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Resonant microstrip antennas usually have very narrow impedance bandwidths. By simultaneously exciting two modes of a circular patch, a two-port patch radiator with image impedance that remains near constant over a wide band can be obtained. This result is achieved by adjusting the coupling so that the two modes present complementary impedances, one in series, the other in shunt, at the same point on the feeder. The patterns can be made unidirectional so that the antenna is well suited for use in series-fed, end-fire, or side-fire arrays. The patterns are stable over a wide frequency band. Hence, the operating bandwidth will probably be determined by efficiency or power gain rather than by impedance or patterns.>
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Mayes et al. (1990) studied this question.
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