship topsides should be kept as free as possib l e of all pinned , snap- linked , and chain - linked metal lic discontin uitie s . A l l metal -to-metal j oints must be c l ass-A -bonded , unless required to be removab l e . Further, the mating of dis simil ar metal s by bolting or riveting must be minimized . The j oining of a l u minum to steel should be accomp lished by welding using bimetal lic bonded j oints . Loose metal lic ite ms such as pipe s , cables , too l s , and portab l e rigging should not be stowed topside e xcept where absolutely nece ssary ( as in the case of anchor chains ) . I n an attempt to minimize the pos sibility o f non linear j u nction intermod­ u lation source s , a l l ships having six or more HF trans mitters must appl y the fol lowing control measures . ( S hips with less than six trans mitters are to suppre s s only those sources positive l y ide ntiied through onboard EMI testing . ) a . Metal lic w a l king rope s and hand safety rope s are not to be used on y ard­ arm s . In stead , non metal lic rails or a l l -we lded rail s are to be used . b . Rigging such as halyard downhau l s , ful l dress rigging , awning line s , life­ boat line s , and other simil ar lines are to be nonmetal lic . Metal lic standing rigging must be bonded to ground as shown in Fig ure 4-3 7 .

c . A l u minu m or a l l - welded stee l liferail s are to be used at a l l deck edge areas not requiring personnel access or c l e ar dec k . Where c l e ar deck edge is required , Kevlar nonmetal lic life lines must be in stal led . ( Ke v l ar is a reg­

istered trademark of E. l . Dupont DeNemours and Co . , I nc . ) Additiona l l y ,

access openings less than six feet wide must be protected with nonmetal lic

rope . d . Life and safety nets and net frame s , where determined to be a source of intermodul ation noise , must be fabric ated from nonmetal lic material ( e x ­

cept i n heat o r b l ast are a s ) o r bonded as shown i n Fig ure 4-3 8 . e . Portable l agstaffs , j ackstaffs , and stanchio n s , where determined to be a source of intermodul ation noise , must be either fabric ated from nonmetal lic material or bonded as shown in Fig ure 4-3 9 .

f . Metal lic aw ning rigging must be disassembled and stowed when the ship is under way , and aw ning stanchions , brace s , and spreaders must be non­ metal lic .

122

SHIPBOARD ELECTROMAGNEICS

T I PE I BONO STRAP I N S TA L L A T I O N HARDWARE

D E C K ( G R OU N D

PO T

E N T I AL )

L I ST OF MATERIAL

ITE" 00.

P&�T

I

C A BL E ,

WE L D I NG ,

2

U · BO L T A S S E M B L Y

HOT(

$ P ( C " ' t &T ' OH

T Y P E TRU 84

MIL·C·915/21

I I

NOTE S :

I .

B O N D I NG C AB L E S H A L L L E NGTH .

BE

'1 E A S U R E D AriD

CUT

TO P R O P E R

O N E E N D S H A L L B E EQU I P P E D W I T H A L U G T E R M I N AL

I NSTALLED THE

S M E AS

A TYPE

I

BOND S T R A P .

THE UPPER

FND SHALL BE ATTACHED TO THE W I R E · ROPE STAY BY C L E A N I NG B O T H C AB L E S A T PO I N T o r C O N T A C T A N D A P P L Y I NG M I L · T · 2 2 3 6 1 A N T I · S E I Z E COMPU N D T H E N C L M P I N G T H E C A 8 L E S B Y T H E M E T H O D S H OW N . OVERALL WEATHERSEAL ING SHALL

Figure 4-37 S tanding Rigging , Bonding

B E PROV I DE D A S S P E C l r l E D THE R E I N .

SHIPBOARD ELECTROMA GNEIC INTERFERENCE

1 23

L A S H I NG R O P E

MARG I N ROPE

A L T E R NA T E M E T H O O S NOT E S

J & 4

L IST

ITEM NO I

OF

MATERIAL

PART

UONIJ S T R A P ,

TYPE

2

ROND I NG C A D L E

J

U - U OL T

�

� WA GE �l Evf

S P E C I fi C A T ION

HOTE

I

2.1

CRES

CRES

4

2

NO T [ S :

I.

A T YP E NE T

I

D O N O S T R A P S H AL L

f R ME

I NCRr ASED

H I NG E .

THE

I N L E NGTH ,

T YPE

BE I

I N S T AL L E D A C R O S S E A C H BOND S T RAP MAY BE

I f NEEDED,

T O AL LOW

NETS

TO

RA I S E A N D L O W E R .

2.

SWAGE

SL E f V l

UuND I H G C A U L E

J.

AS

(OR S IM I LAR D E V I C E )

AN A L T E R NA T E M E T H O D ,

AROUND

THE

SHAL L

B E C R I MP E D

TO

Arm W E L D E O T O N E W f R AM E .

MARG I N R O P E ,

A C R E S U- BOL T M A Y B E

I N S T AL L E D

L A S H I N G R O P E , A N D BOND I NG

CAUL E .

4 .

AS A S E C O N D AL T [ R A T [ , MAY B E W l L D E D

THE

L UG O f A

T O A U - DOL T A N D

WHE R E N E T S A R E R E Q U I RE D f OR MA I N T E N A N C E , A T Y P E

Figure 4-38 Metal l ic Life and S afety Net s , B onding

TYPE

I

I N � T A L L E D AS

BOND S T R A P SHOWN .

TO BE R E MO V E O P E R I OD I C A L L Y I I B O N D S T R A P MA Y B E I N S T A L L E D .

i 24

SHiPB OA RD ELECTROMAGNEICS

LIST ITEM NO I

OF NO T £

PART 00 '1 0 ) T R A P , T Y P E

I I

fI O T E S . P R O P E R 'J E A I H [ R S ( A L I N G O f sOria S I R I, P S T U O T E R M I 111,L S H A L L OE P R O 'I I O E O A S S P E C i f i E D T H E R E I ll .

Figure 4-39 Metal l i c Flagstaff or J ackstaff, B o ndi ng

g . Meta l l i c i nc l i ned l adde rs must be grou nded as shown i n F i g ure 4-40 or rep l aced w i th l adde rs made of nonmeta l l i c materi a l . Metal l ic v e rt i c a l l ad­ de rs are cons ide red sati sfac tori l y grou nded when t i g h t l y secured bolts are used. C l i mber safety rai l s are cons ide red sati sfactori l y grou nded w h e n i n stal l ed w i t h we lded brackets . B rackets c l amped to l adde r ru n g s must u s e a type I I bond strap at t h e s e poi n t s , w i th the we lded e nd o f the bond strap attached to the h u l l struc ture and the detac hable e nd bol ted to the s afety rai l . h . Portable l i fera i l s are to be con structed of nonmet a l l ic material except i n heat o r b l ast areas . I.

Armored cables must not be u sed for new de sign s h i p s . On s h i p s where armored cable a l re ady e x i sts , the cable w i l l be re loc ated i n s ide the mast as shown i n Figure 4-4 1 or w i t h i n a w i reway e n c l o s u re as de picted Figure 4- 3 3 .

j . Expansion j o i n ts must be bo nded a s i l l u strated i n Figure 4-42 . k . T i l t i n g ante n n a pl atforms must be bonded as shown i n Figure 4-43 .

10

1 25

SHIPBOARD ELECTROMA GNEIC INTERFERENCE

'1

If' NO I

L IST

I

1

M A T E RIAL

OF

PART

I

1 1 1 ' 1 1 ', I k A I'

I

Uk

I II'

� P l C l f l C A T lO H

1 I

NO r [ 1

NO I I � I NU I NI O - I k l Au I A U O I W5

�I IAI. l

Ul

UUNUI O

I U GkllUND

U Y 1 1 1 1 I N > l A l I A I I O N 0 1 A U O N U � I R A P AC k O S " 1 1 1'1 I U O N O I U P A N D U N L UIl I I UM I' I N N I O MOU N I . S I II A I ' \ A k l P k l l I W k l ll W I l I R I 1 " [ I A I l U l k MUS I U [ 1'1 k I l I U I C A l l Y k - M l I � 1 U , I V I' [ I I U O N O S I k A P S S H A l l U [ PI l I L N I I A I

I I rU

I V PI

I N " A l l L Il

�1I 1 1' 11111 1

Ilk

I I

1l0ND S l k A P S

\ l k UC l l l k t

I ' k l l l ' l k W [ A I I I I W S I A l I N" �IIAI l

UL

IH[

HL

Wl l o r D

0 1111

L AU D l R

SIiAl L

A N Il HOl T l D

UI

UONU S I U AP S I IiU

10

I L R M I NA l

Pk l J � I U I U .

Figure 4-40 Metallic Inclined-Tread Ladders , Bonding

126

SHIPBOARD ELECTROMAGNEICS

C AB L E E X I T S f R OH HA S T A T

4 So OR AS APPROPR I AT E CABLE

S H I E L D G R O UND I N G D E V I C E

K I C K P I PE S OR S T U f f i NG

S H I E L OED CBL E

T UB E S

6, 7, , S

S E E F I GU R [ S -- U N S H I E L D E D C A B L E

--

20'

TYP .

.j

A D D O N C AS L E S H I E L D I N G ( CONDU I T )

6

HAS T B ANO I NG

C ABL E S

S E C T I ON A - A

S/S" : o

=:��-�

)- 7 / 1 6 "

6"

-� �

D I A TO SU I T

1- 1/4"

�

� T

0

-6 '

10 I I I I f ' �

2

COVI H

J

U Ol I ,

4

WA\ItI R , S I L · l I NC P L A I E D SPL I T

MI U S IL MI D S TL

MA l It

,

J/,," · I L U H C

. 0" LA\ K I I LAIlL E

I

M U U N I I NG b A H

H A NGI R

1 0 . 2 . PL

S [ L · Z I N C PL A I E D 2A M E X HD

H UlItlf R

�

4 U . � . PL .

I 8"

Ttl:

I MII -S-28 MIL-S· ln�

.2

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�

1

- 1/2"

[ 1� -

S/S"

HO l f S :

OOTE

1 .2

MI

I

C A B L E HANG E R

SPECifiCATION

1J(j l l ll l l H

2

- 1/2"

1 /4 "

L I S T O F MATERIAL

I

S

D I A T O S U I T , DR I L L

" 1� l -- 1 / 2 "

PA�T

DOUBL E R

P U N C H B A N D I NG HOL E S TO S U T

� U H T I NG BAR

I T [ .. 0.

�

8

2 �9l

.2

I.

I A UR I C A I I O N OE T A I L S A R E

I Y P I C AL

D I M E N S I ON S A N D M A Y B E

MU D l f J [ D T O S U I T O T H E R S H I P S AS R E QU I R E D .

2. J.

l OR A L U M I N U'� MA S T S .

C O V [ R S AND D O U B L E R P L A T E S S H A L L B E

f A U R I C A I E D f R OM A L U M I N U M MA H R I A L . NUMB E R

AND S PAC I NG Of C A B L E H A NG E R S W I L L

BE

D E T E RM I NE D

� Y CA U L [ R E QU I R EM E N T S ANO MAS T S I Z E .

· R · QOO

2 J

4

4.

S.

MOUN I I NG O A R S S H A L L B E Of MAS I .

I NS T A L L E D B Y WE L D I NG TO

E X H R NA L AC C E S S HOL E S A R E A R E L AR G E E NO U G H T O P E R M I T AND M A I N T E N A N C E .

Figure 4-4 1 M ast Cables Located Within M ast , Typical

NOT REQU I R E O I N T E R NA L

I NS I DE

I N MASTS THAT

CABLE

I N S T A L L A T I O NS

127

SHIPBOARD ELECTROMAGNEIC INTERFERENCE

H I N G E D OR F I X E D CO V E R

L IST

I T E MI

.0 I

l UaNa

) I HAP

PART

I YP[

OF

I

MATERIAL

I

I

SPE C I F I C A T I O N

I

JI

HOlE

2

NO T C S :

I .

UONO S I R A�S AT

2.

StiAL I

I N T I HVAl S Ilr

or

UC

IN � T A L l. l O A C H O S S ( x P A N � ION J O I N T �

AP�HOX I�A l l l '

,

r[er

AND LOCA T [ D ON

T II [ J O I N I NU l l X P OS t O T O I t I [ W l A I I I [ H .

TlI[

� I U[

I I I[

l C N e l t i or

T tiC

UOIIO S T R A P SHAL L

P l R � I T M A X I M UM l X C U H � I O N 0 1

IHl

ur

SUrf l C l l N T

TO

[ X � A N S ION JO I N T .

Figure 4-42 Expan s i on Joints , Bonding

I.

Large o r long portable metal l i c items o r equ ipment such as fog nozzle s , dav i t s , and person n e l stretchers stowed w i t h i n 50 feet of an H F antenna must be i n s u l ated from contact with the ship h u l l structure by i n s u l ated hangers , c l i ps , or brackets . I n s u l ating materi al may be weather-re s i stant , heat-shri nkable tape or tubi ng , rubber matti ng , plast ic s , epoxy , i be rglass , or other s i m i l ar materi a l s .

m . Masts , mast braces , k i ng posts , and s i m i l ar deck structures bol ted i n place must be grounded by type I bond straps spaced equal ly around each struc ­ ture as seen i n Figure 4-44 . Care i n preparing the s u rface for good bondi n g i s very i mportant . S u rface preparation for i n stal l ation of we lded or brazed bond straps ( type I and type I I ) and welded studs must be accompl i s hed b y cleaning t o bare metal the areas where bond strap l ugs are to be we lded , brazed , or bolted . Cleaned areas and all threaded hardw are must be coated w i th an antiseize compound prior to in­ stal l ation of bolted bond straps . B ond stra

��!�

i n stal l ation h ardware such as nuts bolts , washers , and studs ·· 2 � t�. For topside areas , moun t i n g _��� �o b� .e jJh_e! -inch o� 8-� a _ hard ware must b e corrosion-re s i stant steel except where a l u m i n u m studs are req u ired . In areas other than tops ide , the mounting hardw are ( e xcept studs) must

12X

SHIPBOARD ELECTROMA GNEICS

SEE OETAIL

"A"

V I EW A . A

HAND O P E RAH D

P L A T F O R M SH A F T B O N D S T RA P AT TACHME N T PO I N T ( A N T E N N A I N H O R I Z O N T A L PO S I T I O N , 90° ) S E E DE TA I L

"A "

BOND S T R A P GROUND A T T AC H M E N T PO I N T

p,-----------

�> , / I

"

B O N D S T R A P A T T A C HM E N T

4 5°

PO I N T

E L E C T R I C / H Y DR A U L I C O P E R A T E D Df T A I L

IT[WI 00. I

I

I

L I S T O F MATERIAL " �T U O N O S T R AP ,

T YP(

J

I

S P [ C I F ICAT ION

I

I

"OTE I

NO T l S :

I .

U O rw S T R A P L ( N G T I I A N D M( T H OO o r AL L OW l O R M A X I MUM

TRAV[L

or

I N S T AL L A T I O N S H AL L

A N T l NNA

T I L T r N G M ( C HA N I SM .

Figure 4-43 T i l t ing Antenna Mount s , B onding

( AN T E N N A I N

V E R T I C AL

"A"

PO S I T I O N )

129

SHIPBOARD ELECTROMAGNEIC INTERFERENCE

I fEM HO. 1

)

LIST

I HOND

OF

I

MAT ERIAL

PART I yP[

S T R AP

I

I

S P E C I F ICAnON

I I

HOlE 1 ,2

ND H S : 1 .

B O N D S T R A P � S H A L L O N L Y BE BOL T E D

2.

I N S T A L L E D ON MA S T S WH I C H A R E

I N PLACE .

B O N D S T R APS

SHAL L

BE

I NS TAL L E O

I N ACCORDANCE W I TH THE

fOL L OW I NG : MAS T 0I A M E T E R

NO .

20

I N C H E S O R L AR G E R

20

I NC H E S

LESS

T H AN

TO B

8

nOND S T RA P S

Of

BONO S T R A P S

I NC H E S

I NC i t E S SHALL

8E

E Q UA L L Y S P A C E D A R O U N D M AS T .

Figure 4-44 Metallic Masts , Bonding b e plated steel . Studs in other than topside areas m a y b e either alu minum or plated steel as appropriate . Methods of attaching unwelded bond straps are shown in Figure 4-45 . Bond straps are to be installed so as to permit immediate inspection and replacement , and mounted in such a m anner that vibration , expansion , contrac­ tion , or relative movement wil l not break or loosen the strap connection . In-

1 30

SHIPBOARD ELECTROMAGNETICS

G R O U N D PO T E N T I A L DR BO N D E D

I TE M

G R O U N O P O TE N T I A L OR BONDED

I TEM

M E TH O D 2

1

M E THOD

& NUT

BOL T

STUD & NUT

SEE

NOTE

2

GROUND P O T E N T I AL OR B O N D E D

I TEM

M E THOD 3 E X I S T I NG BOL T , S T U D OR T H R E A D E D H O L E

IT(M MO. 1 2

L I S T O F MATERIAL

I BO N D

PART STRAP

WASHE R

L O C KW A S H E R ,

4

NUT

ODL T

6

S T UD ,

I I

MOTE

SPECifiCATION I I I

OR

I 2

IV

T YPE .

F O R O O N D S T RA P A T T AC HM E N T S H A L L B E A C O L L A R

TO P E R M I T W E L D I N G ,

5

FF-N-836

5

MENTS Of M I L - S - 24 1 4 9 :

MIL-S-1222

5

MIL-S-24149

3

STEEL

SURFACES .

I N S T A L L A T I OllS

AL UI� I N U M S T U D S

4 . f O R BONO S T R A P 2 .

THE

STUDS ,

OR T H R E A D E D H O L E S MA Y

-

- TYPE _

P R O V I DE

BETWEEN

THE

T H R E A D E D H A R O WA R E ACCORDANCE W I TH

USED

SHAL L

TilE

BE

I I

TO

�ONO S T R A P

f O L L O W I N G R E QU I R E ­

T Y PE

V,

CLASS

IV,

4,

CLASS

CRES

1

P R E P A R E D AIIO S E AL E D

REQU I R E MENTS O F

5.5.2,

IN

5.5.1,

AND

5.5.4.

INS 1 ALLAT ION .

I ll S T A L L , T I O N P R OC E D U R E S

SHALL

BE

TYPE

TO

1/8" - IS

STUD S I ZE STEEL

S T UDS

TO

f O R A T T AC H M E N T FOR A T T AC H M E N T

SHALL CONfORM TO THE

NO T E S : E X I S T I N G OO L T S ,

STUDS

STUDS U S E D FOR

ALUM I N UM S T U D S

l .

S T U D S S HA L L C O R R E S P O N D

T H E I�AT I N G S U R f A C E ,

MS35425 , and

S H O U L D E R OR COL L A R

STUDS USED

A L UM l ilUM S U R f AC E S A N D S T E E L

F F - W - 84

SPL I T

1.

5

F F - W- 92

fLAT

J

S

TYPf

FOR B O L T E D BONO S T RAPS

fOR A C L E A N M E T A L - T O - M E TAL

BONO S T RAP AND THE

CONTACT

s.

f O R S H I PBOARD E X T ER I O R AND

APPL I C A T I ON S ,

I T E MS

5 SHALL O E CORROS I ON R E S I S T A N T S T E E L .

MAT I NG S U R F A C E .

Figure 4-45 Methods of Attaching Nonwe lded Bond Straps

2,

1, 4,

SHIPBOARD ELECTROMA GNEIC INTERFERENCE

131

stallation of bond straps must not interfere with the structural integrity of cabinets or enclosures , or weaken any item to w hich the strap is attached , or restrict the movement of hinged or movable items . Where con venient , existing bolts , studs , or threaded holes may be used for bond strap install ation . The lug ends of type I and type I I bond straps w hich have been welded in place must be weather-sealed by priming and painting the lugs and welded areas . The cable j ackets of these type bond straps do not require painting ; painting the j ackets , however , wil l not affect the bond strap performance . Type I I and type I I I bond straps installed on threaded studs or fastened by bolts must be weather-sealed by coating the lugs and associated hardware w ith M I L - S - S 1 7 3 3 sealing compound . A fter installation , painted areas affected are to be restored to the original paint i nish . Bond straps installed in areas other than topside do not req uire weather-sealing or painting . Antiseize compou nds used between metal surfaces to be bonded preserve grounding conductivi t y . These compounds main tain the q u ality of grounding by preventing ox idation or corrosion in the ground path . The compou nds are used on l y in areas where metal-to-metal contact through the compound can be main­ tained u nder pressure such as with threaded bol ting . A fter appl ication of the antiseiz i ng compou nd and attachment of the bond strap , the union must be sealed with M I L - S -45 I S0 sealing compound to prevent the anti seize material from melt i ng and run ning u nder high temperatures . Examples of potential topside non linear j unction intermodul ation interfer­ ence sources and RF bonding are show n in Figures 4-46 through 4- 5 1 .

4-2 .3

Nonmetallic Topside Material Techniques

In the previous section it was noted that two primary means for the reduction of h u l l -generated intermodulation interference are: ( I ) to replace potential me­ tal lic noise contributors with nonmetal lic items; and (2) to use insu l ation material for isolation of the offending source from the metallic hu l l . These methods are not new i n concept . Improved materials and install ation techniq ues have been implemented continually throughout the years , however , and are proving re­ mark ably effective in lessening intermodu l ation and broadband noise . Traditional naval lifelines made of metal are notorious generators of in­ termodul ation interference . Long , relatively free of deck obstructions , clasped to vertical metal posts , these lifelines act as natural parasitic anten na elements to in tercept RF energy such as HF transmissions prevalent aboard ship . The coupled energy is then conducted along the lines to terminal points of con nectors and tunbuckles , making and breaking contact at the stanchion hooks . The re­ sultant rapidly intermittent metallic contact creates arcing and intermodul ation EM ! . Furthermore , in addition to being generators of noise , lifel i nes in the ield of view of microwave an ten nas perturb the radiation patterns; those i n H F ields

SHIPB OARD ELECTROMAGNEICS

132

(a)

(b)

Figure 4-46 Typical S hipboard Unbonded Potential Noise Sources ( Intermittent

Metal Contact )

SHIPBOARD ELECTROMAGNEIC INTERFERENCE

133

(a )

(b)

Figure 4-47 Typical S h ipboard Unbonded Potential Noise Source ( I ntermittent

Metal Contact )

1 34

SHIPBOARD ELECTROMAGNEICS

(a )

(b)

Figure 4-48 Bond Straps Across Lifel i ne Connections

SHIPBOARD ELECTROMA GNETIC INTERFERENCE

(a )

(b)

Figure 4-49 Bonding of Rotatabl e Joints

135

SHIPB OARD ELECTR OMA GNEICS

136

(a )

(b)

Figure 4-50 Bonding of Ship Exhaust Stacks and Pipes

SHIPBOARD ELECTROMAGNEIC INTERFERENCE

(a)

(b)

Figure 4-5 1 Ineffective Bond Strap

137

1 38

SHIPB OA RD ELECTR OMA GNEICS

create RF burn hazard s to personnel; and all metal connectio n hardw are is subj e c t t o c o rrosion , thereby aggrav ating t h e production o f i n te rmodulation noise b y nonli near j u nction s . Because o f these unfo rtuante tendencies shipboard lifeline s have been a foc a l point o f E M ! e ngineering practic e s for many y e ars . Long-term appl ication and e v aluation of v ario u s material and design tech­ niques for imp roving life line EMC have been continuous . The e arliest expe ri­ me nts i n v olved the use of prestretched , double- braided nylo n , and , later , mylar­ type nonmetallic rope s . These alte rnat i v e s to meta l l ic lifelines w ere deemed un sati sfactory , however, as the materi al stretched and sagged . Glass -reinforce d p l astic l i nes seemed t o offer good promise , b u t t h e s e too proved inadequate after e x tended hardships of s h i pboard wear and tear . An ex ample of a dangerously worn iberglass l i fe l i ne that has suffe red severe abrasion at a stanchion J - hook i s pictured i n F i g u re 4-5 2 .

Figure 4-52 Won Nonmetallic Lifeline

SHIPBOARD ELECTROMAGNEIC INTERFERENCE

139

Contemporary plastics have been much improved i n recent years i n their resi stance to the marine en v i ronment , i n weight-to-strength ratio, and i n their low - stretch characteristic s . As a con seq uence, their serv ic ab i l ity has been w e l ­ comed , and Kev l ar l i felines current l y are bei ng employed a s the standardi zed 26 nonmetallic lifel ine for US naval sh ips . Note , however , that emphasis i s being g iven to reducing the use of l ifelines and guardli nes to a min imum aboard ship. I nstead , i xed lifera i l s of wel ded stee l or aluminum are used w herever practicab le . The present objective i s to: ( 1 ) in stall welded liferails ( i . e . , having no hinged or moving connection s ) i n a l l dec k areas except those requ iring remov able stanch ions such as at replen i s hme nt stations and safety net s ; ( 2 ) u se Kevlar nonmetal l i c l ifeli nes w here c lear dec k edges must be maintained ; and ( 3 ) u se polyester rope i n place of metal chain for short guard l i nes . I n such a manner all possi ble E M I sources normal l y created in shipboard liferails and lifeli nes are elimin ated . As mean s of reducing antenna radiation pattern disturbances , however, nonmetallic l iferails and lifelines have not fared so wel l . Recent stud ies have concluded that , rather than being tran sparent to microw aves , nonmetallic rai l s s u c h as those seen i n Figure 4 - 5 3 interac t w i t h electromagnetic energy as much or more than do metal rai l s of eq u i v alent size and form . I n fac t , i nd ications are that , with nonmetal lic ob structions , radar an ten na sidelobes are enhanced while 27 main beam level s are reduced . For this reason current tops ide des ign prac tices recommend that

�

�JoY�ve antennas be pl aced on a pedestal high enough to radi ate clearly over rai l i ngs , as shown in Figure 4-54 .

I

Figure 4-53 Nonmetallic Lifelines in A nten na Field of V iew

SHIPBOARD ELECTROMAGNEICS

140

Figure 4-54 Nonmetallic Lifelines Below Antenna Field of View

There are many other instances where, in addition to the special case of lifelines, nonmetallic materials are used to reduce the occurrence of shipboard intermodulation noise generation. For example, it is now commonplace to use nonmetallic guy wires, life nets, lag boxes, inclined ladders, stanchions, lag­ staffs, jackstaffs, and utility boxes. Moreover, several ever-present topside items such as fog nozzles, booms, davits, personnel stretchers, and pipes are isolated from the metal hull by insulated cradles and brackets. Examples of insulated devices are shown in Figure 4-55.

4-2.4

EMI Filtering Techniques Good design, maintenance, grounding, bonding, and shielding practices

quite often are still not suficient to prevent some forms of EM! from reaching and degrading the performance of shipboard electrical and electronic equipment. Of course these EM! control techniques should be diligently applied to reduce the potential sources of interference to a minimum. Yet, in spite of the above engineering procedures, conducted interference sometimes will ind a way to gain entrance. It is in such cases that ilter devices can help.

SHIPBOARD ELECTROMAGNEIC INTERFERENCE

(a)

(b) Figure

4-55 Insulated Brackets and Cradles

141

SHIPBOARD ELECTROMAGNElCS

142

4-2.4. J

Filter Classiication and Characteristics

Electronic ilters are generally either (1) the reactance circuit type that employs discrete resistor, capacitor, and inductor (RLC) components specially conigured to pass currents at certain frequencies and to block currents at other frequencies, or (2) the lossy line type that, rather than rerouting or relecting

unwanted signals, absorbs and dissipates them. Reactance ilters use series and parallel RLC combinations in familiar L, T, and pi networks. Using circuit resonance characteristics, these networks pre­ sent a high impedance to interference lowing in the desired signal path while shunting the interference to ground via a very low impedance branch. In contrast, lossy ilters are constructed of such materials as silver-coated ferrites that act strongly to attenuate undesired frequencies. In either case, the ilters are so designed to discriminate against unwanted signals and to inhibit their conduction in the path of the desired signal. Filters are incorporated by manufacturers as an integral part of equipment and systems to achieve speciied performance requirements; our main interest here, however, is after-the-fact ilter applications to rid ship systems of EMl known to have disrupted or degraded mission op­ erations. Filters normally are classiied as low pass, high pass, band pass, or band

rej ect, depending on the intended method of excluding interference frequencies (as functionally illustrated in Figure 4-56).Jo�_ pass ilter,s are most often used

in EMC and are usually available in pi networks consisting of a series inductor and two capacitors in a three-branch circuit.

28

--

Because they ordinarily are inseted in an active circuit in such a way that all circuit energy has to low through some part, ilters must accomplish their function without impairing normal operations. Ideally, there would be no adverse effect at all on the desired signal upon addition of the ilter, but in practice, a small amount of signal attenuation does occur. Therefore, one important measure of a ilter's quality is its

insertion loss;

i.e., how much it attenuates the desired

frequencies. Filter quality is determined also by how greatly it attenuates the undesired signals, and over what range of frequencies. If the ilter does not provide suficient restriction of undesired energy over the stopband of interest, it is simply not adequate to the purpose, no matter what its other merits. Having selected a ilter to achieve the desired EMI control, there are yet other characteristic features which must be considered. For example, voltage and current ratings of the ilter must be suficient to allow operation under all expected circuit requirements. Large voltage deviations and steep transient pulses have to be accounted for, as well as all environmental conditions that the ilter must withstand under prolonged usage. As part of the overall operating circuitry, adequate ilter ratings and characteristics are essential to ensuring high systems reliability.

143

SHIPBOARD ELECTROMAGNEIC INTERFERENCE

)

!!

21

1

1

z

Q

�

" � 5 -

� " u r .. " )

21 �I �I al I

� � c

..

� C :, z " �

AT TENUATES ALL

u z

FRE UENCIES � THIS SECTIN

•

) : �

) " ) )

z -

:

PASSA>

STOPBAND FREUENCY _ fc

FREQ UECY . PASSBND A. LOW PASS FILTER

B.

�

PASS

I

ASSES ALL

FREQUENCIES IN THIS SECTION

z

Q

� c J z " � � c

ALL FREQENCIES

N

THIS SECTON

1

I I I

I I

I

! �

I

c

I II I I I I :

fC I

fC

i

, � � C

4-2.4.2

�

n " z

'0 a� i

, U r .. , , J, 1C X �

c m , , c L)

Z

o

ATTEUATS LL FREQUENES N

•

fC

FREUECY D

!

�

" II ' , eX C L, z .. -

1' � ! f

T HIS S ECTN

fC I

2

�� 5�

� ..

STHD

, c L

THIS SECTIH

C BAND PAS S FILTER

Figure

n "

I -

FREUECY

HIGH PASS FILTER

2 •

BAND RE�CT FILTER

4-56 Functional Characteristics of Filters

Shipboard Filter Applications

A typical application of ilters is in shipboard powerline circuits: 400-Hz main power distribution sources are commonly used aboard naval ships, along with interference-susceptible loads. Good iltering is a must to prevent pickup and conduction of harmonic interferences generated by power supply rectiiers. M : ore signiicantly, many of the powerline EMI problems are associated with ground system currents lowing throughout myriad ship structures, including the hull, decks, framework, pipes, cable shields, conduits, equipment racks, and cabinets. Because of the variety of possible sources, these structure currents are complex and dificult to predict or measure. Nonlinear loads fed by the ship's primary power are frequently the source of ground currents. Variances in line­ to-ground impedances create unbalanced line-to-ground voltages, hence differ­ ences of potential between ground points, to set up structure currents.

SHIPBOARD ELECTROMAGNETICS

144

Harmonic product generation is another prevalent cause of ground current interference experienced in ship systems. Nonlinear loads such as solid-state rectiiers produce

a

high content of harmonics that then become part of the

structure currents. Because they originate in the ship power system, structure currents are low-frequency (i.e., 60-Hz and 400-Hz fundamentals) and, left uniltered, are a prime cause of performance degradation in low-frequency elec­ tronic equipment. Coursing through racks, cabinets, chasses, and cables, struc­ ture currents will make an unwelcome appearance when picked up by susceptible electronic circuitry. At the other end of the spectrum from low-frequency electrical power sources of EMI are sophisticated microelectronic devices that generate RF dis­ turbances. Digital switching in logic circuits, for example, create subtle inter­ ference signals containing harmonic components extending well up into hundreds 29 of megahertz. Coupled intenally to chassis terminals, the interference easily reaches interconnecting cables, which act as antennas to conduct and radiate the harmonic energy as stray EM!. In this manner the cables become emitter sources of interference from such seemingly innocuous digital devices as personal com­ puters, pinters, and modems. Methods to suppress interference from these sources must be concentrated on prevention of high-frequency currents ranging from 30 MHz to 1 GHz from lowing into the circuit wiring and onto the ground shields of attached cables. One highly effective means of controlling harmonic radiation from digital equipment is to use low pass ilters to block EMI curents at the cable connectors. Recent examples of ilters being employed routinely to preclude disruption of shipboard operations are those installed in the Central Control Station (CCS) lubrication oil pressure and level indicator monitoring systems of newer frigates and destroyers. In the frigates a single ilter is inserted between the pressure indicator transducer output cable and the transducer body. In the tank level indicating transducer, two ilter kits are used, one at the input and one at the output. In just these two types of CCS transducers aboard frigates there are 24 different ilter conigurations. Two samples are pictured in Figure 4-57. The two primary sources of EMI which cause interference to the transducers are: (1) electrical broadband noise, and (2) HF communications transmissions in the 2-30 MHz band. Electrical broadband noise is created by the continual making and breaking of electrical contacts in nearby equipment and is charac­ terized by high-intensity spurious products coupled onto the transducer cables. In the case of the HF transmissions, interference is coupled onto topside cables and conducted down to below-deck areas, where it is picked up by susceptible transducer cabling. In both cases the effect is to drive the transducer output signals into erroneous readings at the CCS, thereby causing false alarms.

145

SHIPBOARD ELECTROMAGNEIC INTERFERENCE

OUTPUT

INPUT

INPUT SIDE

OUTPUT SIDE

(CONNECTED TO TRANSDUCER)

(CONNECTED TO CABLE)

(a)

OUTPUT

INPUT INPUT SIDE

OUTPUT SIDE

(CONNECTED TO TRANSDUCER)

(CONNECTED TO CABLE)

(b)

Figure 4-57 Shipboard Pressure Transducer Monitoring System Filters

SHIPBOARD ELECTROMAGNEICS

J4n

By using reactive components to impede interfering energy in series with the desired dc signals and to shunt the unwanted energy to ground, these ilters prevent both the broadband electrical noise and HF interference from reaching the transducer circuits. An RLC double-pi network used in pressure transducer ilters on destroyers is depicted in Figure 4-58. Other everyday examples of ilters used to avoid or control EMl in ship­ board electromagnetic systems design are bandpass circuits of transmitter and receiver multicouplers; notch ilters in EW equipment to suppress the fundamental frequencies of continuous wave radars (termed notch ilters because they are extremely narowband rejection ·ilters used to exclude an unwanted fundamental frequency and to pass the rest of the band of interest); and band-pass-band­ reject ilters incorporated to protect IFF systems such as that illustrated in Figure 4-59.

I

C4

Rl

... . B -.._ MS3101E

MS3106E/

OUTPUT TO

MS3108R

CABLE

..

INPUT TO TRANSDUCER

PART

DESCRIPTION 0.02 MICROFARAD NPO DISC CAPACITOR

CI

0.01

C2

C3,C4,C5 ,C6 Ll, L2

MICROFARAD NPO

DISC

CAPACITOR

O.OOl 'IICROFAAD NPO DISC CAPACI TOR 6800 47

Rl

MICROHEN RY INDUCTOR

OHM,

l

/2

WATT METAL FILM RESISTOR

NOTES:

l.

Wire

2.

Emerson

3.

Tolerances should not

size 22

gau ge

stranded copper

& Cumming Po tt ing Compound

STYCAST

"

FREQUENCY (MHz)

#26S1, CATALYST #9 exceed

lO%

Figure 4-58 Pressure Transducer Schematic and Characteristics

••

SHIPBOARD ELECTROMAGNEIC INTERFERENCE

147

AHTENNA ",7 A,·1771U,.

AHTENNA &·6 ANTENNA e-,

TOPSIDE

J

O .N'INo I-I 0.'" tHIn NO 14

IFF AND RADAR REPAIR ROOM

Figure 4-59 Filters Used to Protect Shipboard IFF System

4-2.4.3

Filter Installation Precalions

In addition to judicious selection of the correct ilter to fulill the required need, proper installation is essential to achieve the desired effect. In most cases it is best to place the ilter in or on the apparatus that is generating the EMI; i.e., mount it at the source. It is important to establish as low as possible an RF impedance between the ilter casing and ground. Consequently, the methods used to mount a ilter become critical at high frequencies, where an improperly in­ stalled ilter can result in impedances to ground suficiently large to develop EMI voltages and to reduce the ilter effectiveness. To maintain optimum bonding to the ground plane structure, both the surface on which the ilter is to be mounted and the surface of the ilter itself must be unpainted and thoroughly cleaned. Mounting ears and studs must ensure irm and positive contact to establish and maintain an RF impedance as close to zero as possible. Adequate separation of input and output wiring is imperative, particularly at high frequencies, as radia­ tion from wires carrying interference signals can couple over directly to the output wires, circumventing the ilter. Additionally, where chassis wall mounting isolation is not feasible, shielded wire should be used to assure adequate isolation. Figure 4-60 depicts various methods of correctly installing powerline ilters.

148

SHIPBOARD ELECTROMAGNElCS

CABINET, PANEL OR CASE

C ABINET, P A N EL OR CASE

SPECIALLY ADDEO SHI ELD

POWER INTO EQUI PMENT

PWER

...

--_

POWER

LINE

:: w � J .

GOOD METAL-TO­ METAL CONTACT

POWER INTO EQUI PMENT

. \\\\\\\\\\

GOOD METAL­ TO-METAL I�_CONTACT

P OWER

CABINET, PANEL OR CASE

Figure

4-2.5

LINE

SPE CIALLY ADDEO SHI EL D

POWER INTO

";" EQUIPMENT

1���P�O�W�ER INTO I EQUIPMENT

4-60 Powerline Filter Installation Methods

EMI Blanking Techniques

A unique method often used aboard naval ships to prevent reception of high-power local interference is a form of time domain synchronization called electronic blanking. This technique originated in the early 1950s when it was learned that ship EW passive intercept receivers were experiencing severe in­ terference from onboard radar systems. Operating close by and simultaneously with the EW receivers, the radar transmitters emitted signals so intense that ordinary frequency domain practices such as iltering could not provide suficient receiver protection, nor could the problem be eased by more careful selection of installation locations so as to provide adequate isolation between source and victim. In the limited topside volume available, the radars and EW intercept receivers simply could not be separated widely enough to preclude high-power mutual coupling of EMI. It was evident that the only feasible alternative was to cut off, or "blank," the EW receiver at the moment of radar energy intercept. Since radar pulses are short in duration relative to the interval between pulse transmissions, there is adequate time to permit "look-through" of the EW receivers. By making use

149

SHIPBOARD ELECTROMAGNEIC INTERFERENCE

of energy supplied directly from the radar pulse circuitry to activate electronic gate switching, the Navy fabricated a blanking device to interrupt EW reception. The original 1953 experimental model, developed by the Naval Research Lab and named the AN/SLA-IO Blanker, is pictured in Figure 4-61. This ive-stage ilter using individual diode gates is the direct ancestor of highly sophisticated modem-day shipboard blanking equipment. In later design philosophy it was found more expedient to have a pretrigger signal derived from each interfering source feed into

a

central blanking unit.

The blanker generates and sends a blanking pulse to the victim receiver, inter­ rupting its operation in synchronization with the anticipated interference emis­ sion. Present technology favors blanking at the intermediate frequency stages of the intercept receiver, circumventing the need for several frequency selective devices. In this manner all interfering signals are blanked, irrespective of indi­ vidual frequency.

Figure

4·61 AN/SLA-I0 Original Experimental Model (1953) Blanker (Photo courtesy of Naval Research Lab)

There is, of course, a distinct disadvantage of blanking: It Interrupts, actually tuns off, the receiver system operation for the duration of the interfer­ ence. Carefully programmed synchronization minimizes the loss of reception (off-time) due to blanking intervals. Nevertheless, excessive blanking time can become a problem, particularly when a large number of onboard emitters cause various shipboard receive systems to be disrupted for a signiicant portion of their operation. As an actual example we will examine the case of a recent naval warship combat systems design (the case is representative, but by no means the severest). It will be seen that this blanking scheme involved complex electronic programming to ensure success. Beginning with the preliminary design phase of the ship, efforts to achieve maximum EMC included establishing an EMCAB. One of the principal purposes of the EMCAB was to evaluate the total ship environmental effects. Within the constraints of topside volume ("real estate"), mission requirements, and means of controlling EMI, the ship topside design was continually reined and assessed.

SHIPBOARD ELECTROMAGNEICS

150

Part of the EMCAB initiative included an analysis of each of the several emitter­ receiver combinations in terms of direct and indirect EMI coupling. The analysis considered the ongoing topside design, documented leet experiences, inspection board surveys, and EMl test and corrective action team repots. As a result of the analysis, solutions were identiied and action was taken to preclude EMl between the many emitter and receiver pairs. For some of the directly coupled interference paths, blanking was determined to be the only viable solution, even though it was conceded that blanking is ultimately undesirable because it inter­ rupts receiver performance. In accordance with the EMCAB assessment, a precise blanking plan was developed. The plan required that blanking be provided for ive onboard victim systems, including EW, air control radar, and telemetry data receivers. Eleven high-power radar and air navigation emitter sources, some with multiple oper­ ational modes and varying pulsewidths, would supply an aggregate of 15 pre­ trigger signals to the programmed input channels of a central blanker. The blanker system would determine which of the input pretriggers to combine into a series of blanking pulses for each of the ive outputs, and would deine the proper timing and duration for each output pulse. System off-time was also computed to summarize the effects of blanking for each of the victim receivers. A inal analysis of the blanking plan concluded that the effect of blanking on the performance of each of the ive receive systems was within acceptable tolerances. It was noted. however, that in the case of one EW system, the blanking cutoff time approached one-ifth of the overall receive operational time. It was felt that any further blanking might cause that system to suffer noticeable deg­ radation (a threshold compromise in that blanking prevents a more serious form of degradation). Blanking, the EMCAB acknowledged, is an EMI control method of last resort, to be applied only when no other solution is possible.

4-2.6

Topside Systems Arrangement Techniques It should be apparent to the reader by this time that shipboard EMC involves

many facets of systems engineering design, installation, maintenance, and EMl corrective practices. We have seen that achieving EMC requires an understanding of the shipboard EME with all its potential sources and victims of EM!. It requires a thoughtful management program and a vigorous EMI control plan. It requires thorough knowledge and proicient application of such EMl suppression techniques as shielding, grounding, bonding, iltering, and blanking either to prevent or to relieve system performance degradation. Notwithstanding all of the above, there is yet another crucial factor necessary to establish shipboard systems EMC. That factor, to be addressed now, is topside electromagnetic systems arrangement; i.e., the optimum placement in the ship topside of high­ power emitters and ultrasensitive sensors to ensure both EMC and the effective reduction of EM I.

SHIPBOARD ELECTROMAGNEIC INTERFERENCE

4-2.6.i

151

Antenna inteference Characteristics

Modem warships may have well over a hundred antennas. Simultaneously transmitting and receiving varied fons of information on frequencies as low as 10 kHz to above 30 GHz, each of the antennas is essential to the fulillment of mission requirements. Because of the constraints in available topside space, however, it is dificult to select suitable locations so that the antennas may perfon well. The intrinsic electromagnetic nature of antennas is itself a major cause of the problem. An antenna functions at its best when well isolated from any of its own kind, from any other electromagnetic devices, and from any objects nearby which may interfere with good performance. Unfortunately, isolation aboard ship is virtually impossible, interference is everywhere evident, and antennas are peculiarly sensitive to interference in many forms. Shipboard antenna interference may be categorized primarily as that due to blockage, coupling, RF emission, and high-level radiation. a. Blockage-When an antenna must be placed near a large object such as a mast, or potions of the superstructure, or other antennas, a corresponding sector of its intended coverage will be shadowed out. If the antenna is to be used for receiving, that blocked sector is unusable and is either sur­ rendered as such, or a second antenna is added to ill in the gap (comple­ mentary coverage). If instead the antenna is employed for transmitting, its radiated energy, unless prevented from doing so, will impinge upon the offending obstacle, causing relections and scattering (and quite likely coupling and reradiation), thereby distorting, perhaps signiicantly, the radiation patten. b. Coupling-If the nearby obstacle possesses certain electromagnetic char­ acteristics (for example if it is made of metal, or is another antenna), mutual coupling with this parasitic element will result, altering the imped­ ance as well as the patten. The effect on the system may be so great as to drastically reduce the antenna's utility. c. RF E m issions

-

Reception of undesired emissions is one of the most fre­

quently encountered forms of interference aboard naval vessels. It is the natural consequence of a relatively small platform crowded with so many radiators. Unwanted signals are generated on ship as hannonics, inter­ modulation products, noise spikes, and broadband noise, to be picked up by onboard sensors used for receiving distant, and oten much weaker, signals. d. High-Level R ad ia tion- In many instances, particularly on warships, high­ power radiators are required for carrying out ship missions. Often the emitters are microwave, posing biological hazards to personnel, but even at lower frequencies the high energy levels are a threat to onboard fuel and explosives. A common problem is that of HF transmitting antennas

SHIPBOARD ELECTROMAGNEICS

152

inducing currents in nearby metallic structures to cause RF bums to per­ sonnel coming in contact. Restricted locations must be allocated aboard ship in which to place these high-power emitters so as to minimize the dangers of RF radiation. Clearly, aboard naval ships, all the above conditions inherently must exist. There can be no true isolation of antennas. There will always be sources of interference close by which will adversely affect antenna (and, therefore, mis­ sion) performance. Consequently, it is this high potential for interference that the systems engineer must anticipate, and with which he must cope, during the design and integration process.

4-2.6.2

Preliminay Antenna Arrangement Considerations

The engineering problem facing the antenna systems designer is to place each antenna in the topside: (I) to provide good coverage, that is, to avoid blockage of the radiation patten; (2) to realize maximum intended range for each antenna's purpose of communications, or navigation, or radar target search, detection, acquiring, tracking, illumination, and weapon control; (3) to avoid being susceptible to EMI; and (4) to avoid being a source of EMI or a radiation hazard to ship personnel, ordnance, and fuel. Only the most careful thought in placing the antenna can produce a topside integration which effectively achieves all of these objectives. Shipboard antennas fall generally into one of three categories: 1. Omnidirectional antennas used mainly for communications, navigation,

and passive reception to satisfy the need of ships and aircraft to maneuver independently of each other and ixed radio stations.

2. Directional antennas used for transmitting and receiving spatially concen­ trated energy in one direction at a time, e.g., radar, weapons control, and SATCOM, to radiate to or obtain information from remote sources. 3. Directional antennas used to determine bearing of incident radiation; e.g., direction inding, navigation, and EW. To accommodate these three classes of antennas, four speciic approaches are taken: a. Broadband excitation of the masts and superstructure, as in the case of high-frequency, fan type, wire-rope antennas. b. Probe excitation of ship structures as with VLF tuner whips; e.g., LORAN and OMEGA. c. Tuned independent antennas such as 35-foot whips with base couplers. d. Directional, independent antennas and arays such as those used for radar and weapons control.

SHIPBOARD ELECTROMAGNEIC INTERFERENCE

4-2.6.3

153

The Topside Systems Design Team

Designing the topside of a modem naval ship is an exercise of compromises involving several engineering competitors. Each competitor justiiably seeks to protect a special interest and to design the best possible subsystem within that sphere of interest. At the sane time, however, each creates an impact, often severe, upon the other topside interests. Therefore, no competitor, whether weap­ ons, navigation, communication, helicopter operations (helo 0ps), or other, should be allowed to optimize at the express disregard of others. In the mid-1970s, it was recognized that a formalized procedure must be instituted to attain joint agreement among the several competing engineering elements represented in the design of a ship's topside. Accordingly, NA VSEA established the Topside Design Integration Engineering Team, or TDIET, to provide an engineering committee for new ship designs and for major moden­ izations and conversions. Regularly scheduled TDIET meetings bring together design specialists: principal engineers and architects from combat systems, hull structures, weapons arrangements, navigation, lighting, iring zone coverages, topside electromagnetics, mast design, stack exhaust dynamics, safety, and var­ ious other technical areas. The TDIET is responsible for developing a topside systems arrangement that satisies the ship performance requirements. The EMC systems integration engineer, as chairman of the TDIET, is charged with ensuring topside EMC, with EMI suppressed to the minimum effect possible. In this manner, the step­ by-step derivation of candidate topside arrangements, including rationale for trade-offs and iterations, is documented, and the resultant design substantiated. The challenge is to arrange each item in the ship's topside so that each will adequately meet its mission requirements, but in the early stages of design, the platform itself is generally undeined. Some relatively ixed boundaries, such as length and width of the proposed hull may be known, but not the height. Therefore, the topside volume is luid and undeined. For example, it could not be known whether there will be one mast or two. Moreover, whether a mast will be a self-supporting pole or tripod or quadripod depends upon the number, size, and weight of the antennas it must support, some of which are massive. Other factors, such as bridge clearance restrictions, also affect mast height. The quantity of antennas to be mast-mounted may in tum depend upon what restric­ tions are placed on deck locations; i.e., deck zones must be kept clear for the iring cf guns or the launching of missiles or the operating of helicopters, and for the replenishing of fuel or the handling of cargo. How high the antennas must be placed on the mast may be governed by such widely diverse factors as what radiation coverage must be provided, what weight and moment tolerances are allowed, and how much physical isolation from other antennas is required.

SHIPBOARD ELECTROMAGNEICS

154

It is evident from this single example that no pat of topside design can be done independently. Interaction with all other pats is imperative, and is, therefore, the rasion d' etre for the TDIET. The objective of the TDIET, as heat of the design process, is to reach agreeable compromises so that each topside element achieves satisfactory per­ formance individually with minimum degradation to (and from) all other topside elements. The result must be a totally integrated topside system working in complete harmony. Of course, the effort involves numerous trade-offs and iterations to arive at alternative topside arrangement options. Detailed analyses are performed to predict weapon and electromagnetic performance, and at the same time to min­ imize EMI and radiation hazards. To aid in the analyses, computer modeling is done to determine systems performance factors such as coverage, blockage, and range. Radiation hazard restrictions are determined and an EMI maxtrix is de­ veloped to project potential sources and victims for the TDIET to mitigate (see Chapter 3, Figure 3-1). Concurrently, to ensure and enforce EMC of the combat systems during the design process, the EMC ystem engineer also chairs the EMCAB. Everyday debilitating or annoying EMI problem

being experienced in the leet, and the

particular resolution applied. are fed back to the TDIET and EMCAB sessions. In this manner, the designers are able to avoid or corect deiciencies so that the EMI problems will not be perpetuated in the topside design.

4-2.6.4

HF AnrennQ System Integrtion

Given the proposed outlines of a new hull, the antenna designer is con­ cerned immediately with the interrelationships of major topside items: the height and hape of the superstructure. placement of the deck weapon systems, location and form of the stacks. quantity and physical structure of the masts, and available installation space for antennas. At the early stages of topside design, none of the above are fixed. Placement of large, high-power HF antennas on deck will affect performance of the weapons. and vice versa. The quantity and weight of antennas poposed for mast mounting may determine the number of masts and will certainly inluence the shape and height of any mast. Height and geometry of the superstructure above the main deck may inluence greatly the radiation characteristics and impedance of certain antennas. Thus, each item affects the location and performance of the other . In the beginning, only gross arrangements can be suggested with alternatives proposed as options. An obvious irst step in the preliminary systems integration is a serious attempt to reduce the total number of antennas required. The most likely candidate is HF communications. Broadbanding and multicoupling are now used routinely

SHIPBOARD ELECTROMAGNEIC INTERFERENCE

155

to reduce the overall number of shipboard HF antennas. By dividing the HF band into three overlapping segments, namely

2-6, 4-12,

and 10-30 MHz, and

by using broadbanded antennas (e.g., wire-rope fans, trussed or twin whips), several transmitters or receivers may be operated into a single antenna. Not only is the number of antennas reduced but, since multicouplers are also iltering devices, electromagnetic interaction is lessened. a. HF Antenna Scale Modeling-Using an antenna modeling range, scaled models of the ship with variable antenna conigurations are subjected to carefully controlled measurements to determine the feasibility of the pro­ posed arrangement or to recommend the best alternative. These models are usually s scale, are made of sheet brass (see Figures

4-62

and 4-(3)� .

and inlude all topside structural elements inluencing the HF antenna characteristIcs. Base

on tests made on the range, c anges to tie model's

. topside may be made quickly and easily to expedite the HF antenna systems design. The emphasis during modeling is on design of the broadband HF transmitting antennas, normally one each to cover the

2-6, 4-12,

and 10-

30 MHz frequency bands for each ship. The main objective is to provide, and integrate into the ship hull, HF communications antennas with eficient, omnidirectional radiation characteristics by attaining a 3: I VSWR through­ out the entire frequency band. Such eficiency is achieved by the physical form and resultant topside placement of the antenna itself, and by exacting calculations to derive the inductive-capacitive L-, T-, or pi-type matching network inserted in the transmission line at the antenna feedpoint for maximum transfer of energy. Through modeling experimentation, test, and analysis, the component values of the variable inductors and capacitors and their coniguration in the matching network are so accurate that only inal tuning adjustments need be done at the time of installation in the actual shipboard environment. The antenna range, with its rotating lead sheet turntable simulating the sea "ground," is used also during modeling for radiation pattern measure­ ments. Polar plots are made at speciied band frequencies and varying elevation angle cuts over 360 degrees of azimuth. The resulting plots (see Figure 4-64) graphically illustrate the degree of coverage along with pattern nulls and perturbations of the proposed ideally omnidirectional antenna for a particular frequency and elevation radiation angle. Full-scale measure­ ments at sea have over the years conclusively validated the scale modeling results. The brass modeling design and testing of broadband HF receiving antennas is less demanding than for the transmitting antennas. Naval HF rec�ivi). antennas are not required to be highly eficient.

0- theO'trary,

it is

156

SHIPBOARD ELECTROMAGNEICS

Figure 4-62 Scaled Brass Ship Model on Antenna Range

Figure 4-63 Scaled Brass Model Topside Antennas

I I ,

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POINT PLOT REPRESENTS HORIZONTA� POARITY

10 270,

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FREQUENCY

Figure 4-64 Modeled HF Broadband Antenna Radiation Pattern Plots

UARTER· WAVE MONOPOLE

IIT IS 45 dB RELATIVE TO A

UARR·WAVE MONOPOLE

dB REATIVE TO A

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/58

SHIPBOARD ELECTROMAGNETICS

desirable that they be ineficient to the extent of just matching the threshold of the external minimum atmospheric noise level to the receiver system

internal noise. The receiving antenna is thus made as insensitive as possible to locally generated high-level interference of the shipboard environment, while still being an effective receptor for its intended purpose. The primary aim of the modeling engineer then is to choose locations with good all­ around reception while placing the receiving antenna on the ship as far as possible from the HF transmitting antennas to provide maximum isolation and electromagnetic decoupling. Additional RF protection to the receiving system is afforded by employing HF receiving multicouplers having highly selective ilter networks. b. Computer Modeling-By the mid-1980s, antenna systems design and in­ tegration engineers began making good use of computer-aided graphics to provide visual assessments of ship antenna and weapons placement rapidly. The designer is quickly able to determine the validity of varying options selected in topside siting. The ship hull is displayed in modular, three­ dimensional form (see Figure 4-65) and, when a particular location for an antenna or weapon is chosen, polar and rectangular plots graphically show

Figure 4-65 Computer-Modeled 3-Dimensional Modular Ship Hull Fom

SHIPBOARD ELECTROMAGNEIC INTERFERENCE

159

the radiation coverage or iring zone coverage available. Likewise, block­ age due to superstructure and deck obstacles is immediately evident from these computerized plots (see Figures 4-66 and 4-67). Using computer programs in this manner, the systems designer is readily able to illustrate those arrangements which are viable for futher considerations. From there, additional computer-aided techniques are used to predict system

perfor­

mance in terms of range, frequency, power, gain, and expected EMI to and from each emitter and sensor.

FrD

M L OAGE IOlUCLlTURI:

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DATi: PILI: RUN:

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150

165

Figure 4-66 Computer-Modeled Sensor Coverage (Blockage) Polar Plot

SHIPBOARD ELECTROMAGNEICS

160

OM OICL COVEAGE NOYENCLATURE:

lK-16

704- 1

TIN:

X, Y,Z:

1100-"

(148,,125,788) 93A2 la-NOV-84

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Figure 4-67 Computer-Modeled Sensor Coverage (Blockage) Rectangular Plot

4-2.6.5

EMC Consideations

Once a fairly firm complement of the number and types of antennas required is obtained, the next step in the topside design and integration process is to begin tentative placement of antennas and to anticipate the impact that the arrangement will have in terms of overall ship's predicted performance potential, EMI, and RADHAZ. In fact, it is this competition with other systems (and structures) that is most dificult to resolve. A irst ap roach that might come to mind is to locate all a�tennas as high as pos s ible, in the ar, for omnidirectional tra ;s � n _ !n.i!eception. �he masts and ardaf11s would seem fhe best choice. Unfotu­ nately, as seen in Figure 4-68, there are problems with this choice: a. Communications engineers, weapons control engineers, radar engineers, navigation-aid engineers, and EW engineers all have the same hopes.

SHIPBOARD ELECTROMAGNEIC INTERFERENCE

161

b. It is undesirable to collocate transmitting and - receivin antennas in the o same frequency band ; ne mode, ela;er tra�smit or re� �i� h;s to be ' placed elsewhere. l ormaly , the transmitti�g �;t�nnas are inst�iled in th� VICInity 0 e 'transmitter equipment room in order to minimize cable length attenuation losses. ) c. Some antennas, particularly transmitting antennas in the low portion of the HF band, do not function well when high above the water, their radiation pattens being apt to split up, forming multiple lobes in the elevation plane. To compound the problem even further, the yardarm and masts are used also to support lag halyards (which frequently become entangled in the antennas), commissioning pennants, navigation lights, and wind speed indicators. As a result, only antennas that absolutely require such locations can be mast-mounted. For example, air-to-ground UHF communications antennas, TA­ CAN, and direction-i� ing antennas are instal ed hig� abo��..�� sea to_$�� t�e maximum possible line-of-sight raD�d to h ve.an az'IluthaLIadj�jiol pattern �hlc is as nearly circulii- s sible. For large, heavy antennas, other locations must be sought, and competition for real estate begins to get quite dificult: On any ship there are areas which are immediately eliminated; e. g., helicopter operation areas, vertical replenishment zones, gun arc-of-ire zones, missile launching zones, cargo and boat handling zones, and visual navigation zones. Additionally, antennas should not be installed on stacks or next to fuel handling areas or ordnance stowage areas.

Figure 4-68 Ship Mast Antenna Congestion

SHIPBOARD ELECTROMAGNEICS

162

Isolation between antennas is maximized to the greatest extent possible. Separation of communications receiving antennas from high-power transmitting antennas is necessary to prevent overloading the receivers and the generation of intermodulation products within the receivers. Isolation not adequately afforded by physical separation is compensated by frequency separation, iltering, and blanking. It is also advisable, and in some cases essential, that isolation be provided between antennas of different functions; e.g., communications and radars, or search radar-to-navigation radar. A typical case is the requirement for UHF satellite communication antennas to be located well away from ship-to­ ship UHF transmitting antennas. 4-2.6.6

Candidate Antenna Systems Arrangements

As a result of working closely with all the various engineering participants-hull, machinery, deck arrangements, weapons arrangements, and electrical-candidate topside conigurations are proposed. The options fulill each of the requirements to the greatest extent possible; it is, however, recognized that no single solution is capable of meeting all requirements. Trade-off studies determine those options most nearly meeting requirements, with the risks inherent in selection of each. Recommendations are made, with documented rationale for the selection, including the identiication of any risks and deiciencies of the resultant system. 4-2.6.7

Post-Design Phase

During both the shipbuilding and the active leet life of the ship, revisions are made ranging from simple additions of platforms and structural reinforce­ ments to major changes in ship equipment complement. Such modiications more often than not will affect antenna characteristics, usually adversely. Examples of topside changes which may seriously degrade antenna per­ formance include addition of deck houses; extensions to bridge wings; modi­ ications of mast and yardarm conigurations; additions, deletions, or relocations of antennas; and changes in weapons systems. Since each antenna has been tailored to its speciic environment, such alterations may have a dramatic effect upon topside EMC. How well the total integrated shipboard electromagnetic system will func­ tion in the support of ship missions is only determined when put to the test of actual operations. At that time the quality of the overall topside design, including the initial planning, the model range studies, the EMC analyses, the coordinated iterative efforts to reach compromises, and the EM performance assessments, in derivation of the topside systems arrangement will be evident.

SHIPBOARD ELECTROMAGNEIC INTERFERENCE

4-2.7

163

TEMPEST Electromagnetics

This chapter would not be complete without some consideration being given to a peculiar type of electromagnetic concen where, rather than noise interference, undesired emission of intelligence information must be entirely suppressed. The highly specialized discipline to accomplish this suppression is known by the short name TEMPEST. Not an �cronY}J1, TEMPEST is an un­ classiied term for the detection, evaluation, and control of conducted and radiated signals emanating from communicationand data proc�s�ing equipme�t. Th e ' c '' being processed by the system eqUipment. The techniques us�d in TEMPEST p�Wices to s�ess_�ec:im�g�tic emissions a� ge1e;ally ��al�n! to !hose used in the reduction of EMI; viz, shielding, grounding, bonding, iltering, and ' signal isolation. Thereore, the subje -is of lOtereS -as an integral part o 'bOard electromagnetics. Because of their classiied nature, however, the details of TEMPEST test procedures, design techniques, and emission levels cannot be discussed here. Only a supericial overview can be presented for introductory purposes. The Chief of Naval Operations document that implements national policy on the control of compromising emanations for facilities, systems, and equipment used to process classiied information is OPNA V Instruction C551O. 93. The Department of the Navy Supplement to DOD Directive S5200. 17 addresses speciic physical security criteria applicable to shipboard. M IL-STD-1680 trans­ lates these DOD instructions into installation criteria relevant to shipboard equip­ ment and systems. Known as TEMPEST installation criteria, speciic measures are required to minimize the possibility of compromising electromagnetic em-

=�:':��:: ; ���'' 'e strict protective control. Red zones are ;

physicall

s nauthorized access. Obviously the encyption d�vice s it elf is a Red/Black interface unit, and extraordinary precautions must be ex­ ercised to control access to Red intelligence information. These precautions include utmost control of conducted and radiated emissions. Unfortunately, a compromising path or source may be very subtle, requiring great effort to detect or suppress. Isolation is the primary objective. lsola!ion ..�we�n Red and Black in­ formation must be complete at every level in design .� ui ment, and system. The most notable �o bDs are po� sU2ply_syste�s and� e ov� l

SHIPBOARD ELECTROMAGNEICS

164

grounding scheme; so it is these that require most attention. Additionalry, Red and Black cabling must be segregated and adequately shielded. Where feasible, iber-optic cables are used to provide maximum isolation and energy decoupling. Other techniques app ied to achieve suficient isolation include: (1) iltering ot Black lines that penetrate Red areas; (2) maintaining separation of Red and Black equipment racks; (3) separating control cables from power cables within equip­ ment racks; (4) providing 3600 bonding of cable shielding at the backshell; and (5) using double shielded outer braid on cables. These are but a few of the generalized practices used in TEMPEST en­ gineering. It can be seen that they exactly parallel those of shipboard EMI control. For speciic technical details of TEMPEST, readers with an established need to know should consult the MIL-STD-1680 installation criteria document. REFERENCES I.

2. 3.

•

"Electromagnetic Compatibility Program within the Department of the Navy, " OPNAVINST 2410.31 D, Ofice of the Chief of Naval Operations, Washington, DC, 6 August 1984, p. I. B. E. Keiser, Principles of Electromagnetic Compatibility, Artech House, Dedham, MA, 1983, p. 35. Electromagnetic Compatibility Installation and Maintenance Theory and

NAVSEA STD-407-5291779, Department of the Navy, Puget Sound Naval Shipyard, Bremeton, WA, March 1981, p. 2-2. R. V. Carstensen, Electromagnetic Inteference Control in Boats and Ships, Don White Consultants, Gainesville, VA, 1981, p. 1.3. Keiser, op. cit., p. 35. "Electromagnetic Interference Reduction, " Electronics Installation and Maintenance Book, NS 0967-LP-000-0150, Department of the Navy, Washington, DC, June 1972, p. 8-1. G. C. Salisbury, "Analysis of Very High Order Frequency Components of the Intermodulation Interference Spectrum," Report TR 1852, Depart­ ment of the Navy, Naval Electronics Laboratory Center, San Diego, CA, 26 October 1972, p. 5. Practice,

'.

5. 6.

7.

8.

The Electronics Material Oicer's Guide to Shipboard Electromagnetic

NAVSEA STD-407-5287556, Department of the Navy, August 1980, p. 2-9. Keiser, op. cit., p. 113. Carstensen, op. cit., p. 5.25. M. Smith and F. O'Conner, "New Shielding Potential with Metallic Fab­ ric," Interference Technology Engineers Master, Robar Industries, West Conshohocken, PA, 1985, p. 334. C. D. Storms, "EMIIRFI Shielding For Structural Foam Business Machine Components," Interference Technology Engineers Master, Robar Indus-

Interference Control,

9. 10. I I.

12.

SHIPBOARD ELECTROMAGNEIC INTERFERENCE

165

tries, West Conshohocken, PA. 1983, p. 53. 13.

Ibid., p. 54.

14.

Ibid., p. 54.

15.

G. A. Kulik, "Electroless Plating For EMI Shielding," Interference Tech­ nology Engineers Master, Robar Industries, West Conshohocken. PA.

1984, pp. 180-188. 16.

J. H. Ling, "EMI Shielding: Selection of Materials for Conductive Coat­ ings," Interference Technology Engineers Master, Robar Industries. West Conshohocken, PA, 1986. p. 186.

17.

W. Gindrup and R. R. Vinson, "A Logical Approach To EMI Shielding Materials," Inteference Technology Engineers Master. Robar Industries. West Conshohocken, PA, 1986, p. 184.

18.

Electromagnetic Compatibility Design Guide for Avionics and Related Ground Support Equipment, NAVAIR ADII15, Department of the Navy.

Washington, DC, July 1980. p. 5-30.

9.

G. Olenskyj, "Selection of EMI Shielding Gasket Materials," Interference Technology Engineers Master. Robar Industries, West Conshohocken. PA,

1984, p. 62. 20.

E. J. Carlson and W. S. V. Tzeng, "Galvanic Corosion Measurements On Various EMI-Gasketed Joints," Interference Technology Engineers Master, Robar Industries, West Conshohocken, PA, 1985, p. 312.

21.

/2.

Olenskyj, op. cit., p. 62. Handbook of Shipboard Electromagnetic Shielding Practices, NA VSEA

S9407-AB-HBK-010, Department of the Navy, Washington, DC. 1982. pp. 6-1 through 6-26. 23.

Ibid., p. 2- \.

24.

Hull Generated Intermodulation Interference Reduction Techniques for Forces Aloat, NS0967-266-10 10, Department of the Navy, Washington,

15.

DC, October 1971, p. 31. Shipboard Bonding. Grounding, and Other Techniques for Electromag­ netic Compatibility and Safety. MIL-STD-13100. Department of the Navy.

8 Februay 1979. 26.

R. S. Key, "Kevlar Lifelines," Deckplate. Vol. 6. No.4. July-Aug 1986. p. 17.

27.

D. F. Herick and M. S. Geoghegan, "The Effects of Life Rails on Radar Antenna Performance," Final Technical Report Task B029, Naval Surface Weapons Center, Dahlgren. Virginia (Circa 1985). A. W. Laine. Ph.D., "EMI Filters." Interference Master. Robar Industries, West Con hohocken. PA. 1985. p. 244.

JJ.

H. Hazzard and R. Kiefer, "EMI Reduction Using Capacitive Filter Con­ nectors." Interference Technology Engineers Master. Robar Indu trie .. West Conshohocken. PA, 1986. p. 240.

Chapter 5 Shipboard Electromagnetic Radiation Hazards (EMR) 5-0

THE RADIATION HAZARDS PROBLEM IN GENERAL

In recent years there has been a rapidly increasing public awareness of potential biological ham resulting from unwitting exposure to electromagnetic radiation. The problem has been especially dramatized in the case of microwave frequencies. Although public anxiety at times has been fostered by alarming reports in the media, the desire for clear and forthright information on the subject of RADHAZ neverthe less is j ustiied. We live in an electronic age, immersed in an electronic environment, and, while reaping the material beneits, we must be kept well informed of possible adverse consequences. Afl uent societies such as ours make use of virtually the entire electro­ magnetic spectrum from powerline energy frequencies to x-ray frequencies. The dominant high power usages for the civil population are electrical utilities and radio and television broadcasting. Numerous other heavy demands on the spec­ trum incl ude: (l) commercial airlines air-to-ground communication, navigation , and air trafic control radar; (2) law enforcement communications and radar surveil l ance; (3) commercial shipping, recreational l ying, and boating com­ munications and navigation radar; (4) medical specialized equipment for detection and treatment of disease; and, perhaps of greatest signiicance, (5 ) military radar and communication networks that dot and web the landscape from shore to shore. Even in the home such radiating devices as microwave ovens and automatic remotely controlled garage-door openers are now commonplace. As a resu lt , the general public throughout the industrialized world is quietly (and invisibly) being subj ected to continuous, increasing levels of electromagnetic radiation (EM R ). The question is, do these radiation exposure levels pose a threat to health? More importantly to our subj ect matter, how do we preclude exposure to EMR from being hamful to shipboard personnel? 167

lX

SHIPBOARD tL:'-CTROMAG

EICS

Our special i nterest here is i n the biological effects of non i o n i z i n g R F radiation-th at i s , rad iat ion other than that from , say , nuclear fu s ion reaction s , weapons conta i n i ng nuc lear warheads, or from inte nti onal med ical use of such ionized energy as u l tra v i o let and x ray. In d i s t i ng u i s h i ng between ion i z i ng and non i onizi ng radiat ion it should be poi nted out that , in the pot ion of the e lec­ tro magnet ic spectru m above the i n frared segment , energy from u l traviolet rays , x rays , and gam ma rays reacts with l i v i ng matter with such force as to i o n i ze , or electrical l y charge , organ ic molecu les. In so doi ng , chemical bonds are de­ st ructed , cau s i ng damage to t i ssue and poss i b l e di sruption of biological fu nc ­ tions. I Non ion i z i n g radiation , on the other hand at the microwave and lower freque nc ies lacks s u fi c ient i ntensity and conce n tration of energy to i on i ze organic material , but st i l l may i nteract i n an -e l u s i ve , i ndirect man ner. The effects o r rad iat ion for our pupose therefore w i l l be understood to mean the res u l t s of non i on i z i ng rad iat i on. Of paticu lar concen is the probl e m of e lectromagnetic e nergy of such i ntensity as to affect (l ) h u man t i ssue , (2 ) lammab le fue l vapors , and ( 3) explos i v e s ; i. e. , to cause biological damage , ign ition of fue l , and det­ onat ion of ordnance aboard ships.

5-1

BIO LOGICA L EFFECTS OF RADIATION

Electromagnetic energy i mp i nging upon a human body may be relected , absorbed , or tran s m i tted through the t i ssue , or some comb i nat ion of these ? The biological result of t h i s contact is the s ubject today of exte n s i ve study , debate , and controversy. Remaining unsettled are prec isely which frequencies , what energy l e ve l s , what radiation condi tion s , and what mechan i sms actual l y cause i n terac t ion. And , g i ve n that an i n teraction occ urs , which biological e ffects are harmfu l , w h i c h are perhaps even beneic ial , and which are harmless or i ne ffec ­ 3 tual? I n fact , a biolog ical effect very l ike l y may have n o sign i i c ant health conseq uences. On l y when it produces i nj urious or degrad i ng alterations to the health of an organi sm is the effect a biological hazard. The degree of ham depends upon such i n tere l ated factors as the frequency of radiation , energy i nten s i ty , polar i zat i on of the ield. and duration of e xposure. I nte ns ive i n vestigations over the years have estab l ished that b iological damage to l ivi ng ti ssue wi l l result from penetrati v e heati ng if critical leve l s of radiated power de ns ity and length of e x posure are exceeded. S uch themal dam­ age affects v u lnerab le body pats , including the ski n , muscles , bra in , and central nervous system; the effect is most severe , however , for de licate organs with little abi l ity to d i s s i pate heat , such as the lungs , l iver , testes , and potions of the eye. Futhermore , radiation levels may be so low as to cause no apparent harm to tissue , yet be adeq u ate to rai se the whole-body temperature or to generate l ocali zed hot spots w it h i n the body. 4 In such i nc idences phy s iological control mechan i sms of critical body functions may suffer.

169

SHIPBOARD ELECTROMA GNEIC RADIA ION HAA RDS

The body's ability to dissipate heat successfu l l y is dependent upon such factors as the ambient temperature and air c irculation rate of the environment , c lothing being won , power density of the radiation ield , amount of radiation absorbed , and duration of exposure to radiation. Temperature regulation in the body is accomplished primarily through sweat gland evaporative cooling and by heat exchange in peripheral circulation of the blood. The regulation process i s complex and adverse effects produced when high temperatures are induced i n t h e body may result i n decreased system eficiency. B ecause of the body's limited ability to lower heat through perspiration and blood circ ulation , only a moderate increase above normal temperature can be tolerated. Where areas of the body are cooled by an adequate blood low through the vascular system, there is less likelihood of tissue damage resulting from abnormal temperature; i n body areas having relativel y l i ttle blood circu lation , however , the temperature may rise considerabl y from lack of means for heat exchange. Consequently , biological effects of EMR are more likely to occur where there are radical rises of tem­ perature. Under moderate conditions physiological changes seem to be tolerated by the body's normal capabi lity to adjust and correct. The fear is that when the body is unable to make compensating adjustments to radiation overheating , lasting harm is done. In other words , thermal damage to tissue may be irreversible in those cases where the body is unable to replace the tissue through natural process , resulting in lasting side effects. The human eye is a case in point . Certain parts of the eye's vascu lar system are inefi cient for the circulation of blood and the exchange of heat to the surrounding tissues. The lens of the eye in particu lar appears to be very sus­ ceptible to thermal damage. U nlike other cell s of the body , the transparent lens cel l s of the eye cannot be renewed. When the cells making up the lens become damaged or die , opacities or cataracts develop. The loss of transparency i s usual ly a slow process and the individual begins to suffer impaired vi sion. I t can be readily appreciated , therefore , why there i s such concen for preventing radiation overexposure to sensitive organs of the human body. More disturbing even than thermal effects are recent revelations that various nonthermal problems may be observed from experimental microwave radiation tests; i. e. , i n certain circumstances chromosomal damage i n l ive animals is bei ng reported. 5 Some experts are concened that these and other biological changes break down the body's immune systems , cause behavioral changes , and promote the development of cancer. Moreover, there is growing evidence that 60-Hz powerline electrical and magnetic ields produce biological effects in h umans , albeit as yet apparently not harmful. 6 Is there a danger, then , of being exposed to EMR , and , if so , how much and where? To date , the results of thousands of studies over the past 40 years seemingly con i rm tha! d�s ite intense use of the entire electromagnetic spectrum, -

-

--

�

-

-

-

--

-.

-

-

'

-

170

SHIPBOA RD ELECTROMA GNEICS

the general public is in no danger of being exposed to h amfu l leve l s of radiation . The U S Govenment's Environmental Protection Agency (EPA) has perfomed investigations at nearly 500 locations in 15 cities that show, even in those e nvironments subj ected to high-power radio broadcast and television transmis­ sions, that 95 percent of the population is exposed to extremely weak level s of radiated power density; that is, no higher than 0.1 microwatts per square cen­ timeter . 7 (Weak ie lds are described as those radiation levels that do not produce temperature increases in animal s above normal body luctuations: in general, poer ensities below 1 milliwatt er s u are centimeter (mW/cm2) over a fre­ ' quen ran e of 30-300 MH;. �derate levels of 1-5 mW/cmf are toleraleLby human bein s for short duration . 8) ,. The EPA stud� rre a e wel l with many others, all of which indicate that the American public, although living in an environment illed with myriad forms of EMR, is exposed to e nergy levels hundreds of times below current U S guide lines o f safe, permissible intensity levels. Based o n these data it might be concl uded that nonionizing radiation poses no threat of h arm to the general public in today's highly industrialized society . However, as has been pointed out earlier , the debate and controversy go on and will perpetuate until the i ndings are no longer incon siste nt , inconclusive , or ambiguou s , even though "no clear cut ,, damage to hu man beings from low -level radiation has been demonstrated . 9 There are , though, two classes of people known to be subj ect to potentially hazardou s le vels of EMR: occupational workers and military personnel . In the case of occupational workers there is a large variety of apparatus in use that radiate e lectromagnetic energy . These devices include microwave food proces­ sors ; industrial plastics heat sealers ; chemical analysis equipment; medical dia ­ th ermy , detection, and therapeutic eq uipment; science and research l aboratory eq uipment; radio and television broadcast eq uipment; and microwave telecom­ munication transmitting equipme nt-all of which should require adherence to federal , state , and local regulations to avoid potentially harmfu l leve l s of radiation expos ure. These concerns , however , are outside the realm of our particul ar interest . We wish to address the speciic problem of protecting military personnel from the hazards of EMR.

5-2

SHIPBOARD HA ZARDS OF ELECTROMAGNETIC RADIATION TO PERSO N N E L (HERP)

We have seen in previous chapters that , with an extraordinary density of high -power emissions , the shipboard environment is conducive to the generation of EM! . It is likewise true that the large number of high-power emitters radiating highly conce ntrated energy in and around so coni ned a platform makes naval surface ships among the most potentially hazardous of e lectromagnetic environ­ ments in which people must live and work.

SHIPBOARD ELECTROMA GNETIC RA DIA ION HAA RDS

17 1

Quite aware of the severity of the problem and, desirous to prevent any chance of overexposure to its shipboard personnel, the Navy began in the 1 950s to establish and enforce safe radiation exposure limits. Little was really known at the time of the nature and effects of EMR interacting with the human body. But, anxious about exposure to high-power microwave ields in particular, each of the military services was eager to support research and experiments which would aid in the derivation of guidelines to ensure adequate protection of per­ sonnel. Industry, science, and other Govenment agencies also felt the need of setti ng standards.

5-2.1

Origin of Radiation Exposure Limits

As an outcome of studies done in 1 95 3 at the U niversity of Pennsylvania, the irst tentative recommendations for safe radiation exposure limits were made. Proj ecting the ant £ ated results of heatin� anic tissue and9 f possible damage from overexposure, and inco oratin a safety factor of 1 0, a power density of LO mW/cm 2 was proposed. 10 The US Navy quickly accepted this limit. It was applied to all frequencies between 1 00 MHz and 100 GHz without any restriction on duration of exposure. In 1 966, the American National Standards Institute (ANSI), a private organization which publishes voluntary regulations, formally issued the 1 0 mW/cm2 limitation in its ANSI C95. 1 Standard. When, in the mid- 1 970s, ANSI rei ned the standard to constrain the e ,Qosure duration to 6minute inte alsl19 10 ere the freguency of inter�� t.o 1 0 MHz, the Navy complied immediately. This new standard gained wide acceptance in the U nited States as the single most important nonionizing EMR exposure standard. It remained the Navy's accepted HERP level for 30 years, until in the 1 980s it was abruptly revised.

5-2.2

Emergence of Modern Radiation Exposure Standards

During the three decades from the 1 950s to the 1 980s there was relatively quiet acceptance of and adherence to the ANSI power density radiation exposure limits. However, as we noted, among the public there began to arise sharp interest, concen, and controversy regarding j ust what constitutes safe levels of exposure. Continuing studies only added fuel to the debate. Publicity served to bring the issue to the attention of worried politicians and high-level oficials throughout the Govenment. The interest and visibility did not go unnoticed by either the scientiic community or the military. Relecting the mood of concen for public safety, ANSI, in its 1 98 2 periodic review of standards, updated the C95. 1 guidelines to account for current theory

172

SHIPBOARD ELECTROMAG

.TICS

of energy absopti on . I mproveme nts i n rad i ation e x posure mea ureme nt tech­ n iques over the years had re sulted i n a determ i n ation th at the e nergy absorbed by a n i mal t i s sue is a d i rect function of rad i ation frequency , polarization of e nergy (i . e . , orie ntation of the e lectromagnetic i e l d component ) , and phys ical ize of the i rrad i ated boy . For example , a normal size adu lt hu man tanding in a vertical l y pol arized rad i ation ielJ i s an e fi c i e nt receptor of e lectromagnetic energy , due to body re sonance , in the 70 MHz to 1 00 MHz VHF range-a low range that was not i n cluded in the ori g i nal power de ns ity e x posure l i m its adopted by the Navy duri ng the 1950s and earl y 1960s . AN S I , therefore , real izi ng the frequency-dependence of e x posure i nte n sity , rad i cal l y modi ied its standards to l i m it the absorption of rad i ation e nergy . Care fu l to keep its former safety factor of 1 0. AN S I offered a new C95 . 1 - 1 98 2 standard of 0 . 4 W/kg of whole body weight averaged over a n y 6-mi n ute period , and , at the same time , i mposed stricter ex posure l imits (lower power dens ity leve l s ) above 1 0 MHz . Accordingl y , for the body re sonance freq uencies of 30 2 to 300 MHz , the l i mit i s now much more restrictive , 1 m W/cm , a value one­ tenth the previous level . Also , the overal l frequency range of the standard was e x panded by extending the l o wer frequency end to 300 kHz. To d i ffere ntiate th i s new concept from the old , the exposure limit terminology was changed from the former "power den sity " to the c u rrent "speciic absoption rate , " or S A R , i n the derivation of freq uency-dependent pem i s sib le exposure leve l s (PEL s ) o f radi ation dosage . N ote how this concept of time rate o f absoption p e r m a s s of tissue now correctly concedes that the potential for EMR hazards i nc l udes fre­ quencies we ll be low microwave .

5-2.3

S h i pboard Perm issible Exposure Criteria

The N avy , e mbrac i n g the 1 98 2 ANS I philosophy of frequency-dependent rate of energy absoption , proceeded to make a fe w modiications to the new standard so as to establish its own pre ferred safeguards . The radiation frequency range was broadened still futher to cover the e lectromagnetic spectu m all the way from 1 0 k Hz to 300 GHz; i . e . , from V LF to infrared . Additionally , two separate categories of e x posure leve l allowances were dei ned : ( 1 ) restricted access, which , based on criteria deve l oped by the A merican Conference of Governmental Industrial H ygenists (ACGIH) as an occupational standard , applies to shi ps at sea and excludes persons le ss than 5 5 inches i n height , and (2 ) unrestricted access, which confoms to the ANS I standard and applies to the general public irespective of body size , and includes ships in pot . The e x posure l i mits for these two categories are identical up to 1 00 MHz , but d i ffer widely above 1 00 MHz . I n adopti ng the new S AR exposure l i mits , and i n keeping w ith its policy to avoid un necessary risk of EMR to personnel (or , when risk is unavoidable ,

173

SHIPBOARD ELECTROMA GNEIC RA DIA lON HAARDS

to ensure that any exposure is with i n safe l i mits and as low as reasonably achievable), the Navy issued its new radiation exposure criteri a on 30 Ju l y 1 985 as OPNAY Notice 5 1 00 . 11 Directed as a uniform , Navy- wide protection criterion, the Notice req u ires appl ication to al l phases of eq uipment design, acqu isition, i nstal l ation, operation, and mai ntenance . Tables 5 - 1 and 5 -2 itemize the various PELs as averaged over any s i x - m i nute i nterval for both the restricted and the unrestricted access categories, respecti vel y . Figure 5 - 1 depicts the exposure leve l s i n graph ic form . 100

100mW/cm 2

RESTRICTED 10 ---

2 10 mW/cm

900

� F2(MHz) --

N

E

u

� E

NOTE:

10 kH z

Figure

Restriction excludes personnel under 55 Inches In helQht.

3

MHz

30

MHz

100

MHz

300 MHz

I

GHz 1.5 GHz

300

GHz

5-1 Whole B ody Radi ation Hazards Exposure Lim its

There are some spec ial exceptions to the normal PELs , as stated below: a . Personnel who , as patients, undergo di agnostic or therapeutic procedures in medical or dental treatment fac i l ities are excl uded . b . Dev ices operating at or be low I GHz with an output power of 7W or less are excl uded. c. The deri ved PEL criteria i n Tables 5 - 1 and 5 - 2 may be exceeded under spec ial circu mstances , provided it can be de monstrated by measure ment that: I.

The whole-body SAR does not exceed 0 . 4 W/kg when averaged over any 6- minute period . 2 . The spatial peak SAR (hot spot) does not exceed 8 . 0 W/kg averaged over any one gram of body ti ssue . 3 . The peak e lectric ield intensity does not exceed 1 00 kY/m . 4. Personne l are adequate l y protected from e lectric shock and RF bums

174

SHIPB OARD ELECTR OMA GNEICS

Table 5 - 1 . Eq uivalent Permissible Expos ure Levels for Restricted Areas 1.2.3,4

Frequency

Power Densiy

(MHz)

(mWI cm2 )

0.01-3 3-30 30-100 100-1,000 1,000-300,000

100 900/f2 1.0 j/100 10

Electric Field Strength Squared

Magnetic Field Strength Squared

400,000 4,000 4,000 400 40,000

2.5 0.025 .025 .025 .25

(y2/m2)

(900Il) fl100)

(A2/m2)

(900Ij2) f/l00)

I

Restricted acce s s area s are controlled to exclude persons les s than 55 inche s in height. 2Yalue s in these tables were derived u sing the impedance of free space of 400 ohms. This value is rou nded up from the generally accepted value of 377 ohms to allow for ease of calculation s. Also , j is in M H z. 3Whe n both the electric i eld and magnetic ield are measured , use the more re strictive value. 4Tables apply only to whole body exposure s and are b ased on the overall SAR of 0.4 W/kg averaged over 0.1 hour (six min ute s ).

through the use of electrical safety matting , s afety clothin g , or other isolation techniq u e s. The power de nsity P ELs listed in Tables 5 - 1 and 5-2 are derived for far­ i eld plane wave conditions and apply only where a strict far- i eld re lation ship between the electric and magnetic ield components exists. In the Fres nel zone near i elds ( such as for HF commu nications transmitting antennas aboard ship) both the electric and magnetic ield stre ngth limits , rather than the power den sity values , must be u sed to determine compliance with the P ELs. Futhermore, it is important to note that in all cases , expos ure levels must never exceed an electric ield maximum inte nsity of 100 kY/m. As pat of the shipboard safety measures , radiation hazard waning sig n s are req uired a t all access points to areas i n which expos ure leve l s may b e exceeded. T h e format of t h e signs follows that suggested b y the ANSI C95.11982 standard as shown in Figure 5 - 2. The waning symbols consist of b l ack wave fronts radiating from a stylized point source antenna on a white backgrou nd e nclosed in a yellow and red hash- bordered triangle. (This color scheme deviates somewh at from the ANSI standard in order to e n sure proper aware ness of the

/75

SHIPB OARD ELECTR OMA GNEIC RA DIA ION HAARDS

Table

5-2. Equivalent Permissible Exposure Levels for U nrestricted Areas 1.2.3.4

Power Electric Field

Magnetic Field

Strength Squared

Strength Squared

(M H z)

(mWI cm 2 )

0 . 0 1 -3 3-30 30-300 300- 1 ,5 00 1 ,500-300,000

100 900112 1.0 /300 5. 0

400,000 4,000 4,000 4,000 20, 000

2. 5 0. 025 . 02 5 . 025 . 12 5

Density Frequency

(y2/m 2 )

2 (90011 ) /300)

( A21 m 2)

(900112 ) f/300 )

1 U nrestricted access areas are not controlled a n d al l persons m a y e nter. 2Yalue s in these table s were derived using the impedance of free space of 400 ohm s . This value is rou nded up from the generally accepted value of 3 7 7 ohms to allow for ease of cal c ulations. A lso, 1 is in M H z. 3W h e n both the electric i eld and mag netic ield are measured, use the more restrictive value . ' ables app l y only to whole body exposures and are based on the overall S A R o f 0. 4 Wlkg averaged over 0. 1 hour ( six min utes).

sign in all shipboard lighting conditions, from low- level red light to y �llow sodium vapor light. ) For areas where access to radiation levels greater than 1 0 times the PEL may exist, warning signs are to be considered insuficient to e n sure adeq uate protection. I nstead, additional waning devices and controls such as l ashing lights, audible signals, and variou s physical constraints such as guardrails and interlocks are required to prevent the chance of overe xposure .

5-2.4

S h i pboard EMR Hazards Protection Tec h n iques

The requirements of OPN A YNOTE 5 1 00 to protect naval personnel against overe xposure to nonionizing radiation from 1 0 kHz to 300 GHz m u st be imple­ mented in every U S Navy surface ship and in all new ship designs . The immediate shipboard problem, in order to abide by the more stingent radiation exposure limits, is to e n s ure maximum safety to person nel with minimu m adverse effect on ship mission operation s within the e xisting topside systems arrangement . This is irst met by e n forcement of the restricted access criteria: ships at sea, after all, are operated under strictest access control and do not have personne l less

176

SHIPB OAR D ELECTR OMA GNEICS

IOTE :

ALL 01

II BLACI BACla.OUID

LETTERIIG

WHITE

E L LOW

ED

RADHAZ ,

TO

PERSONNEL

'

/

"

�

O. 2S

IICKES

T

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,

Figure 5-2

AN S I Standard Rad iation H azard W arn i ng S ign

than 5 5 i nches ta l l aboard . Neverthe less, i n the performance of routi ne operational duties c re w members do work i n and around areas where high - power tran smitting ante n nas are i n stal led . S ome of these ante n nas , patic u l arl y those u sed for HF communications, radiate high- level energy omnidirectional ly throughout the whole tops i de . There fore, such commo n l y mann ned ope n deck areas as l ookout and watc h statio n s , rep l e n i shment-at- sea stations, s ignal and search light position s, catw alks , and passage w ays are with i n the ield of rad i ation . Isol ation by spatial separation i s sim p l y not a viable sol uti on i n so l imited a pl atform volu me . Finding sat i s factory solutions is a conti n u i ng chal lenge . I mpotant i rst steps are to make sure that al l s h i pboard personnel are well i n fomed about pote ntial rad i ation hazards, and to e n force e x posure lim its . W an­ i n g signs m u st be posted, and danger zones m u st be clearl y marked by circles painted around al l transmitting antennas that pose a threat to safety . Radiation hazard adv i sorie s m u st be i ss ued which spec i fy safe operating conditions for allowable leve l s of e x pos u re to personnel working i n the area . The next step i n volves a deta i led e lectromagnetic mapp i ng of surface ship topsides through s pecial i zed radiation level env ironmental surveys . 12 I n this manner source e m i tters and areas of excessive e x posure level are ident i ied as

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potentially hazardous . Unsuspected sources of hazardous radiation, such as en­ ergy leakage from waveguide langes or faults and from mal functioning radiation cutout devices which al low radiation past the l i mits , may be detected as part of the survey . Procedures for control l i ng personnel exposure are then determi ned. The i nal step requires i mplementation of speciic methods to reduce the potential for creation of hazardous conditions . These techniques include the relocation of antennas , reduction of transmitter power , carefu l management of operational frequencies , and erection of non metal l ic l i ferail barriers around haz­ ard zones . Systems engineeri ng techn iques to reduce the l ikel ihood of radiation hazards to personnel must be applied as discussed below . 5-2.4.1.

Ship Design Criteria to Control EMR Hazards

The foremost naval ship design element that determines the EME is the placement of and relative proximity among the several transmitt i ng antennas. During the process of attain ing the optimum arrange ment of the antennas, the radiation exposure concerns are considered in balance with all the other leading design needs such as weapons arrangement , combat systems performance ob­ jectives , operational and mi ssion effectiveness , and we ight and moment con­ straints. Designers keep in mind al l the whi le that the permiss ible exposure leve ls of Table 5 - 1 must be adhered to duri ng operations at sea , and those of Table 52 when in port or when carry ing passengers less than 5 5 i nches i n he ight . I n fact , the design goal o f a l l n e w naval shi p construction programs wil l b e the more stringent exposure levels of Table 5 - 2 . As part of the topside systems design various sources of information are used. S h ipbuilding speci ications , ship draw i ngs , equipment techn ical manual s , technical repots , ship operations personnel , and onboard topside surveys a l l w i l l assist in determ ining the fol lowing design factors: • • • •

•

•

Types of electromagnetic systems to be instal led . Maximum on-ax is power density and PEL distances from each emitter . System operational requ irements of all potential EMR hazard sources . Location of all potential EMR hazard sources and the re lationship to all normally occupied areas . Types of radiation- limiting mechanisms , present or proposed settings , and methods of override. Ship design characteristics for pitch and roll .

For the purposes of EMR analyses , shipboard emitters are classi ied as stationary , rotating , or directed beam. The design requ irements di ffer somewhat for each category.

178

SHIPB OA RD ELECTROMA GNEICS

a.

Stationary Em itter Design

Requirements-Stationary emitters generally are used aboard naval ships for H F , VHF, and U H F omnidirection al com­ munications transmiss i son . H F antennas are the most dificult to cope with in systems EMR design as they must be installed in areas that preclude physical contact by shipboard personnel . VHF and U HF ante nnas ordinarily are pl aced h igh in the superstructure or on the ship masts to provide clear all-around transmi ssion . Since these systems general ly are line-of-sight and therefore are of low gain and rad i ating low power , they are not nomally an EMR problem . HF transmit antennas ( radi ating in the military 2 M H z to 30 M H z band) must have a m i n i mum 4 - foot hori zontal and 8 - foot vertical physical clear­ ance i n all occupiable topside areas . HF whip antennas that radiate more than 250 watts must provide a 1 2- foot m i ni mu m phy sical cle arance radius from any potion of the ante nna to any occupiable tops ide are a . b . Rotating Emitter Design R e qu ir e me n ts S h ip board rotati ng antennas in­ cl ude se veral type s of 2- D and 3 - D air search .. surface search , navigation , and air- trafic contro l radars. some of wh ich radi ate extreme ly high lev l s of p u l sed energy . The se antennas are usual l y mounted on a platform high on the sh i p mast or on a pedestal in nonoccupiable are as . Because they rotate they produce intermittent exposure and are therefore seldom an EMR probl e m . Ne vetheless. the topside design must con i rm that no radiating antenna wi l l be placed in a locat ion that w i l l cause the PEL to be e xceeded in any normally occupiable are a , particu l arly when it m ight be in its highest duty cycle mode or al lowed to radiate i n a nonrotati ng mode . c . Directed B eam Emitter Design R e qu irements �ap ons control radars , SATCOM tran smit antennas , and EW emitters are example s of directed beam an ten nas used i n shi pboard sy stem_ s o Weapons control radar antennas are very high gai n and ye y.�rrow !!d i ation beamw idt �J con­ �tration of high ene�y . They are pitch- and rol l - stabi lized , and ' because of we ight and performance considerations are frequently mounted lower than rotat ing emi tters and are there fore near to normal ly occupiable areas . Most of these radar ante nnas therefore employ rad i ation cutout devices such as mechan ically operated sw itches , computer software , or mechanical stops wh ich prevent rad iation into se lected areas . S ATCOM and EW tran s­ mit anten nas have s i m i lar characteri stics and require like design consid­ erations . Directed beam emi tters are the most common source of EMR problems , and their problems can be the most di ficult to resolve . For design purposes , directed beam antennas must be pl aced as high above normal ly occupiable areas as is electrically and mechanical ly prac­ tical sti ll to sati s fy all other sy ste m performance req uirements . A n EMR cam cutout sche me m u st be developed for each directed beam antenna to prevent the irrad iation of any normal ly occupiable area with EMR levels -

-

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179

that exceed the PEL. At the same time , directed beam antenna coverages must not be degraded by an EMR cutout device that does not have an override (battleshort) capability. 5-2.4.2

EMR Hazards Measurements and Analysis

EMR ield measurements are required to be taken aboard naval surface ships regul arly to ensure the safety of shipboard personnel. The purpose is to determine that permissible levels of radiation exposure are not exceeded in normally occupiable areas near ship radar and HF transmit antennas , beyond the limits of radiation cutouts , or from RF leakage in below-deck compartments. In all events the risk of overexposure must be prevented. Overexposure for Navy purposes is dei ned as any human exposure to nonionizing EMR that exceeds the pemissible exposure level by a factor of i ve. Overexposure requires im­ mediate medical attention , and a report of the incident must be submitted within 48 hours. The most commonly used test instruments for taking RF ield measurements are those that have broadband isotropic monitor probes integral with a radiation level indication meter. 13 The isotropic probe allows near-equal response to energy arriving from any direction except along the instru ment handle. Hence it is not necessary to rotate the probe in any manner to strive for a maximum reading. Power from the electromagnetic ield under test is dissipated in the isotropically spaced thermoelectric elements of the probe's lossy media. A low-level dc voltage is subsequently generated and conducted to the instrument preampliier. Typical EMR meters require two probes to cover a frequency range from 10 MHz to 26 GHz , and measure average power lu x densities varying from 0.2 mW/cm 2 to 200 mW/cm 2. Meter response time , the time needed for the meter to reach 90 percent of its steady-state reading , can introduce a signiicant error if the radiating antenna under test is rotating or scanning, or if the test probe is moved quickly through a narrow radiation beam. If either movement is too rapid the meter wil l not have time to reach its fu l l value and wil l indicate too Iow a reading. A meter response time of less than I second is preferred , and must not exceed 1 . 5 seconds. a. Preliminay Test Procedures-Prior to commencement of actual test meas­ urements , a thorough inspection of the test areas should be conducted. Waveguide systems above deck and in radar equipment compartments should be checked for loose lange bolts , cracks , or other faults that might allow escape of energy. The condition of lexible waveguide sections requires special scrutiny. Where energy leakage is suspected , the locations should be noted for follow-up tests. For directed-beam emitters that use radiating cutout devices to pre­ vent ill umination of selected safe zones , the cutout mechanisms should be

I�O

SHIPB OARD ELECTROMA GNEICS

checked so as to be sure that they are functioning in accordance with the set limits. H F transmit antenna installation sites should be examined to con i rm that they incorporate required EMR safety practices. Likewise, all areas subject to EMR overexposure should be inspected to make sure that hazard waning signs are posted properly. Finall y, shipboard administrative safety procedures for the operation and checkout of potentially hazardou s radiati ng systems should b e examined t o certify that they confom to speciied requirements. Personnel performing the EMR tests must avoid any possibility of radiation overexposure. If the measurements are to be taken while the ship is dockside, there must be suficient clearance in the vicinity of the ship to precl ude EMR e xposure to persons on the dock and on adj acent ships and piers. Test engineers should at all times be thoroughly familiar with the antic ipated power de nsity levels at the various ield points to be tested. Navy technical manual OP 3 565 lists the PEL distances and maximum al lowab le ex posure t i mes for various operating modes of shipboard emit­ ters. 14 The distances and times are based on e xposure to mainbeam radia­ tion , though it is se ldom necessary for test personnel to be in the fu l l power mai nbeam whi le taking EMR measurements. T o b e sure that ex­ cessively high leve l s of radiation are not prese nt , EMR hazard monitors should be used to qu ickly check the test area. When e xcessive levels are detected , the tran smitter power should be se lective ly reduced and the test data the n extrapolated i n the same ratio. I n al most al l case s shipboard EMR tests are performed under sim­ ulated operating condi tions, with the emi tter deliberate ly stopped at or pointed to de sired azimuth and elevation angles. The test location and le ngth of time for personne l to take the measurements while the antenna is radiat ing should be predetermined. Judiciou s choices of these test con­ dit ions are necessary as they must be representative of those which could be e ncountered duri ng wide vari ations of actual operations. Measurements made u nder unrealistic test conditions could result in critical EMR over­ e xposure situat ions going undetected , u nreported , and uncorrected. There­ fore, even maximu m pi tch and roll conditions should be simu lated so as to test to the worst case operations. Te st point locations should be selected which typify normally oc­ cupiable areas in the topside , such as on bridge wings , at lag bags , at signal searchl ights , and at lookout stations-wherever it is reasonable to assume shipboard personne l would be present. b.

EM R Test Measuremen t Gu idelines I . Station ary Em itter Tes ts -The

primary obj ective of this portion of the shipboard EMR survey is to determine the maximum power density in H F trans mitting areas and the PEL contours in normally occupiable

SHIPBOARD ELECTROMA GNEIC RADIA ION HAARDS

IBI

spaces nearby. This is to be done for each HF transmitting antenna over its range of operating frequencies. The fol lowing general process is rec­ ommended : (a)

(b)

(c)

2.

Rota tin g Em itter Te s ts

Since rotating beam radiators move continu­ ously , they normall y do not present a risk of overexposure. Therefore , by authority of Navy manual OP 3 565 [ 1 4] , radiation tests need not be conducted for rotating emitters unless speciically directed by oficial request. When required , the fol lowing general process is recommended: (a)

(b) (c)

3.

Energize the appropriate transmitter to ful l power using a mode of operation that produces maximum average power output. Re­ cord the power output reading. (If ful l power is not achievable, use reduced power and extrapolate test values by the same ratio. ) In the selected test location , take measurements at heights of 6 feet , 4. 5 feet , and 3 feet; i. e. , at the approximate head , chest , and genital heights of an upright human body. Search for and record the highest E-ield and H-ield levels throughout the test area. If any measured values exceed the al­ lowable PEL for continuous exposure conditions , repeat the tests to determine whether the criteria for intermittent exposure are also exceeded. Note: Large metallic objects near HF transmit antennas wil l capture and reradiate electromagnetic energy. It is possible in some cases that the reradiated energy level wil l exceed the PEL. It is then necessary to measure and record the maximum power density and distance of occurrence around the reradiating object to ascertain at what point the reradiated energy level drops below the PEL. -

Energize the appropriate transmitter to ful l power (or speciic reduced power as recorded) using an operating mode that produces the highest duty cycle. Take measurement data at selected locations , stopping the antenna rotation at the point of maximum power density level. Search for and record the highest power density levels in the test area. If any test value exceeds the al lowable level for continuous exposure , repeat the tests to determine whether the criteria for intermittent exposure is also exceeded.

Directed-B eam Em itters

Antennas in this category are capable of pro­ ducing highly concentrated on-axis maximum , average power den sities from 200 to 400 mW/cm2. These power densities may cause excessive exposure levels at any location subject to illumination by the radiation beam. In fact , EMR overexposure levels can extend to distances several -

SHIPBOARD ELECTROMAG ETICS

182

hu ndred feet beyond the sh i p. S pecial concen must be e xerci sed , there ­ fore , for the safety of indiv iduals on nearby piers and ships duri ng the tests. Most weapons control radar antennas of the directed beam cate­ gory have some form of radiation cutout circu it or mechan ical stops that are set to prevent radi ation i nto speci ied areas. Some of these antennas have j ust two azimuth settings and one elevation sett ing to avoid rad i ation into a selected azimuth sector and i nto areas above or be low a desired elevation. This arrangement , it is important to note , se ldom pemits the radar to operate under max imum rol l conditions w i thout an EMR overex posure occurring at some point. Some weapons radars which have computer-controlled radiation cutout circuits can be programmed to allow numerous settings in both azimuth and elevation. With this capability it is general l y possible to obtain contoured cutout zones around the ship to provide optimum EMR protection w hile af­ fording acceptable system performance. It is recommended that , as part of the E M R testing, measurements be taken to verify that original radiation cutout settings are stil l correct , or , where nece ssay , to reset to new limits as a comprom i se between system perfomance effecti ve­ ness and personnel s afety. The fol lowing general process is recom­ mended for directed-beam radi ation leve l testing: (a)

(b)

(c)

(d) (e)

Detemine the re lationship between each weapons control radar (or other directed-beam antenna) and normally occup i able areas so as to choose test measurement locations properly ; epecial l y note all potential overe xposure areas. Provide for test positioning of the antenna either manual ly with azimuth and elevation hand cranks, or electrically from an op­ erator's console. M ake sure that all personnel are safe l y clear as many such antennas are capable of rapid acceleration and may inlict seriou s inj uy. Measurements to detemine the power den­ sity in selected locations are to incl ude situations w here the an­ tenna is depressed to low angles equivalent to those which result from actual pitch and rol l of the ship. It may be necessary in such tests to bypass cutout limit switche s and temporari l y disconnect the ship gyro infomation input. Train the antenna to the test location bearing , using an elevation which maintains mainbeam radi ation several feet above the test location and test engineers. Prepare for initial EMR measurements by holding the test probe at a height of approximatel y six feet above the deck. Energ i ze the radar system and slowly lower the elevation angle unt i l the PEL reache s the allowable level (per Figure 5 - 1 ) for the

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183

frequency under test. Move the probe from side to side and up and down to verify that the reading is maximum for that paticul ar test site. Record the azimuth and e levation angles and the test data. If unexpected high energy levels are found to be the result of relections from various shipboard objects , document those situations which might prove hazardou s under normal operating conditions. 4.

c.

Wa vegu ide Emission Leak Tests-Checks

for the escape of microwave energy from waveguides should be made to determine the locations of leakage , the radiation leve l s , and whether possible overexposure to personnel may have previously occurred. In the event of possible over­ exposure , it is impotant to document whether the PEL has been ex­ ceeded in occupiable areas , and whether personnel were in the area long enough to have suffered overexposure. Measurements should be carefully performed to determine at what body height the waveguide radiation leaks occur.

EMR Test Measurement Ana lysis-Power

density tests conducted on ship must take into account a number of variables so as to minimize the chance of measurement errors. Test locations , for example , are frequently at a point where the complex electromagnetic ields under measurement are extremely irregul ar. In many instances the tests are made within a few feet of an emitter , and are therefore in the radiation near- ield (Fresnel region ) so that the measured energy luctuates widely from point to point. Pat of the radiation energy may arrive directly from the antenna mainlobe or sidelobe , or it may combine with energy relected from the deck , the bulkheads , the masts , the stacks , or even the body of the test engineer. If these energy components were to all combine additively the test meter would indicate inordinately high leve ls. If, however , the direct and relected energy components were to interfere with one another destructive l y , the meter readings would be quite low. In reality the combining of energy l uctuates somewhere between these two extreme s , and , for that reason , averaging techniques must be applied , as we wil l see. The two classiications of EMR exposure on ship are : con tin uous and in termittent. 1.

Contin uous Exposure - A

continuous exposure EMR environment is one in which an individual may experience a constant level of radiation exposure for six minutes or more. For this case the frequency-depende nt PEL is given in Figure 5 - 1 . Typical shipboard radiation sources that produce continuous exposure levels are HF transmit antennas and wave­ guide energy leaks. Personnel required to stand watches or operate systems at i xed locations for periods in excess of six minutes must be

184

SHIPB OARD ELECTR OMA GNEICS

made aware of the potential for reaching the continuous exposure cri­ teria , although because of a person's movement about the area it is unlikely that actual continuous exposure conditions exist aboard ship . Nevetheless i t must b e stressed that, i n a continuous expos ure environment , levels in excess of those shown in Figure 5-1 are not acceptable. Emission sources producing excessive radiation levels must be repoted and documented , technical or operational procedures must be initiated immediatel y to prevent overexposure of personnel, and the area must be clearl y identiied as an EMR overexposure danger zone . 2 . Interm itte n t Exposure-An EMR situation in which an individual may be exposed to varying le vels of radiation during a six-minute interval is known as an intermittent exposure environment. Most EMR exposure environments (with the e xception of those caused by HF transmitting antennas and waveguide leakages) are intermittent. Generally it is q uite dific ult to determine with certainty the average exposure received by a person during a six - minute period because of movement around the area by the individual, movement (rotation or scanning) of the antenna radiation beam , and variations in power output levels. One recom­ mended procedure for estimating the exposure for a person in an in­ termittent radiation situ ation is the time-weighted-mean average method. To use this method power density and time meas urements are taken at each of several locations where a person is exposed during a six-minute period, so that Ew

Pl tl + P2 t2 + . . . + Pn tn tl + t2 + . . . + tn

where Ew

Time-weighted average exposure for six - minute intervals in mW/cm2

P

Equivalent plane wave power density measured at each specific location in mW/cm2 Time at each specific location , and t I + t2 . . . + tn � 6 minutes

Several measurements should be taken at eac h of the speciied locations to determine EMR exposure levels at head, chest, and genital heights (i. e. , approximate l y 6 feet, 4. 5 feet, and 3 feet). The average of the

SHIPB OARD ELECTROMA GNEIC RA DIA ION HAA RDS

/ 85

readings recorded at each height i s then used i n the E w equation for each of the locations and time periods. If Ew exceeds the PEL for the area , immediate technical and operational procedures must be i nitiated to prevent personnel overe xposure , and the area must be c learly iden­ tiied as an EMR overexposure danger zone. Because the radiation beam of shipboard air search , surface search , aircraft control , and navigation radars is constantly rotating or scanning , any resulting EMR exposure is intermittent. Rotating radar antennas which have relatively wide beams allow longer interval s of exposure per revol ution but produce less concentrated energy than that of nar­ rowbeam radars. The average power density from a rotating radar an­ tenna is approximated by B WP F

360 where PR

Average equ i valent plane wave power density

PF

at a point within the axis of rotating radar main beam in m W /cm 2 Fi xed power density in mW/cm2

Bw

Radiation patten beamwidth in degrees

( I t is interesting to note that rotation rate does not enter into the above equation. At a given test point if the rotation rate of an antenna is changed by a factor , l , the exposure time to the main beam at that point for one rotation is changed by 1 1 l . When the rotation rate is less than the averaging time base for a PEL of six minutes it is not a function in determining the PAR of rotating radars. ) S i milarly , for scan ning beam radars the average power density is approximated by 2B wPF As

where PAS

A verage eq uivalent plane wave power density at a point in the scan beam in mW/cm 2

186

SHIPB OAR D ELECTR OMA GNEICS

Fixed power density in m W /cm 2 Radiation pattern beamwidth in degrees Scan angle in degrees Navy manual O P 3 5 6 5 [ 1 4] gives the PEL distance for i xed beam , rotating beam , and scanning beam antennas for naval shipboard radars. With the e xception of some low-power navigation radars , rotating and scanning beam radar antennas are u s uall y placed on mast platfoms w e ll above normal l y occupiable topside areas. However , in the e v e nt that the mainbeam of rotating and scanning e mitters might illuminate a normal l y occupiable area while locked on target or otherwise stopped , test measurements should be taken to document any potential for over­ exposure . I f the exposure leve ls are found to exceed the PEL , technical or operational procedures mu st be i mplemented immediatel y to precl ude overexposure to personnel , and the area mu st be clearl y identiied as an E M R overe xposure danger zone . One very important case of intermittent e xposure possibility in a normal l y occupiable area i s that of directed-beam radiation during ship pitch and rol l . A good example is a weapons control radar direc tor electrical l y trai ned in azimuth and elevation to track or il l u minate a target automatical l y . Recal l that because of the ir great size and weight , weapons radar antennas ordi nari ly are i n stal led c lose to and on l y slightly higher than normal l y occupiable areas . Ship gyro information is fed to the antenna servo sy stem to stab i l ize the antenna against the effects of pi tch and rol l . General l y , weapons radar directors cannot track a target or be pointed very much be low the horizon l i ne . When the radar beam is on the hori zon , howe ver , added pitch and rol l can actual l y reduce the antenna-to-deck angle suficiently to allow mainbeam irradiation of deck areas . Conseq uent l y , since pitch and rol l cause intermittent con­ di tions of exposure , and weapons radar d irectors u se scanning beams , the exposure level for these antennas i s computed from the 2 B wP F As

average power density eq uation gi ven above for scanning beam radars. d . EMR RF B urn H aza rds RF bum is a uniq ue shipboard personnel hazard caused by E M R in a congested multisystem environment . Distinct from radiated power density ex posure or e l ectrical shock, RF bum is a natural consequence of the coupl i ng of nearby HF transmit energy into topside metal items such as stanchions , king posts , liferail s , crane hooks , booms , -

SHIPBOARD ELECTROMA GNETIC

RA DIA nON HAARD S

I

7

rigging , pi es , and cables . U on casual contact w i th these R F - e x c i ted metal object s , an individual exeriences an i n v o l u ntary reac t ion o the alaming bum or spark . The contact vol tage is itse l f ne ither lethal nor severely dangerous , but the uncontrol led respon se may we l l re u l t in serious bod i l y inj uries from re lex actions of fal l i ng away or stri k i n g other objects when in close quaters . Regard less of the ir intended u se , al l meta l l ic i tems have e lectrical propeties of res i stance and i nductance . A t h i rd e l ectrical feature . capac ­ itance , e x i sts between the items . The mag n i tude of these e lectri c a l prop­ eties deends upon the nature of the metal l i c materi a l . the s i ze . shape . and ph ysical orientation of the objects , the prox i m ity of the objec t s o eac h other, and the degree of grou nding and bond ing to t he mai n s h i p stucture . The effects of the inductance and capac itance v ary w i th frequency and can be rough l y simulated by the s i mpli ied eq ui valent c i rc u i t of F i g u re 5 - 3 . which shows a re lationsh ip of shi pboard k i ng posts and cargo booms . ) 5

\

T .AN S M I T ANTENNA

)

Figure

5-3 Electrical Eq ui valent of Shi pboard Cargo H and l i ng Eq ui pment

The electrical circuit characteri stic i n herent in metal tructure can act to intercept e lectromag netic energy 0 that c u rre nt and ol tage w i l l e deve loed i n the c i rcui t i m edances . At h i g h freq uenc ie t he rea I i c

188

SHIPB OA RD ELECTR OMA GNEICS

components are signiicant, and when the inductive and capacitive reac­ tances are equal (at resonance), maximum voltages w i l l occur. The be­ havior is similar to that of a communications receiving antenna; in fact, metal objects which have the physical and electrical characteristics of H F receiving antennas are q uite commonplace in shipboard topsides. Long metal lic items are very eficient interceptors of RF energy. S ome typical shipboard deck items and their resonant HF frequencies are : • • • • •

Antisubmarine Rocket Launchers 3"/50 Gun B arrel U nderway Replenishment Stanchions 3 5-Foot Metal Poles A-4 and F-4 Aircraft

1 2 MHz 14 MHz 4 MHz 6 MHz 6, 9 , o r 1 8 M H z (depending on the ori­ entation of the aircraft with respect to the an­ tenna)

Shipboard H F communications antennas radiate vertical l y polarized ield s ; therefore , vertical stanchions , pipes , king posts, masts, booms, davits , and cables readily couple the RF energy. The amplitude of the coupled energy is a function of ( 1 ) the length of the metal object with respect to the radiation frequency (wave length) , (2 ) distance between the radiating source and interceptor , (3) leve l of radiated power , and (4) phys­ ical orientation between the polarized ield and the interceptor. Cargo ships especially have many long booms , king posts , and cables, and thus have high incidences of RF bun hazards. However , cargo ships are not alone ; any ship carrying high-power H F transmit antennas is very likel y to ex­ perience the problem. An actual RF bun is caused by an RF current lowing into a person coming into contact with (or near enough to create a spark from capacitive coupling with) an electromagnetically excited metal object. The bun occurs from heat produced by the low of current through skin resistance in the contact are a . The degree of heat range s from w arm to painful. However, the exact leve l at which contact with an induced RF voltage should be classed as an RF bun hazard is not absolute. Experience has shown, for e x ample , that severe buns can occur with the small contact area of a single i nger , whereas with the entire hand at the same point of contact the effect may be unnoticed. For N avy purposes , hazardous RF bun leve l s are dei ned as those voltages which cause pain , visible inj ury to the skin , or involuntary re­ action. The term " hazardous " does not include voltages so low as to cau se

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189

only annoyance , a stinging sensation , or moderate heating of the ski n. The Navy has resolved that an open circuit RF voltage on an object i n an EMR ield in excess of 1 40 volts is to be considered hazardous . The 1 40-volt level is based on tests and measurements which indicate that a person w i l l receive a n RF b u m when coming into contact with that voltage. Misunderstanding of the causes of RF bum is evident from reports of the problem . The most common misconception is that the voltage builds up like static electricity , and is caused by improper transmitter operation. In fact , voltage appears instantaneously when energy is intercepted from the transmitting antenna. It remains only as long as the energy is being transmitted. The amplitude of the induced voltage is proportional to the square root of the radiated power; thu s , a properly tuned transmitter wil l induce higher voltages than a poorly tuned one . As part of naval EMC engineering practices , RF bum should be eliminated from ships as completel y as possible. Several techniques are currently available , including the fol lowing : I.

2.

3.

Hook Insulators-Fiberglass i l ament-wound insu lator l i nks i n stalled between metal cables and cargo hooks are vey effective i n preventing RF bums when contact i s made with the hook itself. (The RF voltage and potential for bum remains hazardous above the insulator links , of course. ) Examples of an uninsulated cargo hook and an i nsulating l i nk used in a cargo hook are shown i n Figures 5 -4 and 5-5 , and lightweight insulators in deck tiedown hardware are shown in Figure 5-6. Heavy­ duty insulator links such as those in Figure 5 -4 are avail able with capacity ratings of 1 5 tons , 30 tons , and 50 tons . Nonmetallic Materials- Use of nonconductive materials for fabrication of such items as l ifelines , guardrails , stanchions , j ackstaffs , and posts has proven very effective for elimination of RF bum voltages. Recal l , too , from Chapter 4 that use of nonmetal lic materials greatly reduces the generation of intermodulation interference. A ntenna Relocation-One of the principal causes of RF bum formerl y occurring i n cargo ships was the common practice of instal ling HF transmit whip-type antennas high atop metal lic king posts. The RF energy generated along the king post and coupled i nto nearby booms and rigging was inten se. It is now standard practice never to instal l HF transmit antennas high on stanchions or superstructure items where an indiv idual can come into contact . S i milarly , there are times when H F antennas have had to be relocated in order to eliminate RF bum hazards. In most cases a m i n­ imum separation of at least 50 feet is required to achieve adequate reduction of coupled HF energy be low the hazardous level.

1 90

SHIPB OA RD ELECTR OMA GNETICS

Figure 5-4

U ninsulated Cargo Hook

Figure 5-5

Insulated Cargo Hook

SHIPBOARD ELECTROMA GNEIC RA DIA ION HAA RDS

Figure 5-6

4.

5.

191

Insulator Links in Tiedown Hardware L inkage

Opera tional P rocedures- I n some instances an RF bum hazard can be eradicated only through the use of operational restrictions . These i nclude operati ng transmitters at reduced power output levels , use of alternative operati ng frequencies , avoiding simultaneous use of transmitting an­ tennas , and avoiding HF transmission duri ng various deck acti vities such as cargo handl ing or replenishment at sea. Effective operational procedures usual l y can be formulated only after carefu l tests and ana­ lyses. B urn G u n Measurements - A fter considerable investigation and exper­ i mentation over many years , the N avy successfully deve loped a test i nstrument commonly referred to as a bum gun to detect potentially hazardous RF bum voltages. Integral to the i nstrument i s a meter that indicates the RF voltage between a metallic object under test and the hand of the individual holding the gun. Ideal ly the voltage level reg­ istered i s a good indication of whether a person would receive a bum if the object under test were touched. In real ity , however , whether the measured voltage w i l l cause a bum i s l argely dependent upon the imped­ ance of the circuit bei ng tested. The impedance can be compared to an intenal power source impedance that determines the ability of the circu it to sustain the voltage and to deliver suficient power to produce a bum. The bum gun has proven to be a reasonably good i ndicator of bum

192

SHIPB OARD ELECTR OMA GNEICS

t

Figure 5-7

U se of B u m Gun to Detect RF Voltages

probability in RF hazard surveys conducted aboard ship. It has been of inestimable value in initiating corrective steps to alleviate RF bum situations. Figure 5-7 show s test engineers u sing the bum gun to measure RF voltages on the metal surfaces of a ship weapons director.

5-3

HAZARDS OF ELECTROMAGN ETIC RADIATION TO FUEL ( HERF)

Perhaps nothing strikes fear in the hearts of seafarers like the repot of a ire onboard. News reports many times over have recorded ghastly scenes of unaway ire damage on the decks of aircraft carriers. As recently as the 1 98 2 Falklands C ampaign the public viewed the charred remains of the once s leek British w arship HMS Sheield i ghting desperate ly to stay aloat. B ecause of the concen for ire , ship crew s are frequently and systematicall y drilled in the practice of ire i ghting and safety procedures. Again , as a result of the peculiar nature of multimission operations in a crowded ship, another EMR hazard is present, known as HERF. A l arge part of Navy shipboard practices to avoid the causes of ire is in awareness and preclusion of HERF. The possibility of having fuel vapors ignite accidentall y by metal-to-metal arcing created from high EMR ields aboard ship has been the subj ect of extensive

SHIPBOARD ELECTROMA GNEIC RA DIA ION HAA RDS

193

study and research. 16 The probability for accidents is highest during fue l handling operations that take place near high-power transmitting antennas. Laboratory e xperiments and shipboard tests have shown that , while it is possible to ignite volatile fuel-vapor mixtures by induced RF energy , the probability of occurrence during fue ling procedures is remote. Several conditions would have to exist simultaneously in order for combustion to be initiated: •

• •

A lammable fue l-air mixture must be present within range of the induced RF arcing. The arcing must contain a suficient amount of energy to spark ignition. The gap across which the arc would occur must be on the order of a half­ millimeter.

Knowing that these conditions must exist has led to HERF control practices to reduce the likelihood of accidental ignition: •

•

•

Care in topside systems design to instal l HF transmitting antennas in sites well away from fue ling stations and fuel vents. U se of pressurized fue ling systems incorporating additives to preclude the formation of fuel-air mixtures at 1 atmosphere on aircrat aboard ship. Use of lP-5 fuel in almost all cases for aircraft aboard ship.

S til l , even though the potential for HERF has been reduced by the above practices , it is yet present when handling the more volatile fuels aboard ship such as lP-4 , aviation gas (A VGAS) , and motor vehicle gasoline (MOGAS). When handling these fuels , personnel must be made ful l y aware of EMR hazards and the importance of fol lowing safety precautions.

5-3.1

The Nature of HERF Combustion

U nder normal operating conditions the handling of gasoline does not pro­ duce a lammable atmosphere except close to vents , at open fue l inlets , or close to spilled gas. When air moves , as with wind across the deck in nearly all cases of ships under way , the fuel vapor is diluted and rapidl y dispersed , greatly reducing the possibility of ignition. The lammability of fuel s is also inluenced by the fue l temperature. If the temperature is too high the hydrocarbon vapor content is likewise too high (i. e. , too rich a mixture) for good ignition. If the temperature is too low , the hydrocarbon vapor content is too little (i. e. , too lean a mixture) to support good combustion. Therefore , each fue l has a characteristic range of temperature , that is , a lammable hazard range , where the vapor-air mixture is best suited for combustion. Approximate typical high combustion temperature ranges for some of the fuels used aboard ship are: •

AVGAS:

-

40° to

+

1 00F

1 94

SHIPB OA RD ELECTR OMA GNEICS

• • •

JP-4: Kerose ne: JP-5:

- 40° to + 1 1 0° to + 1 30° to

+

70°F + 1 65°F + 2 1 0°F

Naval laboratory tests have concluded that arc e nergy is a determining factor for ignition of fuel vapors , and a threshold value of 50 volt-amperes is needed to cause gaso l i ne to ignite. U si ng t hi s criterion , measure ments have been made for v arious fuel i ng scenarios , re lating the 50 volt-amperes to electric ield i ntensity , rad i ated power , and d i stance from the EMR source antenna. From thi s work a general gu idance graph has been derived using a typical H F conical monopole transmitt i ng antenna, as shown in Figure 5 - 8 (from [ 1 4 ] ). Lab experi ments also have determ i ned that a m i n i mum spark gap of about 0.02 i nch ( one-half m i l l i meter) i s required for ignition of a fue l-air m i xture. In the case of shipboard fuel i ng operations , metal-to- metal contact would have to be abruptly separated (making and breaki ng of contact) to create tiny half­ m i l l i meter spark gaps i n a h igh i ntensity E M R ield to draw a spark of suficient length and e nergy to ign i te fuel vapors . Consequent l y i t i s extremely important to ensure that static ground w ires , tiedown cables , and other meta l l ic connections to aircraft , vehicles , and apparatus are properly made before fue l ing ( or defue l i ng ) operation s , a n d that the con nections are n o t d i sturbed until after t h e completion of the operat ions. 5-3 . 2

S h i p board Fuel ing Precautions

Although the total elimination of all EMR arci ng hazards to fuel may not be achievable aboard ship without pl acing unacceptable restrictions on light operations and ship m i s sion s , the fol lowing practices are recommended to m i n ­ i m i ze the ri sk of accidental ignit ion : a . Ne ver energize tran smi tters on aircraft or vehicles i n the vic i n ity of fue l i ng operation s . b . Ne ver make or break any e l ectric al , stat ic ground w ire , or tiedown con­ nection , or any other metallic connect ion , to aircraft , veh icles , or apparatus duri ng fue l i ng operat ion s . Make con nections be fore ; break them ater­ wards . c . Tum off al l radars capab le of mainbeam i l l u m i nation of fue l i ng areas , or i n h i bit the m from irrad iating the area by u se of radiation cutout devices during fue l i ng operations . d . Do not energize H F transmitting ante nnas with i n the quadrant of the ship in wh ich fue ling operations are being conducted.

1 95

SHIPB OARD ELECTROMAGNETIC RA DIA ION HAA RDS

1 00 90 80 70 60

��

A L GU I DA N C E CURVE I N D I CATI N G IGE N E RP?!�NT I A L _F � � L I �� _H� � D

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.2

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o

200 ( 6 0M )

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CURVE

DE N O T E

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I S FO R G E N E R A L G U I D A N C E P R EC I SE

AREAS

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D I S TA N C E I N F E E T A N D M E T E R S F R O M A N T E N N A

Figure 5-8

General Guidance Curve I ndicating Potential Fue l i n g Hazards

1 96

SHIPB OA RD ELECTR OMA GNEICS

5-4

HAZARDS OF ELECTROMAGN ETIC RADIATION TO ORDN A N C E (HERO)

High-power EMR ields in naval ships create yet another potential hazard due to the sensitivity of some forms of electrical l y actuated explosive s , propel­ lants , and pyrotechnics. The hazard e xists on virtuall y all w arships but is perhaps most wori some on aircraft cariers because of the necessity to frequently am and disarm planes with a wide variety of ammunition , bombs , missiles , and rockets. HERO results from the inherent nature of e lectrically initiated i ring mech­ anisms known in naval parl ance as e lectroexplosive devices , or EEDs. The H ERO problem occurs because EEDs are susceptible to being accidental l y set off or having their re liability degraded , by e xposure to RF en vironments. The suscep­ tibility has been found to be most critical during ordnance handling , loading , unloading , assembly , and disassembly operations. Because of the concen for HERO , and for the safety of personnel under all shipboard conditions , the Navy has for many years sponsored an e xtensive testing program to determine the susceptibility leve l s of its ordnance to various forms of EMR. The tests are performed in simulated maximum RF environments which the ordnance and ordnance systems are likely to encounter , from stockpile conditions to l aunch seque nce. From the tests , data are col lected to classify ordnance su sceptibility and to recom mend proper safety precautions. N avy tech­ nical manual OP 3 5 65 Vol ume II prescribes the operating procedures and pre­ cautions necessary for the safe handling , transporting , and storage of ordnance , and to prevent the premature initiation of EEDs in all situations in which e xposure to EMR may exist. 1 7 The following discu ssion is a generalized summary of OP 3 5 65 phil osophy. The reader should refer to the technical manual for speciic detail s regarding ordnance type s , exposure limits , and minimu m safe HERO distances .

5-4 . 1

HERO C l assiications

B ased upon the degree of EMR susceptibility three categories of HERO have been established : H ERO S AFE , HERO S U SCEPTI B LE , and H ERO U N ­ S AFE ordnance. a.

Ordnance-Items of ordnance that are suficiently shielded or protected so as to be negligibly susceptible to EMR e ffects and that require no special RF environmental restriCtions. b. HER O S USCEPIBLE Ordnance- Items of ordnance that are moderatel y susceptible t o E M R effects and require moderate RF environmental re­ strictions to precl ude j eopardizing safety or reliability. HER O SA FE

SHIPBOARD ELECTROMA GNEIC RADIA ION HAARDS

197

c . HERO UNSAFE Ordnance-Items of ordnance that are highly susceptible to EMR effects and require severe restrictions for some or all phases of employment . It is to be stressed that assembly or disassembly of ordnance, or subjecting ordnance items to unauthorized ' conditions and operations, can cause HERO S AFE ordnance to become HERO UNSAFE . 5-4 . 2

HERO Controls in Port and Territorial Seas

Several agreements have been reached between the U nited S tates and other nations with respect to preventing HERO accidents when ships are visiting ports or steaming in territorial seas ; e . g . : a . All operations involving H ERO S U SCEPTIBLE and HERO UNSAFE· ordnance must be curtailed while in port or in territorial seas . b . While sailing territorial seas, a ship must maintain a distance of 1 ,000 yards from shore-based radio and radar transmitters and from radio and radar transmitters on oil or gas drilling rigs . Should it become imperative to go in closer than 1 ,000 yards, only HERO SAFE ordnance may be exposed. c. While visiting foreign ports, ammunition and EEDs that are HERO S U S­ CEPTIBLE or HERO UNSAFE must be protected at all times from ex­ posure to EMR, either by stowage below decks in metal ships or by stowage in shielded closed containers . d . Where stricter national regulations than those above exist, the stricter regulations must be adhered to . 5-4 . 3

Shipboard HERO Controls

Through many years of experience and tests, the following general guide­ lines have been developed to reduce the risk of HERO: a . During the time that an aircraft is being armed or disarmed, its radio and radar equipment must be tuned of. If there are other aircraft in the vicinity of the loading area that are capable of radiating hazardous EMR ields, it must be ensured that these aircraft do not transmit RF energy within safe HERO separation distances . If transmitter equipment in the loading area must be operated for maintenance purposes, it must be ensured that the transmitter is connected to a dummy load antenna . b . A separation of at least 1 0 feet must be maintained between any shipbord transmitting antenna and all ordnance, including HERO S AFE ordnance . For HERO S USCEPTIBLE and HERO UNSAFE items, greater separation distances are required (see [ 1 7] for speciic criteria) . The safe separation

J 9�

SHIPB OARD ELECTR OMA GNEICS

zones apply not only to the ordnance item itself but to any mechanical structure or obj ect to which the ordnance is attached , such as a gun mount , or aircraft , or a missile launcher. There are , however , three exceptions which do allow the collocation of shipboard transmitting antennas , ord­ nance items , and ordnance systems within distances less than 1 0 feet: I.

2.

3.

When , regardless of frequency , an antenna is radiating less than ive watts average power , then HERO S AFE ordnance may be located up to ive feet from that antenna. When an antenna is radiating two w atts or less average power at fre­ quencies greater than 1 00 M H z , then both H ERO S AFE and HERO S U SCEPTIB LE ordnance may be located u p to ive feet from the an­ tenna . When al l loading procedures have been completed , an aircraft with HERO SAFE ordnance may be parked up to ive feet from the vetical projection of a lowered deckedge transmitting antenna . D uring actual loading operations , however , the aircraft must be no closer than ten feet from the vertical projection of the lowered antenna as shown in Figure 5 - 9.

Figure 5-9

Example of H ERO S A FE Distance s on Aircraft C arrier

c. A l l ordnance operations must be planned so that there is a minImUm e x posure of EEDs to the RF environment. Intenal w iring and iring circuits

SHIPB OARD ELECTROMA GNEIC RADIA ION HAA RDS

199

must never be exposed to the RF environment by assembly or disassembly. A l l HERO UNSAFE ordnance must be transported i n completely enclosed metal containers wherever possible. Igniters , primers , detonators , and other items containing EEDs such as electrical ly ired rocket engines , gu ided missile motors , and electronic or electrical fuzes must never be stored together in the same compartment or magazine w ithin ive feet of RF cables , waveguides , or any other radiating or transmitting equipment. Moreover , these items should be stored in metal containers. d. Electrical contacts , electrode primers , and contact pins must not be al lowed to touch any object capable of conducting RF energy during ordnance handling and loading operations. Objects capable of conducting RF energy include aircraft structures , bomb rack breeches , cartridges , and tools. Elec­ trical connections to air-launched ordnance systems must not be made before the ordnance is racked to the aircraft. Electrical connectors to ord­ nance systems are the most l ike ly paths for RF energy to enter. Racking an ordnance item to the aircraft irst and tightening the sway braces before making electrical connections reduces the amount of RF energy induced i nto the intenal circuitry of ordnance items. U mbilical cords and cable connections should be handled only when absolutely necessary. A l l open electrical connectors on ordnance must be covered with nonshorting caps to prevent the pins of these connectors from being touched accidentally. The caps should be removed just prior to connector mating and rei nstalled promptly upon disconnection. e. When handl ing ordnance in the v icin ity of HF transmitting antennas during dockside replenishment , al l loading hooks and metal steering lines must be insulated from cranes , booms , and w ires by the use of nonconductive rope or insulators. During connected replenishment ( CON REP) when phys­ ical contact between the ships has been made with metal cables , ship HF transmitters must not be pem itted to transmit energy while HERO S U S­ CEPTIBLE or HERO UNS AFE ordnance is present on any weather deck. To ensure HERO safety during CON REP ordnance operations , both ships must operate under emission control (EMCON ) conditions. f. It is possible that , when conducting ve\tical replenishment ( VERTREP) while under way , helicopters may ly thro,u gh high intensity mainbeams of radars. If HERO SAFE ordnance is being transferred , a 50-foot sepa­ ration must be maintained between the ordnance and any radiating antenna. If the ordnance is c lassiied as either HERO S U SCEPTI B LE or HERO UNSAFE , but is enclosed within an al l-metal container , it can be consid­ ered HERO S AFE during VERTREP transfer. HERO S U SCEPTI BLE ordnance may in some cases be transferred outside of containers as long as minimum safe HERO distances are maintained (see [ 1 7 ] ) .

SHIPBOA RD ..L E

20

TR OMA G

I..TI

g.

During fl i ght deck ope rat ions H E RO U N S A FE ordnance mu t not be er­ m i t ted on the l i ght deck u n le appropriate EM CON cond i t ion are i n ­ voked . A l l ai rcraft rad io and radar transmi tters m u s t b e off w h i le the pl ane is be i ng loaded or u n l oaded . If other ai rcraft in the load i ng area are capable of rad i at i n g hazardous RF ields they must be proh i b i ted from trans m i tt i ng energy , o r , i f energ i z i n g i s i mpe rat ive for mainte nance reason , the eq u ip­ ment must tran s m i t i n to dummy load antennas . It must be ensured that no RF ields e xceed the max i m u m a l l owable e n v i ronment for H E RO . h . HERO U N S A FE ordnance i s not permi tted on hangar deck s ( w hether hangar doors are ope ned or c losed ) u n le s s appropri ate EMCON conditions are i n voked . EMCON restrictions on H ERO S U S C E PT I B LE ordnance i n hangar bays are t h e same a s those i m posed on l i ght decks for H F tran s­ m i tters . However , operation of aircraft transmitters into dummy l oad an­ tennas is perm i tted . During CON REP, when ph ysical contact has bee n made between ships b y u s i n g metal cables which e xtend i nto the hangar bay , unrestricted operat ions on HERO S U SCETI B LE ordnance i s not perm i tted on the hangar dec k . 1 . B ecause of the e xte nsive amount of high-power commun ications equ ipment instal l ed on maj or command ships and on commun ications re lay ships , u n iq ue H ERO problems can ari se when these ships approach within 24 , 000 feet of other naval vesse l s . Conseq uent l y , the fol lowing precautions must be observed : 1 . When operating w ithin 24 , 000 feet of other ships , H ERO req u i rements must be coordinated w ith those ships to con i m that no H ERO U N S AFE ordnance is present on weather decks or hangar decks ; otherw i se , EM­ CON conditions are to be i nvoked . 2 . When w ithin 3 , 000 feet of another sh i p , H ERO EMCON conditions are req u i red . Radars operati ng at frequencies greater than 1 . 0 G H z should be prevented from d i rect l y i l l u m i nating ordnance or any metal l ic object or stucture attached to the ordnance when w ithin the m i n i m u m safe HERO d i stances . I f H ERO S U SCEPT I B LE ordnance w i l l be i n the mainbeam of a radar and i n s ide the m i n i m u m safe HERO separation d i stance , the radar must be shut dow n . Radars operating at frequenc ies less than 1 . 0 GHz must be tuned off whenever susceptible ordnance w i l l be within the minimum safe HERO separation distance . For the case of commun ications equ ipment rad i ation ields , the safe di stance ield strengths for H ERO S U SC E PT I B LE ordnance can be detemined from Figure 5 - 1 0 . k . HERO U N S AFE ordnance c an be protected from EMR by p l ac ing i t i n a complete l y enc l osed al l - metal contai ner . When exposure of H ERO U N ­ S AFE ordnance c annot b e avoided , it should b e exposed on l y be low decks

J.

201

SHIPBOARD ELECTROMA GNEIC RADIA ION HAARDS

2

1000

"

�

. 0

" � " > z " I: ' J . .

100

: �

i

J : � ) 0 �

!! .

U

: � U J � J

10

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!

� : J >

10 FREQUE NCY - M E G A H E R T Z ( COMMUN I C AT I O N S T R A N SM I S S I O N S )

Figure

100

1 0 00

5-10 Maximum Safe Field Strength for HERO S U SCEPTIBLE Ordnance

in an RF-safe area. It must never be permitted on weather decks unless appropriate EMCON conditions are invoked. HERO S AFE and HERO UNS AFE ordnance can be classiied as HERO U N S AFE by the following: I.

Assembling or disassembling of ordnance systems undergoing repairs , upkeep , or parts exchange. 2 . Testing , such as resistance of continuity checks , using electrical con­ nections to ordnance items. 3 . Exposing unshielded or uni ltered wire leads of primers , blasting caps , impul se cartridges , and other EEDs. 4 . Exposing unshielded ordnance subassemblies such as rocket motors , warheads , exercise heads , and fuzes.

5-4.4

Shipboard HERO Su rveys

The EME of a ship changes with new or modiied radar , EW , HF com­ munication , and navigation transmitter installations. The environment also changes

SHIPB OARD ELECTR OMA GNEICS

202

sign i icantly with changes to ordnance conigurations , inventories , and opera­ tions. B ecause of these environmental changes , the Navy has determined that shipboard H E RO surveys s hould be conducted evey ive years or whenever a major change occurs i n emitter s u i te or ordnance allocation. H ERO survey teams are trai ned and equ i pped to perfom on-site measurements of the RF environment in ordnance operations areas to determine the spec i i c H ERO safety measures req uired for handl ing , storage , and transpot of ordnance items. HERO surveys are performed in re sponse to requests from sh ips. The process beg i n s w ith a presurvey data anal ys i s . An on-site survey is then con­ ducted , and the re sults are anal yzed for conformance to establ i shed safety and re l iab i l ity criteria i nc l ud i ng proper post i n g of standard i zed H ERO wan ing s igns i l l ustrated in Figure 5 - 1 1 . The survey is com p leted w ith the preparat ion and subm i s s ion of a detai led repot w h ic h prov ides the survey indings , anal y s is results, conclus ions . and recommendations . Th is reot becomes the single source of sh i p - s pec i i c tec h n ical data to suppot the i nd i v idual sh i pboard H ERO EMCON re striction d i rect i ves ; i . e . , the so-ca l led H E R O EMCON B ILL . There fore , by perform i n g measure me nts of the EME in a most-to - Ieast order of hazard potential , the overal l re su lts are assessed to re l ate best to the c u rent and future safe ord n ance ope rati ons for the sh i p .

HAZA R D TO

O R D NA N C E

R A D I O "REOU ENCY HAZARD

CHECK WITH COMMAND AUTHORIY B E " O R E P R OC E E D I N G

POINT

F i g u re 5 - 1 1

H ERO

Warn i ng Symbo l

SHIPBOA RD ELECTROMA GNEIC RA DIA TION HAA RDS

203

REFEREN CES I.

2.

3. 4.

5. 6. 7.

8. 9. 10. I I.

1 2.

13.

14.

R . Foster and A . W . Guy , "The M icrowave Prob lem , " ' Scietiic A merican , Vol . 255 , N o . 3 , September 1 98 6 , p . 3 2 . A . W . Laine , " Electromagnetic Radiation i n Prolonged Space Environ­ ments , " Inteference Technology Engineers Master, Robr Industries , West Conshohocken , PA , 1 985 , p. 1 5 2 . E. J . Lener , " B iological Effects of Electromagnetic Fields , " IEEE Spec­ trum , Vol . 2 1 , No . 5 , May 1 984 , p . 5 7 . Radiation Hazards Handbook , Depatment o f Defense ECAC - H D B K- 86005 , Electromagnetic Compatibil ity Analysis Center , An napol i s , Decem­ ber 1 98 6 , p. 2- 1 . Lener, op cit . , pp . 59-60 . M . G . Morgan e t aI . , " Powerl ine Fields and H u man Health , " IEEE Spec­ trum , Vol . 2 2 , N o . 2 , February 1 985 , p. 62 . L . E . Pol isky , " The Commerc i al Appl ication of Non-Ion i z i ng Electro­ magnetic Radiation Hazard Le vel Standards , " EMC Expo 8 6 Symposium Record , Intenational Conference on Electromagnetic Compat i b i l ity , Washington , DC , J une 1 98 6 , p . T24 . 4 . Lener , op . cit . pp . 5 8 - 5 9 . Foster and Guy , op . cit . p . 3 2 . Ibid . , p . 3 3 . " Personnel Protection Pol icy for Exposure to Radio- Freq uency Radia­ tion , " OPN A V N OTE 5 1 00 , Department of the Navy , Washi ngton , DC , 30 July 1 985 . N . T . B aron , " A New Radio-Frequency Radiation Criteria for the U . S . Navy , " EMC Expo 86 Symposium Record , Intenat ional Conference on Electromagnetic Compatibi l i ty , Washi ngton , DC , J u ne 1 986 , p . T24 . I I . M . Z. Netzer, " Erroneous Measurements of Stray Magnetic Radiation . " Inteference Technology Engineers Master , Robar I ndustries , West Con­ shohocken , PA , 1 986 , p. 2 26 . K.

Electromagnetic Radiation Hazards to P ersonnel , Fuel , and Other Flam ­ mable Materia l ,

15. 16. 17.

Techn ical Manual N A V S EA OP 3 565 , Vol . I , Depat ment of the Navy , Washi ngton , DC , 1 5 July 1 98 2 . Ibid . , p . 3 - 2 . Ibid . , p . 7 - 1 . Electromagnetic Radia tion Hazards to O rdnance , Technical M anual N A V ­ SEA O P 3 565 , Vol . I I , Department o f the Navy , Wash i n gton , DC . 1 August 1 986 .

Chapter 6 Shipboard Electromagnetic Pulse (EMP)

6-0

PREPARATION FOR AN EVENTUALITY

We now tum our attention to a most unusual electromagnetic phenomenon, one that is of extreme concen to shipboard electronics but which actually has never been experienced by naval ships except in simulated low-level testing. The phenomenon is electromagnetic pulse, or E M P. So high is the potential for ham done by E M P that one news columnist, while acknowledging that it " is still no more than a scientiic theory mercifully untested, " "awesome, " and a "forbidding new destructive force. " The columnist went on to say: But what EMP means to the rest of us is simply this: If nuclear weapons were to be detonated 200miles above the United States, the electromagnetic pulses from the explosion would almost instantaneously knock out all the electrical power in North America. No television, no radio, lighting, hos­ pital equipment, computers, telephones . Total blackout of the entire con­ tinent . . . . What worries our [military] strategic thinkers, though, is that E M P might be used to knock out America's top level C3 system-command, control, and communications-that is supposed to respond to a nuclear strike with a retaliatory attack. I The news report quoted above is alarming, and, of course, was written in a manner precisely to raise alarm. It is not however, overstated. The analysis is accurate and the concen is genuine-for military, civil, and commercial inter­ ests . The potential for widespread disruptive effects resulting from E M P has been known for more than 2 0 years. In fact, one of the irst public reports appeared in the autumn of 1967, where, in an electronics trade jounal, it was noted that: 205

206

SHIPBOARD ELECTROMAGNEICS

During the high altitude nuclear tests in the Paciic in the early 1960s, " hundreds of burglar alarms " in Honolulu began ringing . " Circuit break­ ers on the power lines started blowing like popcon . ' , 2 Because there were no electrical storms anywhere in the vicinity it was soon determined that intense electromagnetic energy radiated from a high-altitude atomic test 8 00miles from Hawaii had created the unusual disturbances . Scientists conducting the tests were aware of strong electromagnetic efects while observing the overload of sensitive measurement instruments and the upset of communication links. It is only in our modem era of more sophisticated means of deploying and detonating high-yield nuclear devices so as to cause massive, deliberate upset of delicately vulnerable solid-state electronic systems , however, that EMP has been recognized correctly as a " forbidding new destructive force . " One has only to imagine the chaos that would result from electrical shutdown of the highly computerized commercial sector of our society in banking , tele­ communication, power utilities, stock exchange , mass transportation networks , and medical facilities, all from some unseen, unannounced, mysterious electro­ magnetic force from a far-off , otherwise harmless nuclear explosion . A threat of such severity and magnitude cannot be lightly regarded . It has prompted much study and analysis, especially over the last decade . We hope that no society will ever have to experience EMP from a nuclear weapon ex­ plosion. Nevertheless, so long as we endure in an imperfect world we must be fully prepared for the eventuality. Indeed, techniques to harden electronic systems (and, for our purpose, ships) against the effects of EMP are being devised and implemented. 6- 1

EMP CHARACTERISTICS

It is important to be clear about what we mean by EMP. The generation of electromagnetic pulses is, in the broadest sense, a routine occurrence in many ordinary types of electronic systems. A familiar example is the use of radar transmitters to produce narrowband pulsed electromagnetic energy which is pur­ posely radiated outward to search for and track selected targets. The term EMP as generally accepted in the engineering community, however , is not the gentle pulses of energy created in myriad fashion by electronic circuitry and systems , no matter how complex or high in power level or short in duration . Rather , EMP is widely understood to mean an extremely intense, highly threatening , instan­ taneous, wideband pulse of electromagnetic energy originating from a fearful source: a nuclear explosion. To leave no room for doubt of its origination , some scientists and engineers prefer the more precise term nuclear electromagnetic pulse (NEMP). At the present time, however , EMP is still the more commonly used and recognized short form. Therefore, it will be employed exclusively hereafter in our discussion.

SHIPBOARD ELECTROMAGNEIC P ULSE

207

As depicted in Figure 6-1 , there are four basic regions in which electrical and electronic systems may be subjected to the effects of EMP: at or near ground level, in the lower atmosphere, in the upper atmosphere, and at exoatmospheric altitudes. Since our particular interest is in what might happen to shipboard systems, our attention is focused on the effects at ground level.

EXOA TMOSPHERE

Figure 6-1 System Operating Categories

Going a step further, nuclear explosions may be similarly classiied as one of three types: surface, air, or exoatmospheric. Surface and near surface bursts occur nominally at heights of ground zero to about 2 kilometers. Air bursts take place between approximately 2 and 30 kilometers, and exoatmospheric explo­ sions are those which happen above 30 kilometers. Exoatmospheric detonations are frequently referred to as high-altitude EMP, or HEM P. Damage caused by nuclear explosions is a function of weapon size (i.e., yield) and proximity to vulnerable systems. The principal burst effects are blast, heat, shock, and ionizing radiation of neutrons, x ray..acd gamma rays. Should the burst occur near the earth ' s surface or in the low atmosphere in the general vicinity of a ship, the physical damage would be overwhelming, resulting in local devastation beyond the scope of our interest in the effects of EM P. Con­ sequently, it is nuclear detonation in the exoatmospheric region that is of concern to us. It must be assumed that exoatmospheric nuclear bursts are a favored weapon option as they have the potential for dramatically affecting electrical and electronic systems from a very great distance, severely disrupting these systems without doing a pinch of other damage; i.e., in the absence of any of the other nuclear effects such as shock, heat, blast, or ionized radiation. 6-1.1

High-Altitude EMP Generation

Figure 6-2 is an artist 's conception of a nuclear explosion occurring high above a naval leet. Note, however, that such an explosion should not be per-

208

SHIPB OA RD ELECTR OMAGNEICS

ceived as always the result of an enemy attack . It could happen as well from detonation of one of our own, or an ally's defensive weapons; it could be from a nuclear engagement between third-party nations, or, conceivably, even from a nonaggressive high-altitude test in violation of current test ban treaties . The resultant effects on unprotected electronic systems nevertheless would be the same, irrespective of the reason for initiation of the burst . As a matter of fact, it is one of the subtleties of E M P that the immediate reason for and the location of a nuclear detonation may be dificult to discen or predict accurately . Yet it is a reasonable assumption certainly that the motivation for exploding a high-altitude nuclear weapon is to generate a pulse of energy of such intensity as to upset or disable susceptible electronic systems, including those aboard 3 naval warships, over a very large geographic area . In the shipboard case, moreover, it would be unlikely that the burst would take place directly overhead (as suggested in Figure 6-2) because the same destructive effects could be achieved if the explosion occurred from far away.

Figure 6-2 Conceptual Illustration of Nuclear Burst

SHIPBOARD ELECTROMAGNEIC P ULSE

209

Upon explosion at high altitude, all the emitted nuclear products spew radially outward from the burst center. Most are dissipated in the thin exoat­ mospheric medium and outer space. Those directed toward the eath, however, quickly encounter the lower atmospheric regions where the remaining products, except for EMP originators, are effectively absorbed. Figure 6-3 illustrates this event. When gamma rays from the explosion meet the atmosphere they interact in such a manner as to create electromagnetic energy in a process of physics known as the Compton Effect. By this process the newly generated energy is propagated as an electromagnetic ield over great distances from the source.

EMP Generaton

Figure 6-3 EMP Generation

The Compton Effect, essential to the creation of EMP, is described as follows: 4 Gamma rays (and, to a much lesser degree, x rays) emanating as photon energy from the explosion reach the atmosphere and begin colliding with air molecules and dust particles. The collisions are of such force as to dislodge and scatter electrons from the molecules. The ejected electrons, now known as Comp­ ton electrons, are accelerated predominately in the former direction of the gamma rays; i.e., toward the earth's surface, as pictured in Figure 6-4. This process of separation of charge produces an electric ield, and the electron movement con­ stitutes an electric curent, with an associated magnetic ield. However, the process has not yet created classic electromagnetic radiation.

210

SHIPB OARD ELECTROMA GNEICS

COMPTON ELECTRON

GAMMA RAY FROM BURST

SCATTERED GAMMA RAY

Figure 6-4 Compton Scattering Process

The chief mechanism which acts to produce a radiated ield is the delection and twisting of the Compton electrons as a result of the interactive force of the earth's magnetic ield. Modiied by this geomagnetic ield , the Compton electrons begin to follow a spiral path about the magnetic ield lines , as depicted in Figure 6-5. Now possessing both magnetic and electric vector components that vary with time, the electrons , moving as a coherent composite , have been eficiently converted in energy to electromagnetic radiation . The radiated ields are ex­ tremely high in intensity , have a broad frequency spectrum , and, because of the height and extent of deposition , instantaneously cover a very large area of the earth ' s surface. Because of the highly specialized nature of the radiated ield it is quite properly characterized as EMP.

GAMMA RAY

Figure 6-5 EMP Radiation Field Generation

SHIPB OA RD ELECTROMAGNEIC PULSE

6-1 .2

211

High-Altitude EMP Electrical Properties The far-re ac h i ng c o n s e q u e n c e s of a h i gh-a l t i t ude nucl ear exp l os i o n are

i m me d i a t e l y appare n t from Figure 6-6. If a I-megaton n uclear bomb were det­ on ated at appro x i mate l y 300 m i l e s above t h e ce nter of t h e Un i ted St ates. the e n t i re n a t i o n w o u l d s u ffe r t h e e ffect s of EM P with l i tt l e or no ot h e r i n dicat i o n that a n u cle ar b u rst h a d occ ured. L i ke w i se , i f t h e exp l o s ion h appe n e d ove r a l arge body of water s u c h as t h e I nd i a n Ocean or Medi terranean Se a , a l l sh i ps w i t h i n a very l arge area wou l d be affected .

Figure 6-6 E M P Gro u n d Coverage for High-A l t i t ude B u rsts

The rad i u s , R T , fro m source b u rst po i n t to suface tange n t poi n t, and the total area of coverage , AT , are eas i l y determ i n ed from F i g u re 6-7. It i

e v ide n t

t h a t by c o v e ri n g a n area of several m i l l i on s q u are m i l e s , t h e geograph ic range o f EMP e ffects e x t e n d s many orde rs of m ag n i t u de beyond any ot he r n uclear effec ts . Th i s i s t h e major re ason t h at e x oatmospheric e x p l o s i o n s must be an t i c­ ipated . But equal ly i mpo rtant i s the nat u re of the pu l se i t e l f. A l t h o u g h some t i mes l ike ned to t h e e n e rgy in a l i g h t n i n g stroke , EMP is actual l y qu it e d i ffe re n t from any other n a t u ra l or man-m ade e l ec t ric phe nome non. The spe c t rum for EMP i� broadband , e x te n d i n g fro m e x t re m e l y l ow freq u e n c i e s to very h i g h frequ e ncie�. and the pu l se ha a muc h h i g h e r ampl i t ude and faster ri.e t i me t han, fo r exampl e,

SHIPBOARD ELECTROMAG

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a nearby bolt of lightning. While the exact characteristics of EMP are complex and depend upon weapon size, height of burst, and atmospheric conditions, the following properties are considered representative: 5 a. Field Intens iy - Based on free space impedance calculations, EMP energy can reach a peak ield strength of up to 100kilovolts per meter with H­ ield intensities of over 25 0amperes per meter. b. Frequency Spectrum-EMP occupies a broad bandwidth with damaging effects from 10kHz to 100MHz and peak intensities between I and 1 0 MHz. As such, the spectral content of EMP incoporates the frequencies used by a great many commercial and military electronic systems. c. Waveform-High-altitude EMP, as represented in Figure 6-8 (from [3]) , has a sharp rise time of a few nanoseconds and a duration (effective pulsewidth) of a couple of microseconds. d. Polarization-EMP generated from a high-altitude nuclear explosion is propagated downward from the source region in a horizontally polarized plane wave. Local polarization depends on latitude and longitude of the burst and relative location of the sensor. Therefore, EMP energy is emi­ nently suited for interception and collection by large vetical and horizontal bodies of metal, such as a ship hull, and many metallic items on the hull like masts, lifelines, fan antennas, cables, waveguides, pipes, and ducts.

213

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From these unique characteristics it can be appreciated that EMP radiati.n, alth.ugh brief in existence, is billi.ns .f times m.re intense than an ordinary radi. signal. 6-2

SHIPBOARD EMP DAMAGE EFFECTS

Metallic .bjects exp.sed t. an electromagnetic ield will serve as recept.rs .f radiated energy. That is, they will act as a rudimentary f.rm .f receiving antenna even th.ugh they are never intended f.r that purp.se. Generally, the larger the metallic structure, the greater the am.unt .f c.llected EMP energy. Naval ships, .bvi.usly, are very large metallic stuctures. When EMP impinges up.n a ship, s.me .f the energy penetrates directly t. bel.w-deck c.mpartments through hatches, d..rways, wind.ws, hull gaps, and seams. M.st .f the received pulse, h.wever, is transferred t. interi.r electronic systems by siipb.ard antennas (via ass.ciated transmissi.n lines and waveguides), extenal cables, pipes, ducts, and c.nduits, whence it c.uples t. wiring, cabling, and eqUIpment appendages Dr passes through encloure apertures and p..rly shielded barriers in equipment t. inlict (Lsuen surge .f high ringing current like that .f Figure 6-9 .n sensitive electronic circuits. Figure 6-10 symb.lically p.rtrays s.me .f the many paths by which EMP can gain access t. interi.r equipment in a ship.

2i4

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) WINDOWS

SHIPBOARD ELEC TROMA GNEIC P ULSE

215

Electrical and electronic systems are disrupted by EMP in one of two fundamental ways: either through physical damage of discrete components or by circuit upset. In the case of component damage, the usual effect is bunout of microminiature solid-state devices or other forms of electrical degradation of such severity that an element in the circuit no longer operates within its design parameters. Circuit upset, on the other hand, normally results in loss of data transmission or loss of stored memory. Upset is far more likely an event than physical damage because the energy required to upset a circuit is at least an order of magnitude less than that required to bum out a component. 6 Upset of a system occurs when an induced EMP within the circuit time response exceeds the operational level. Appearing as a false transient signal, the EMP can change the state of a logic element, cause loss of clock and synchronization, or erase memory. Disruption of operations can be so severe, especially in the event of stored memory loss, that reprogramming and reloading of data may be required. As a consequence there may be long periods of system outage. Such disruption to mission-critical operations could seriously impair a warship ' s capability to ight, and is, therefore, intolerable. EMP, although a short-term impulse, can have disastrous long-term effects. Circuit components most likely to be adversely affected by EMP are those with low power ratings and critical operating characteristics where small changes produce signiicant effects. Semiconductors are prime suspects, and since they are so vulnerable if left unprotected, it is well to examine the reasons they fail. The preponderant cause of semiconductor failure is thermal overload, which results in junction melt and a short circuit. Bunout of this type generally happens when the EMP imposes a sudden reverse bias on the junction to drive it into breakdown. Failure can result from forward stressing of a junction, too, but the forward-direction threshold is several times higher because of the low impedance and voltage tolerance offered in forward conduction. Other electronic components are susceptible to EMP disabling to a much lower degree. Resistors can change value when overheated by high pulse power. Capacitors can suffer dielectric breakdown from excessive transient voltage, and such elements as switches, relays, coils, and transformers may experience in­ sulation lashover, arcing at contacts, and melting of wiring. The EMP voltage spike may initiate a momentary breakdown path that, once established, is sus­ tained by normal circuit operating levels. 7 Laboratory experiments have dem­ onstrated that the old electronic vacuum tube circuits were more resistant to damage from EMP effects than are semiconductor systems. The transformation

216

SHIPBOARD ELECTROMAGNEICS

of vacuum tube circuits , which were relatively hard to EMP , to delicate transistors and integrated circuit components, was of course never anticipated during the atomic test periods of a quater-century ago . This transition to microminiature , sensitive , low-power , solid-state electronics has resulted in the dramatic increase in- EMP vulnerability. * 6-3

SHIPBOARD EMP HARDENING TECHNIQUES

The subject of EMP was puposely discussed near the end of this book because the methods used to protect and suppress the effects of EMP encompass most of the practices and philosophy discussed previously, e.g., enclosure shield­ ing, cable shielding, bonding, grounding, isolating (decoupling), and compati­ bility with the shipord EME. The methods fonerly exmined do not necessily all have direct application to EMP mitigation because of the unique and severe nature of EMP; nevertheless, EMP hardening techniques are in many respects evolutions of common EMI suppression practices. By way of testimony to the seriousness of EMP, Pinkston [5] has noted that, although there is growing concen by the commercial electronics community and the public services over vulnerability of their systems, it is the aned forces that have responded to the potential threat with immediate action: "EMP is the most consistently speciied environment in the nuclear hardening of military electronic equipment." The main thrust of this action is to provide adequate protection. It is imperative that the nation's defense systems be suficiently hardened against failure caused by such events as logic circuit upset of missile guidance control or interuption of crucial command and control coordination by the bunout of input tages of, say, shipboard communication receivers. The goal of shipboard EMP protection is to prevent the pulse energy from 8 entering areas containing susceptible equipment and systems. This requires effective shielding or isolation of the equipment from the extenal EME, and, at the same time, the use of less susceptible electrical and electronic systems. The all-metal con tuction of ships with thick steel plating and the technique of using continuous-weld seams would appear to provide a near-ideal EMP shield. But the many hull and superstructure penetration required for nomal ship functioning-the antenna transmission lines and cables, ducts, dooways, windows-degrade shielding effectiveness. The manner in which interior com­ partments of a ship are fashioned also affords a good degree of additional shield­ ing; these interior spaces too must have openings and intrusions, however, which reduce the overall shielding integrity. The essence of providing adequate ship­ board EMP protection, then, is properly to control or treat the many openings *By definition. susceptibility is the ability of the system to detect the threat, and vul­ nerability refers to the inability to survive, given detection of the threat.

SHIPBOARD ELECTROMAGNEIC P ULSE

217

and penetrations in order to take advantage of the inherent quality of a ship's metal structure. For purposes of EMP engineering, system resistance to EMP is classiied as either hard or soft. Hard systems are those that are speciically designed to withstand the effects of a nuclear environment so as to continue functioning nomally. Soft systems are those not designed to operate in a nuclear environ­ ment, so they must be protected by the enclosure in which they are contained. Insofar as possible soft systems should be made intrinsically less vulnerable (less collection of energy and less coupling eficiency) to EMP by the use of harder components. Otherwise, the only reasonable altenative is to keep EMP energy from reaching soft systems; i. e. , to reduce susceptibility. General guidelines for minimizing EMP exposure include: a. Shield the system within a metallic enclosure. Reduce to a minimum the number of apertures and aperture sizes. Bond all seams. Use RF gaskets on hatch covers and doors. Use wire mesh or transparent EMI ilm coatings over windows and viewports. b. Route cables inside the ship stucture, inside masts, and inside conduits to the maximum extent possible. Use as few and as short cables as possible. Employ tightly braided or continuous foil cable shields, terminated at the enclosure periphery with conductive backshells. c. Eliminate ground loops if possible, or keep them at bare minimum by proper grounding practices. d. Isolate sensitive intenal electronics such as microprocessors and memory circuits. e. Use nonconductive interface data lines such as iber optics where practic­ able. Otherwise use highly shielded twisted pair lines and redundant data lines. Fiber-optic cables are immune to EMP coupling, so are prefered. f. Choose least-sensitive electronic circuit components. g. Use ilters on interface lines that will withstand EMP transient energy. h. Use terminal protection elements such as amplitude limiting devices and circuit breakers to shunt or disconnect pulse energy from sensitive circuitry. EMP, as a threat to the overall ship mission, must be considered on a total system basis throughout all phases of design and operation. The two major engineering techniques used for EMP protection are cable shielding and use of circuit protection devices. 6-3.1

EMP Shielding and Grounding

One of the most effective methods of hardening a ship against the threat of EMP is to enforce proper shielding and grounding. Cables in particular mu t

SHIPBOARD ELECTROMAGNEICS

218

be prevented from picking up and transferring energy from exposed topside areas into the ship's intenal compartments . Even the complete closing up of all hull apertures would prove futile if EMP transients were allowed to be conducted freely on cables that penetrate to the inside . If the cables are poorly shielded , the EMP energy will couple directly to the cable inner conductors and thence will low to the ship interior , where it will be applied suddenly to the input of equipment to which the cables are connected . Furthermore , part of the energy will radiate from the cables to cause cross-coupling into other systems not even associated with the original exposed cable . By far the best way to reduce the potential for collection of EMP energy is to shield all topside cables completely; i . e . , house the cable conductors inside a metal shroud . Where possible this should be accomplished by restricting cable runs to the ship interior so as to take good advantage of the innate , though imperfect, shielding characteristics of the hull . For cables that must be routed outside , the use of solid metal conduit or trunks is recommended . Well-grounded conduits and trunks will act to intercept the incident EMP and disperse it harm­ lessly over the external skin of the ship . At all points where the conduit penetrates the hull, it must be welded circumferentially at the point of entry (e . g . , deck and bulkheads) on the extenal side . Cables leaving the main deck must also be enshrouded in conduits as detailed in Figure 6-11. To achieve suficient reduction of the hundreds of RF amperes that may be induced on an outer cable shield from EMP, . st 0 B Lattenuation is 9 needed . The most practical way to keep this curent from being applied to below-deck systems is to shunt the energy to the ship ground at each point where the cables traverse a bulkhead or a deck boundary from topside to interior . 6-3.1.1

Cable Shielding Requirements

Navy requirements specify that all cables routed in shipboard topside areas must be shielded from EMP . Coaxial cables and others having an overall inherent shield must have the shield grounded at deck or bulkhead penetration points to remove EMP energy from the cable prior to its passing to the interior . Cables with an overall solid shield are EMP-protected and require no further shielding . Cables exceeding these provisions of exposure, and all unshielded cables and wires, must be enclosed in a solid conduit pipe, in a lexible conduit, or in a metal trunk, with a cable shield grounded to the enclosure points of entry and exit as shown in Figure 6-12. For cable access, wireway trunks must have removable covers using captive bolts on both sides of the cover, with spacing not to exceed 12 inches to ensure proper metal-to-metal contact of the cover to the trunk . Any nonmetallic boxes or covers used for topside cable connections

219

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or ixtures must be replaced by metallic boxes and covers for proper grounding of conduit. The outer shield of solid overall shielded cables must use the pro­ cedures illustrated in Figure 6-13 to ensure corect grounding at weather pene­ tration points. Cables routed inside the ship structure must not be installed within 12 inches of weather doorways , hatchways , or windows , and must be at least 10 feet from hangar doorways , unless the cables are double-shielded or enclosed in conduit. Cables that terminate at hull openings , e. g. , windshield wiper cables , window deicing cables , and door alarm cables , must be placed inside conduits. 6-3.1.2

Waveguides, Pipes, and Metal Tubes Grounding

All waveguide transmission lines , metal pipes , and metal tubes that transit from topside areas to interior spaces must be grounded at each point of pene­ tration , using the methods of Figure 6-13 for pipes and metal tubes and of Figure 6-14 for waveguides. Pipes , to be considered properly grounded , must be welded 3600 circumferentially at penetration points or be threaded with ittings which are welded at penetration points. 6-3.2

Circuit Protection Devices

Of all the many possible paths for EMP to be conducted into sensItive shipboard electronic systems , there is one that predominates by offering wide open access. Not only is it the least resistant route , it is made intentionally so because its very purpose is to intercept and eficiently to collect electromagnetic energy from the environment. That path , of course , is through the many shipboard antenna systems , and especially through those antennas designed to operate below 100 MHz. Since the highest EMP energy products immediately appear at the antenna terminations , the irst system components that require protection are the base insulators , the matching and tuning networks , and the coaxial transmission lines. EMP energy gaining entry by way of shipboard antennas will , if the transmission lines are left unguarded , impose a sudden transient of excessive level at equipment input stages. To prevent this potentially catastrophic occur­ rence , techniques must be devised instantaneously to provide an alternative path for -surge current low , normally in the form of an immediate shunt to ground , wherever the applied level at the input terminals exceeds a speciied threshold. But the moment the overvoltage ceases to exist , nomal system operation must resume automatically. Moreover , the circuit protective device should not in any way adversely affect the performance of the system being protected.

222

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