Radio Antenna Engineering [1 ed.]

Citation preview

Radio Antenna Engineering EDM U N D A. LA PO RT Chief Bngi11eer, RCA I11tr:matio11ol J)frision Radio Corporation o.f . \ 1nrl'ica ;:!,e1àor Jlcmbcr, l11stit11/c o.f Rodio Enginccrs

}'IRST EDITIOX

McGraw-Hill Book Company, lnc. New York 1952

Toronto

Lomlo11 I

'\

RADIO ANTENNA ENGTNEERTNCT

Copyright, 19,52, by thc McGraw-Hill Book C'ornpnny, Ine. Printcd in thc United Statcs of Arncri('a. Ali rights rcscn-cd. This book, or purts thcrcof, uw:, not be rcproduecd in any form without permission of thc publishcrs. Lihrnry of Congrcss Catalog Card Kumber: i>l-12G2-l

THE ~IAPLE PRESS COMPANY, YORK, PA.

Preface

Antenna engineering has dcvcloped into a highly spt'cializcd field of radio engineering which in turn is subdividcd into many special hranchcs. This treatise will deal with antennas macl_e of wircs, masts, and towers for frequencies up to about 30 megacycles. Antennas for higher frequencies are nowadays factory-designed and factory-built, ancl the opcrating and plant engineers are relieved of the design prohlems. There is a vcry extensive experienre with antennas within our range of interest, but unfortunately there is only a relatively small amount of published materia! on techniques. In contrast, there is a vast literature on antenna ancl racliation theory. It is the purpose of this book to attempt, to compile a sufficient amount of useful engineering information to enable nonspecialists to handle many of the orclinary antenna problems that arise in point-to-point, grouncl-to-air, and military communications, and in broadcasting. Some of the more aclvanced antenna designs suggested by very-high-frequency and ultrahigh-frequency techniqucs are includecl because the day is approaching when these principles will have to be applied at the lower frequencit's as the spcctrum conditions hecome more clifficult. Transmission lines are inseparahly related to antennas, so a chapter on this subject is inclucled, together with a chapter on impedance-matching networks. An author of a book on techniques is confrontecl with many difficult situations because he must try to convey a sense of juclgment in significant values and wise compromise in the presence of the many empirica! conditions that surround each individua! problem. The successful solution of an engineering prohlem involves many arbitrary decisions and is largely a matter of persona! ingenuity and resourcefulness in applying sound elec-trical and mechanical principles. For that reason some of our statements made in the discussion of the various topics should not be interpreted too rigorously. Our intention has heen to provide a certain amount of guiding counsel for those who need it even though it was necessary to oversimplify to some 'extent. There are three basic aspects of antenna engineering. The first perv

vi

PREFACE

tain,; to radiation eharal'!Nist i,·s and i11(·lll(l(•,; ali mat \('l',; of thc dist rihution of rndia11t 0110rgy in ,;pat'(' aronnd a11 a11tf'n11a system, a,; wcll a;.; the current di:,;tributions that prod1wc tlte radiatiun pattern. Tltc sceond 1wrtai11s lo ant0rnw. eircuitry :md invoh·c,; such rnattcr8 a,; ,;elf- and mutuai impcdances, cmTcnts, potentials, insulation, and freder sy,;terrrn that will yidd the desirecl current distributions. Third thcrc is the strueturnl engineering which has to do with ali the mcchanical details of support,,, rigging, materials, strPngths, weights, hardware, asscmbly, adjustability, stability, and maintenance. ,vhile each aspect must be separately developcd, the final design mnst bo an integration of thc throo, with a minimum of compromise ancl within reasonable economie limits. The pnrposc of a tnrnsmitting antenna is to project"radiant encrgy over a given wan' path in the most offef'ti,·e and economica! manner. Tho purpose of a receiving antenna is to ahsorb a maximum po,ver from a passing wave field, with the maximum exclusiou of noise and interfcring signals. The transit of a wave fiold between the two clepcnds upon the physics of wave propagation. The antenna enginecr must be famihar with wave propagation to be ahlc to design antenna systems of maximum effective11ess. ,vave propagation is a vast ancl complicateci statistica! ·subject, and for that reason t he space that r·an be devotod to the subjeC't in this book is limitecl to tlw han·st essontiab. Sources of detailecl information are indicateci for refcrencc ancl study. It may be expectecl that future developments in om knowledgo of propagation will have their influence on future antenna design. The design formula;, for thc various types of antennas are presentecl without proof and may \Je regarclocl as rccipes. Their theory aml clerivation may be found in tho literaturc, togcther wìth more completo information of a relateci nature. Also, many data cnrves and tables are taken from recognized sources, aìthough these are sometimcs rearrnnged for greater utility. Some of the information is from nnpublishod sources and includes muoh originai materiai. The appendixes contain reference data of generai use to the antenna ongineer. The nomenclature used for bands of frequencies is based primarily 011 their propagation characteristics. These terms are also approximately in accord with the nomenclature adoptecl by the International Telocommunications Union at its Atlantic City confcrence in 1947. The use of these broad terms has a brevity ancl convenience that is v~ry desirable in writing and talking about frequencies, provided that one thinks about them as having indistinct boundaries. One must recognize rather large overlaps in the bancls of frequencies propagatcd as fo,tecl, and the bands shown are indicativo only. They blend grnclually from onc into thc othor, the amount ancl the extreme ranges varying with tbc state of the ionosphere and ground oharacteristics.

vii

PREFACE

The thrce fre- LONG

1 p f/

BALANCED FEED AT F-F

DIRECTION OF BEAM

Fw. 3.36.

Director-reflector array.

(Data by Brown.)

over ground at a height h would be derivable from the measured horizontal pattern by the relation J(a) = f(/3) cos a sin_ (h sin a) COS (90 Slil a)

The empirica! characteristics of thìs type of antenna make it necessary to follow closely the principle of similitude in attempting to reproduce a prescribed design. The cross-sectional scale must also be retained faithfully. This type of antenna can also be used as a single-mast rotary beam for the higher frequencies. 3.12. Vertical Directivity of Stacked Horizontal Dipoles

It was seen from Fig. 3.15 that increasing the height of a single horizontal dipole above the earth lowers the angle of the first lobe, but only at the expense of forming other higher-angle lobes after the height surpasses one-half wavelength. In practical communication it is necessary

RADIO ANTENNA ENGINEERING

262

for long-distance transmission and reception to focus thc cncrgy at low anglcs ami abo to supprcss partially, if not complctcly, ali highcr lobcs to reduce fading and multipath signals of long dclay. This r·an be clone to a ': certain extent by inneasing the number of cophasccl clipolcs as thc height j: of the lowest clipole is increasccl. The clircctivity of t,rn cophasecl clipolcs spaced something of the orcler of one-half wavclength is such as to reduce the magnitude of the higher lobcs that form as the height of the system is increased. Eventually, the second vertical lobe angle is low enough so ' that two dipoles do not appreciably reduce it, but the third and fourth lobes are effectively reduced. It is then necessary to increase the number of stacked dipoles to achieve the desired reduction of the second lobe. By the time the lowest lobc has been deflected to 5-degree horizon clearance, the lowest dipole is two wavelengths above ground, and if four cophased dipoles spaced one-half wavelength above each other are used, the second lobc has been redùced to 65 per cent of the first in field strength at 15 degrccs ancl the third, fourth, and fifth to 17, 24.5, ancl 25.5 per cent at 25, 40, 54 degrees elevation, respectively. In order to reduce the second lobe stili further, a vertical stack of six or more dipoles would ha ve to be used. This process can lead to very high and expensive structures. Y et such is the problem of obtaining a high concentration of energy at very lmY angles. In the following equation, h is the electrical height of the lowest dipole above perfect ground. The other dipoles are assumed to be Fm. 3.37. Vertical stack of equal-curspaced at half-wavelength interva,ls rent cophased dipoles. withequal currents. The principal vertical pattern, using the method of adding the patterns for individuai pairs formed by each ·dipole and its image (see Appendix V-A), using the geometry of Fig. 3.37, is

J(a) = (sin ·

(h sin a)

+ sin [(h +

180) sin al+ sin [(h + 360) sin al ) ·+···sin {[h + (n - 1) 1801 sin a} N

where N is a normalizing factor and n the tota! number of dipoles in the stack. Each dipole in the stack introduces a term in this equation. ·

-


~

3

i

5

7

7

,o•

9

10 9

W10

-~~-~~~

Il

Il

Il

13 12

13

oa I \ 12

5•

Fm. 3.42. Patterns for collinear cophased equicurrent dipoles with identica! parallel row having a quarter-wave·Jength quarter-phase relation. (Prom RAF Signal Manual.)

n•2

o•

o•

o•

o·

o•

HIGH-FREQUENCY ANTENNAS

271

length have patterns in thc horizontal planc specified by

Patterns for straight rmrn of vertical dipolcs cach having tmit r·utTent, cophased, are sho,rn in Fig. 3.--Hi. BIDIRECTIONAL PATTERN2 COPHASED DIPOLES

UNIDIRECTIONAL PATTERN USING REFLECTING SCREEN SPACED O.i;>,.

0.6

o_J

w Li: 0.4 w

....~. TOREAR

90•

90°

0.2 FIG. 3.45.

Radiation pattern for six cophased dipoles with reflecting screen.

cosine factor of this equation accounts for the tilting, or slewing. It can be seen from this factor that its maximum will always be to one side or the other of the normai to the array when ef, is other than zero. It is also evident that a null is going to appear on the off side of a slewed beam, which, when ef, becomes sufficiently large, causes a split in the pattern. It is this split that sets a practical limi_t to effective slewing, both from loss of gain on the main beam and the growth of the secondary beam to objectionaable size. The most effective form of beam slewing is to introduce an equal phase difference in the current of each dipole in succession. The more uniformly the phase difference is distributed across the array, the greater is the slewing angle before the beam splits. Fig. 3.47 exemplifies two values of beam slewing for an array of four dipoles in line.

o• 000000

n•6

n•7

o•

5•

o o o o o o o ()

50

5•

oc n•8

s•

+--H7

(From RAF

00000000

Radiation patterus for horizontal row of vertical dipolcs with unit current, cophased.

Signal Manual.)

Fm. 3.46.

O O O O O

7

8

~

z... m z z >

-