RL 813 CIRCULAR PATCH ANTENNAS WITH PA.TSIVE CONDUCTING POSTS Dipak L. Sengupta Department of Electrical Engineering and Computer Science The University of Michigan Ann Arbor, Michigan 48i09 and Guey-Liou Lan RCA Laboratories Princeton, NJ 08540 Summary It is known [1,2] that the use of passive conducting or tuning posts within the input region of a probe-fed circular pr.tch antenna may generally increase the resonant frequency, provide impedance matching or circularly polarized radiation. The input impedance behavior of the antenna based on modal considerations is found useful for the understanding of the performance and for the design of such antennas under various conditions. Consider a single tuning post placed at p = p, ~ = o within a circular patch antenna fed Vrom the iback with a coaxial probe located at p = p, p = 0 degrees, as shown in Fig. 1. It can be shown that in the absence of the post, tile dominant (TM ) mode E-field at a given p and variable p may be considered to 'onsist of two degenerate mode fields, referred to as the A- and B-mode, eacn having a resonant frequency same as that of the antenna (i.e., f ) but the variation of each field in the ~-direction being orthogonal to the other. With a post present, the two modes become distinct having distinct resonant frequency fA (:fo) and fB, (fB > fo), and each mode having a distinct input impedance; under these conditions. the input impedance of the antenna is the sum of the two modal impedances. The measured input res<-tarce vs frequency for an antenna having one post at = 135 degrees ana variable p is shown in Fig. 2 which clearly indicates that with sufficiently large p the two modes 1 resonate at two distinct frequencies. It is found that f f 415 A 0 MHz in the present case and fB > fo and its value significantly depends on p. Width p fixed and D variable, the input resistance vs. frequency for the same antenna is shown in Fig. 3 which indicates the predominance of A-mode for ~ = 90 lZ-grc:s and th&e of the B-mode for 0 = 180 degrees. These modes can be controlled by proper choice of 40,), and the number of posts to obtain the desired characteristics RL-813 = RL-813

froml the antenna. In general, with a single post at::- ts the modal c input resistance may be represented by RA (o) RA sin2 o in o RB(o) B 2 in 0 0 0 where RA( o), RB( o) are the maxinum (resonant) resistance of the Aand B-modes, respectively. In general, it ha' been found that the input resistance of the antenna depends mainly on O, and hence such post(s) can be used for matching; location and number of posts can also be used to increase the resonant frequency of a given circular patch antenna; finally, with proper arrangement of posta a single probe-fed antenna can be used to produce circularly polarized radiation. Detailed results including far field patterns will be discussed during the presentation. References: [1] G-L Lan and D. L. Sengupta, "Tunable Circular Patch Antennas," Electronics Letters, Vol. 21, No. 22, pp. 1022-1023, 24th October 1985. [2] G-L Lan and D. L. Sengupta, "Analysis of Tunable Circular Patch Antennas, Program and Abstracts, 1985 North American Radio Science Meeting, University of British Columbia, Vancouver, Canada, p. 93, June 17-21, 1985. [3] G-L Lan and D. L. Sengupta, "PosL-runad Single-Feed Circularly Polarized Patch Antenlas,", Antennas and Propagation Symposium Digest, Vol. 1, AP-S International Symposium, University of British Columbia, Vancouver, CAnada, pp. 85-88, June 17-21, 1985. o FEED * 1POh;r -— a (a) Fig. 1. Circular patch antenna with ore post. Feed at p=p, (=0~. Post at p= p1, = Q40.

U z H V) W-3 80 70 60 50 40 30 2Q CIRCULAR PATCH (a) (b) (c) (d) (e) ANTENNA WITH ONE POST p = 0.00 cm p = 0.50 cm p = 0.65 cm p = 0.70 cm p = 3.00 cm to = 135~ - - (e) - 10 0 9 fr 1400 1420 1440 1460 1640 1G0 163j FREQUENCY ( MhIlz) Fig. 2. Input resistance versus frequency for different distances p.(P1 is the distance between the post and the center line.) "o = 1.2 cm, a = 3.8 cm.

I.4 CIRCULAR PATCH ANTENNA WITH ONE POST 80 - -, p = 0. G5 cm Gu - -. i,' — \ ' 'I- R = 1800!\/ - / \ I'; \ 1, P 1.0 cm, a = 3.8 c.e v f u f vy a s, 1 ': c, a - I1 cm, a = 3.8 cm.