IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. AP-34, NO. 8. AUGUST 1986 977 Analysis of an Aperture Coupled Microstrip Antenna PETER L. SuLLWAN, MEMBER, IEEE, AND DANIEL H.SCHAUBERT, SENIOR MEMBER, IEEE Absfruct-A microstrip patch antenna that is coupled to a microstripline by an aperture in the intervening ground plane is analyzed. Conpled integral equations are formulated by using the Green’s functionsfor grounded dielectric slabs so that the analysis includes all coupling effects and the radiation and surface wave effects of both substrates. A Galerkin moment method solution of the coupled integral equations agrees quite well with measured data. Design data are contained in parameter studies, many of which are verified by experimental results. 1 T *b INTRODUCTION v i t h Aperture NEW FEED CONFIGURATION for microstrip antennas has beenproposed by Pozar [ 13. The feed and a rectangular patch antenna are shownin Fig. 1. The feed consists of an open-ended microstripline that is located on a A dielectric slab below the ground plane. The microstrip antenna is formed on a separate dielectric slab above the ground plane and the two structures are electromagneticallycoupled through an electrically small aperture in the groundplanebetween them. A related structure that used waveguide as the feed [2] has been reported, but not was successfully impedance matched. The new design is particularly advantageous when applied to millimeter wave monolithic phased arrays. In this application the associated active elements such as phase shifters and amplifiers are formed on gallium arsenide, which has a high dielectric constant (er = 12.8). However, it is preferable to mount the antenna elements on a low dielectric constant substrate in order to increase the bandwidth, the radiation efficiency and the angle off broadside at which scan blindness occurs [3]. With the two-layerdesign, the antennas are located on a separate substrate, which yields optimal array performance andeliminates the competition for surface space between the antenna elements and the feed network. In addition, the ground plane shields the antenna half-space from spurious I radiation emitted by the feedlines and active devices. Finally, (b) aperture coupling obviates problems associatedwith probe Fig. 1. Aperturecoupled patchantenna. feeds at millimeter wave frequencies, such as complexity of construction and large probe self-reactances [ 13. In this paper, the microstripline fed aperture coupled patch antenna is analyzed to determine its input impedance. Design data are provided by means of parameter studies that have been performed using the analysis and verified experimenManuscript received September 1 1 , 1985; revised February 28, 1986. This tally. In the next section, the theoretical basis for the analysis work wassupported by Rome A r Development Center, Electromagnetic is presented. The third section contains results of the analysis i Sciences Division under Contract F19628-84-K-0022. and experiments. P. L. Sullivan was with Department the of Electrical andComputer I A Ground Plane Engineering, University of Massachusetts, Amherst, MA. He is now with Bell Laboratories, North Andover, MA 01845. D. H. Schaubert is with the Department of Electrical and Computer Engineering, University of Massachusetts, Amherst, MA 01003. IEEE Log Number 8609025. FORMULATION THE ANALYSIS OF A schematic of the antenna and feedline is shown in Fig. 2 with impressed and induced currents indicated. The ground 0018-926X/86/08oO-U977$01 0 1986 JEEE .oO Authorized licensed use limited to: ULAKBIM UASL - MIDDLE EAST TECHNICAL UNIVERSITY. Downloaded on November 21, 2008 at 08:39 from IEEE Xplore. Restrictions apply. . . 978 z IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. AP-34, NO. 8, AUGUST 1986 represented by their integral expressions such as Eb(Ms)= E I SI, 1s ~ f G ~ ~ ~dblx0,x , ( YY apenure YO, 0) o dY0 Region b - 2Ms(xo, Yo) h (5) Region a where GiMyx the y-directed electric field at (x,y , d b ) due to is an infinitesimal x-directed magnetic current at (xg, yo, 0) radiating in the presence of a grounded dielectric slab. This and the other Green's functions needed for the analysis are obtained by using spectral domain methods so that m r t sf, -m ejk,(x-Xg)ejky(Y-YO) dkx dk,. (6) Expressions for all the required kernel functions are given in the Appendix. Three coupled integral equations are obtained for the three unknown currents &&,and 1 by enforcing the boundary , conditions: 1 ) E" = 0 on the patch antenna, 2) B = 0 on the " microstrip feedline, and 3) H" is continuous through the aperture. (Note that the assumption M; = - M: ensures that (b) E is continuous through the aperture.) Al of the other " l Fig. 2. Antennaandfeed with incident and inducedcurrents.(a)Original boundary conditions for the structure are satisfied because of problem. (b) Equivalent problem. the Green's functions that are used. A Galerkin moment method solution of these equations is plane and dielectric substrates extend to infinity in thex and y formulated by choosing expansion functions for the undirections. The electric surface currents on the patch antenna knowns. For the patch current, and feedline and the electric fieldinthe aperture are all N. I a s s u m e d to be y-directed ( e Fig. 2(a)). The analysis of this se structure follows similarly that in [4], but requires consideran=l bly more effort to treat the dielectric substrates and additional where interaction terms to treat the microstrip feedline. By invoking 1 sin k:(hb-ly-ynl) the equivalence principle the aperture can be closed off and q x , y ) =9, replaced by magnetic surface currents M, just above and W, sin k:hb below the groundplane(see Fig. 2(b)). Continuityof the tangential electric field through the aperture is ensured by making the magnetic current above the ground plane equal to the negative that of below. The known incident current distribution on the feedline is denoted by Znc, the scattered feedline current by and the patch current by s . , Denotingthe space below the ground plane (z < 0) as region a and the space above the ground plane (z > 0) as region b the total electric and magnetic fields in each region Because the aperture is assumed electrically short the can be written as a summation of fields due to the various magnetic current can be represented only asingle piecewise by currents as follows: sinusoidal (PWS) current mode. Noting that the aperture is sf, Eh"=E,(~~f.,c)+E,(5f)+E,(M~) ( 1 ) always centered about the origin H ~ = ~ , ( ~ ~ ) + ~ , ( ~ f ) + ~ = ( ~ , ) n;is(x, y)= vapn;jr"p(x,y ) (2) where T/"P, - vo Hr=ab(Jb)-Hb(Ms). (4) (12) Each field on the right side of (1)-(4) is the field due to the specified current radiating in the presence of a dielectric slab and ground plane wt the aperture shorted. These fields are ih 1 sin k,(La42-Ixl) &Pyx, y ) = i, Wap sin ka&,12 - La42< x O (56) Im k 2 = ( k i - p 2 ) 1 / 2 ,Im { k 2 } < 0 , Re { k 2 } > 0 p2=k:+k;. T,"=klb COS (klbdb)+jk2 Sin (klbd) (57) (58) (59) Tk=efk* COS (k1bdb)fjklb sin (klbdb) 112. (60) z = (/.LO/EO) o The required kernel functions are (61) I> for G&,,,,(x7 .32 cm .48 cm Y 7 db 1 0 y 7 db): x7 o .375 cm .613 cm 1.083 cm 1.056 cm Qi.,yy(kx, O y ) Zk - Fig. 10. Calculated input impedaxe as a function of feed substrate thickness. Tabular datagive feedline width andstub length used to maintain 50 fl characteristic impedance and stub length of 0.22 X for each value of , d,. Other antenna parameters are: E : = 2.54, d b = 0.16 cm, L, = 4.0 cm, W, = 3.0cm. x& = O.Ocm,yo, = 0.0 cm, La, = I.Ocm, W,, = 0.11 cm, E : = 10.2. -j 4T2ko (c:k:- k:)k2 cos ( k l b d b+ j ( k i - k : ) k l bsin (klbdb) ) T,"T k x sin ( klbdb) (62) Authorized licensed use limited to: ULAKBIM UASL - MIDDLE EAST TECHNICAL UNIVERSITY. Downloaded on November 21, 2008 at 08:39 from IEEE Xplore. Restrictions apply. 984 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. AP-3, a, AUGUST 1986 NO. .4 - -2 .3- db ( 4 .2 - (cm) -1 .I - Cr= 2.54 , 50 100 Rres Fig. 11. Relationshipof resonant resistance, antenna substrate thickness,and aperture length. Apertureoffsets arc zero and feedlines are50~.a:~~=~~=2.54,d,=0.16cm,L~=4cm,W~=3cm,W,,=0.1545cm,L,=2cm.b:~~=~~=10.2,d,=0.16 cm, Lp = 2 cm, Wp= 1.5 cm, W,, .1 cm, L, = 1.1 cm, c, d: E : = E : = 2.54, d, = 0.16 cm, Lp = 4cm, W, = 3 cm, L, = 2 = cm. REFERENCES D. M. Pozar, “Microstrip antenna aperture-coupled to a microstripline,” Electron. Lett., vol. 21, no. 2, pp. 49-50, Jan. 1985. D.H. Greenlee, M. Kanda, aild D. C. Chang, “The characteristics of iris-fed millimeter wave rectangular microstrip patch antennas,” Nat. - ~;klbk2 cos (kl&)+j(k;(e:- 1) - kib) sin 7’;Tk (klbdb) Bur. Stand. Tech. Note 1063, U.S. Dept. Commerce, Oct. 1983. D.M. Pozar and D. H. Schaubert, “Scan blindness in infinite phased arrays of printed dipoles,” IEEE Trans. Antennas Propagat.. vol. AP-32, pp. 602-620, June 1984. R. F. Harrington, “Resonant behavior of a small aperture backed by a conducting body,” IEEE Trans.Antenna Propagat., vol. AP-30, pp. 205-212, Mar. 1982. R. W. Jackson and D. M. Pozar, “Full-wave analysis of microstrip open-end and gap discontinuities,” ZEEE Trans. Microwave Theory Tech., vol. MTT-33, pp. 1036-1042, Oct. 1985. P. L.Sullivan and D. H. Schaubert, “Analysis of an aperture coupled microstrip antenna,” Dept. Elec. Comput. Eng., Univ. Massachusetts, Amherst, ANTLAB Rep. 851, July 1985. D. M. Pozar, “Input impedance and mutual coupling of rectangular microstrip antennas,” IEEE Trans.Antennas Propagat., vol. AP-30, no. 6, pp. 1191-1196, Nov. 1982. K. R. Carver and J. W. Mink,“Microstrip antenna technology,” IEEE Trans. Antennas Propagat., vol. AP-29, pp. 2-24, Jan. 1981. (63) for qfM.&’ Qk-(kxY Y Y ObOY Yo, 0): ky) - -j - 1 4n2koZ0 klbT:Tk - [jkEk&(+ 1) + (€;k;-k;) x (klbkdE,b+ 1) sin (klbdd COS (klbdb) +j(e;ki sin2 (klbdb)-k:bcos2 (kIbdb))}] (64) for GgMYx(x,, 41x0, Y O ,0 : Y ) QgMyx(kX9 ky)= - QLJxy(kx, ky). (65) Equation (65) follows from reciprocity. The corresponding expressions for region a are obtained by replacing €:by E; and db by d, throughout. ACKNOWLEDGMENT The authors wish to thank D. M. Pozar for his many helpful suggestions and for providing his computer program, which aided in the development of the computer program that was used here. Our thanks also to R. W. Jackson for his suggestions and assistance in developing the numerical analy- Daniel H.Schaubert (S’6844’74-SM’79), for a photograph and biography please see page 85 of the January 1984 issue of this TRANSACTIONS. sis. Peter L Sullivan (S’85-M’85) received the B.S. . degree in zoology in 1978 and the B.S. and M.S. degree in electrical engineering i 1984 and 1985, n respectively a l from the University of Massachul setts. He is currently employed byBell Laboratories, North Andover, MA, where he is responsible for constructing a bistatic FM-CW scatterometer and for performing scatterometry studies to enhance the data base used to analytically predict interference due to ground scatter in microwave communication links. Authorized licensed use limited to: ULAKBIM UASL - MIDDLE EAST TECHNICAL UNIVERSITY. Downloaded on November 21, 2008 at 08:39 from IEEE Xplore. Restrictions apply.
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