ANALYSIS OF A SUSPENDED PATCH ANTENNA EXCITED BY AN ELECTROMAGNETICALLYCOUPLED INVERTED MICROSTRIP FEED

Qiu Zhang, Yoshiro Fukuoka, Tatsuo Itoh and Luo Su*

ABSTRACT

A suspended patch antenna electromagnetically excited by an invertedmicrostrip line has been studied based on a full-wave transmission lineanalysis in conjunction with a microwave network theory. The inputimpedance calculated has been compared with measured data.

INTRODUCTION

A suspended microstrip patch antenna is a useful modification of aconventional microstrip antenna Ill because the effect of the ground planeis reduced and an improved radiation characteristic can be expected [2].One of the easiest ways to feed this suspended antenna is to insert aninverted microstrip line beneath the antenna (Fig. 1). In this way, thereis a possibility of changing the coupling characteristics by adjusting thelength of the inserted line. The electromagnetic coupling of this sortwas first proposed by Oltman, and several theoretical and experimentalstudies have been done 13,4]. These theoretical analyses use transmissionline models to characterlze the antenna system, and are all based on theassumption of homogeneous medium. However, as the coupling mechanism isstrongly affected by the existence of the dielectric sheet, it is neces-sary to include this effect in the analysis for accurate prediction of theantenna characterlstics.

The approach in the present paper uses a transmission line modelobtained by the application of the full wave spectral domain method tocharacterize each line section. In this way, the inhomogeneity and non-symmetry of the structure can be correctly taken into account and, there-fore, the analytical model is much more realistic, In addition, stripsand patches with any widths can be analyzed.

METHOD OF ANALYSIS

The side view of the antenna structure is shown in Fig. 2a in whichthe hypothetical extensions of the strips are included. These extensionsare introduced to account for the open-end capacitance [5,6]. In thepresent analysis, the antenna system is separated into three regions.Region I is an inverted microstrip line used as the feed line, Region IIIis a suspended microstrip line representing the "uncoupled" portion of theantenna patch, and Region II is the overlap of the antenna and the feedline.

The first step of analysis is to obtain the propagation constants andthe ch-aracteristic impedances in all three regions by means of the full-

Dept. of Electrical Engineering, The University of Texas at Austin,Austin, TX 78712

*On leave from the Research Institute of Electronic Techniques of theChinese Academy of Sciences, Guangzhou, China

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wave spectral domain method [7,8]. It is important to recognize thatRegion II is a nonsymmetric coupled transmission line in an inhomogeneousmedium. In this region, there exist two independent modes with differentvalues of propagation constants (.even-like C mode and odd-like rr mode).For each mode, we have two characteristic impedances corresponding to theupper and lower strips. In the equivalent circuit, all of these imped-ances and propagation constants are incorporated in the derivation of the4-port Z-matrix representing Region II 9]. Use of this Z-matrix distin-guishes the present work from those previous ones t3-4].

The equivalent circuit model is shown in Fig. 2b, The port 4 is leftopen and the ports 2 and 3 are connected to radiation conductances, Theradiation conductances at the port 3 is calculated by the method presentedin [10]. At the port 2, the radiation conductances are calculated for twodifferent modes by the same method and added together, In this case, forthe r mode, the effective height of the line is taken as the distancebetween the antenna and the feed line, since the field is concentrated inthis region. From these terminations of the 4-port, the input impedanceat the port 1 can be calculated.

Once the input impedance is calculated, the resonant frequency of theantenna system is found at which the imaginary part of the input impedancebecomes zero. At this frequency, a good matching can be reached if theinput impedance of the antenna system is equal to the characteristicimpedance of the feed line (Region I).

RESULTS

The dimensions of the antenna system are shown in Table I. Theresonant frequency of the patch antenna without the feed line is 2,56 GHz,which is calculated by the spectral-domain method 12]. The feed line isdesigned so that its characteristic impedance becomes 50 ohms. Calculatedvalues of input impedances for various lengths of inserted feed lines arepresented in Fig. 3. From the figure, it is seen that a good matching canbe obtained when the inserted length is either 0 or 3,2 cm (9Q1/Z = 0 or0,8).

In order to check the accuracy of our method, we performed experimentsfor various values of Zi, The VSWR was found high as predicted except forthe two values given ab-ove. For Z1 - 3.2 cm, the frequency dependence ofthe input VSWR was plotted in Fig. 4. The VSWR curve shows good agreementbetween theory and experiment. The high VSWR observed in the experimentfor other inserted lengths indicates that no effective coupling is accom-plished. This again agrees with our theory. As a special case, when theinserted line length becomes zero, the VSWR curve is calculated and com-pared with results reported in U2]. The coupling is only due to the smallextension parts of the antenna and the feed line. Therefore, the trans-mission line approximation is not expected to be very accurate. Never-theless, the present results in Fig. 5 show reasonable agreement with thosein 12]. Other data including the radiation patterns will also bepresented.

CONCLUS IONS

An equivalent transmission line analysis of electromagneticallycoupled patch antenna was presented, and it was shown that a low input

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VSWR is obtained by adjusting only the length of the feed linre insert.The method in this paper is simple and can be used for design purpose.

REFERENCES

[1] A. G. Derneryd, "Linearly polarized microstrip antennas," IEEE Trans.Antennas Prop,agation, Vol. AP-24, pp. 846-851, Nov. 1976.

[2] J. Rivera and T. Itoh, "Analysis of an electromagnetically coupledpatch antenna," Proc. APS Symposium, pp. 170-173, 1983.

[3] H. G. Oltman, "Electromagnetically coupled microstrip dipole antennaelements," Proc. 8th European Microwave Conf., pp. 281-285, 1977.

[4] R. S. Elliott and G. J. Stern, "The design of microstrip dipolearrays including mutual coupling, Part I: Theory," IEEE Trans.Antennas Propagation, Vol. AP-29, pp. 757-760, Sept. 1981.

[5] E. 0. Hammerstad, "Equations for microstrip circuit design," Proc.5th European Microwave Conf,, pp. 268-272, Sept. 1975.

[6] S. S. Bedair and M. I. Sobhy, "Open-end discontinuity in shieldedmicrostrip circuits," IEEE Microwave Theory and Tech., Vol. MTT-29,pp. 1107-1109, Oct. 1281.

[7] T. Itoh, "Spectral-domain immittance approach for dispersion char-acteristic of generalized printed transmission line," IEEE Trans.Microwave Theory and Tech., Vol. MTT-28, pp. 733-736, July 1980.

[8] R. H. Jansen, "Unified user-oriented computation of shielded, coveredand open planar microwave and millimeter-wave transmission-linecharacteristics,"' Microwaves, Optics and Acoustics, Vol. 3, pp, 14-22,Jan. 1979.

[9] V. K. Tripathi, "Asymmetric coupled trnasmission lines in an inhomo-geneous medium," IEEE Trans. Microwave Theory and Tech., Vol. MTT-23,pp. 734-739, Sept. 1975.

[10] H. Sobol, "Radiation conductance of open-circuit microstrip," IEEEMicrowave Theory and Tech.., Vol. MTT-19, pp. 885-887, Nov. 1971.

ACKNOWLEDGMENT

This work was supported in part by US Army Research Office ContractDAAG29-81-K-0053 and in part by a contract from Texas Instruments Equip-ment Group,

TABLE I

Length of patch antenna 4.0 cmWidth of patch antenna 1.65 cmWidth of feed line 1.23 cmHeight of feed line 0.297 cmThickness of dielectric sheet 0.297 cmDielectric constant 2.23

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Fig. 1 Suspended patch antenna excited by anelectromagnetically coupled invertedvmicrostrip feed

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