Ieee Microstrip Antenna Technology

Citation preview

2

zyxw zyxwvutsrqpo zyxwvut zyxw IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. "-29,

NO. 1, JANUARY 1981

Microstrip Antenna Technology

KEITH R. CARVER, MEMBER, IEEE,

AND

JAMES

w.MINK, MEMBER, IEEE

reported in the literature until the early 1970's, when a conductingstripradiatorseparatedfromagroundplanebya dielectricsubstrate was describedbyByron [ 4 ] . Thishalfwavelengthwide and several-wavelengthlong strip was fed by coaxial connections at periodic intervals along both radiating edges, and was used as an array for Project Camel. Shortly thereafter,amicrostripelement was patented byMunson [SI anddataonbasicrectangularandcircularmicrostrip patcheswerepublishedbyHowell [ 6 ] . Weinschel [ 7 ] developed several microstrip geometries for use with cylindrical S-band arrays on rockets. Sanford [ 81 showed that the microstripelementcouldbeused in conformalarray designs for L-band communication from a KC-135 aircraft to the ATS-6 satellite. Additional work on basic microstrip patch elements was reported in 1975by Garvin e t d . [ 91, Howell [ 101, INTRODUCTION Weinschel [ 111, and Janes and Wilson [ 121. The early work HE PURPOSES of this paper are to describe analytical and by Munson on the development of microstrip antennas for use experimental design approachesformicrostripantenna as low-profieflush-mountedantennas on rocketsand miselements, and to provide a comprehensive survey of the state siles showed that this was a practical concept for use in many of microstrip antenna element technology. A companion antennasystemproblems,andthereby gave birth t o anew paper [ 1 ] discussed microstrip array design techniques. Taken antenna industry. together,thesepapers provideareference forthecurrent Mathematicalmodeling of the basicmicrostripradiator state of development of microstripelementsandarrays of was initiallycarriedoutbytheapplication of transmissionelements at a time when advancements in this relatively new line analogies t o simple rectangular patches fed at the center technology are being reported primarily in a wide variety of of aradiatingwall [ 131, [ 141. The radiation pattern ofa technicalreportsandprivatecommunications,and to a les- circular patch was analyzedandmeasurementsreportedby This Carver [ 151. The first mathematical analysis of a wide variety ser extent in this TRANSACTIONS andotherjournals. paperbeginswitha reviewof thestate of printedcircuit of microstrip patch shapes was published in 1977 by Lo e t d . materialstechnology as itaffectsthe design of microstrip [ 161, who used the. modal-expansiontechnique t o analyze antennas,andthen describesseveral theoreticalapproaches rectangular, circular, semicircular, and triangular patch shapes. to the analysis of rectangular and circular patches, as well as Similar comprehensive reportson advanced analysis techniques patches of other shapes and microstrip dipoles. Design curves werepublishedbyDerneryd [ 141, [ 171, Shenand Long are presented for both rectangular and circular patch shapes, [ 181, and Carver and Coffey [ 191. By 1978 the microstrip andfor linearly and circularlypolarizedelements. A dis- patch antenna was becoming much more widely known and cussion of thebandwidth andefficiency of the elements is usedinavariety of communicationsystems.This wasacpresented with the patch size, shape, substrate thickness, and companiedbyincreasedattention by the theoreticalcommaterial properties as parameters. Several practical techniques munitytoimprovedmathematicalmodels whichcouldbe are outlined for modifying the basic element for such special used for design. In October1979,the first international purposeapplications as conformalarrays,feedsfordishes, meetingdevoted to microstripantennamaterials,practical dual-frequencycommunicationsystems,etc.Thepapercondesigns, array configurations, and theoretical models was cludes with suggestions for future critical needs in the further heldat New Mexico State University(NMSU),LasCruces, development of the antenna. undercosponsorshipof the U.S. ArmyResearchOffice and Themicrostripantennaconceptdates back about 26 NMSU's Physical Science Laboratory [ 201. years t o work in the U.S.A. by Deschamps [ 21 and in France The terms stripline and microstrip are often encountered by Gutton and Baissinot [ 3 ] . Shortly thereafter, Lewin [ 991 in the literature, in connection with both transmission lines investigated radiation from stripline discontinuities. Additional and antennas. A stripline or triplate device is a sandwich of studies were undertakeninthelate 1960'sbyKaloi,who three parallel conductinglayersseparated by twothin distudied basic rectangular and square configurations. However, electric substrates, the center conductorof which is analogous otherthanthe originalDeschamps report,work was not to the center conductor of a coaxial transmission line. If the center conductor couples t o a resonant slot cut orthogonally in theupperconductor,the device is said t o bea stripline Manuscript received March 5, 1980; revised July 22, 1980. radiator [ 21.1. Althoughtherearemanyvariations on this K. R. Carver is with the Physical Science Laboratory, NewMexico printed-circuit stripline slot antenna, these are outside the State University,Las Cruces, NM 80003. J. W. Mink is with the U.S. ArmyResearchOffice,Research Tri- scope of this paper and will not be considered further. angle Park, NC 27709. By contrasta microstrip devicein itssimplestformcon-

Absfruct-A surveyofmicrostripantennaelementsispresented, with emphasis on theoretical and practical design techniques. Availablesubstratematerialsarereviewedalong with the relation betweendielectricconstanttoleranceand resonant freqnency of microstrip patches. Several theoretical analysis techniques are summarized, including transmission-line and modal-expansion (cavity) techniques as well as numerical methods such as the method of momentsandfmite-elementtechniques.Practicalprocedures are given for both standard rectangular and circular patches, as well as variations on those designs including circularly polarized microstrip patches.Thequality,bandwidth,andefficiencyfactors of typical patch designs are discussed. Microstrip dipole and conformal antennas are summarized. Finally, critical needsfor fnrther research and development for this antenna are identified.

T

zyxwvutsr zyxwv 0018-926X/81/0100-0002$00.75 0 1981 IEEE

zyxwvutsrqpon z zyxwv

MICROSTRIP CARVER AND MINK:

3

ANTENNA TECHNOLOGY

GROUND

PLANE

TOP VIEW

where fo is theresonantfrequency of amicrostripantenna assuming a magnetic wall boundary condition, E , is the relative dielectricconstant, Sf is the changein resonantfrequency, and & E , is the changeinrelativedielectric constant.For example, if the operating frequency of the antenna is t o be predicted t o k0.5 percent using E , = 2.55, the required accuracy is & E , = 0.025. Howevera typicalquoteddielectric constant accuracy for materials of this type is 8 ~ ,= k0.04. The relative frequency change for small dimensional changes may be expressed in terms of linear dimensions or in termsof temperature changes as follows:

zyx zyxwvutsrqp

TOP VIEW

T is the temwhere a, is the thermal expansion coefficient, ature in degrees Celsius, 2 is the frequencydetermining length of the microstripantenna. An uncertainty of less than 0.5 percent in the operating frequency with a temperature variation of 100°Cwouldrequire thethermalexpansioncoefficient a, t o be less than 50 X 10-6/oC.Commonly used materials are adequate in terms of thermal expansion. While thicknessvariationinthesubstratematerialcanhavean effect upon the operating frequency, this factor is much less importantthanthedielectricconstanttolerance. With this background one can determine the suitability of various dielectric materials for use in printed circuit antennas.

zyxwvutsrq zyxwvuts

Fig. 1. (a) Rectangular microstrippatchantenna. (b) Circularmicrostrip patch antenna. (c) Open-circuit microstrip radiator. (d) Micro-

strip dipole antenna.

sists of a sandwich of two parallel conducting layers separated by a single thin dielectric substrate [ 221. The lowfr conductor functions as a ground plane, and the upper conductor may be or circularpatch,aresonant asimpleresonantrectangular dipole, or a monolithically printed array of patches or dipoles and the associatedfeed network. Sincearraysofmicrostrip patches and dipoles were considered in the companion article on microstrip arrays [ 11 , this paper will concentrate on basic microstrippatchesanddipoles.Fig.1showsarepresentative collection of microstrippatchanddipoleshapesandtheir associateddielectric substratesandground planes.Practical microstripantennashavebeendevelopedforusefrom 400 MHz t o 3 8 GHz, and it can be expected that the technology will soon extend t o 60 GHz andbeyond. Since mutual coupling between microstrip elements is considered elsewhere in [ 88 ] , it will not be discussed in this paper. 11. MATERIALS FOR PRINTED CIRCUITANTENNAS Thepropagationconstantfora wave inthemicrostrip substrate must be accurately known in order t o predict the resonantfrequency,resonant resistance, andotherantenna quantities. Antenna designers have found that the most sensitive parameter in microstrip antenna performance estimation is the dielectric constant of the substrate material, and that the manufacturer’stolerance on E , is sometimesinadequate. The change in operatingfrequency of athinsubstrate microstripantennaduesolely to asmalltolerance-related change of the substrate dielectric constant may be expressed as

Available Microwave Substrates Therearemanysubstrate materials on the market today with dielectric constants ranging from1.17 to about 25 and loss tangents from 0.0001 to 0.004[ 10214 1041. Comparative data on most substrates (2.1 E , < 25) are given in Table I [ 23 1, [ 241.Polytetrafluoroethylene(PTFE)substratesreinforced with either glass woven web or glass random fiber are very commonly used because of their desirable electrical and mechanical properties,and becauseofawiderangeofavailable thicknessesandsheet sizes. For Woven webmaterials,thicknesses range from 0.089 mm to 12.7 mm and sheet sizes up t o 9 1.4 cm X 9 1.4 cm. Glass random fiber is available in thicknesses from 0.508 mm t o 3.175 mm and in sheet sizes up to 40.64 cm x 10 1.6 cm. The discontinuous nature of the fiber and the relatively soft and deformable polymer matrix allow one to formthismaterial on complexsurfaces.Stress relief may be accelerated by heating the material. Also, this material is available in shapes other than sheets, such as rods or cylinders. For applicationsrequiring high dielectricconstants, E , < 10.3) are frequently alumina ceramic substrates (9.7 used.TypicalcommerciallyavailablesubstratesareK-6098 teflon/glasscloth ( E , Z 2.5), RT/duroid-5880PTFE (E, 2.2), and Epsilam-10ceramic-filled teflon ( E , IO).