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Impedance characteristics of circular microstrip patches

Microstrip antennas are highly resonant structures and, for simple patch configurations, the bandwidths of each mode is quite small. As an antenna, their most useful mode is the TM11 mode that radiates in the broadside direction with a good gain. All other modes, TMnm, radiate conical beams with a null in the broadside and have a pattern peak that moves towards the endfire direction by increasing the mode index η (in the azimuthal direction). These antennas are normally excited by using a feeding probe from a coaxial line, or a co-planar transmission line. The excitation coefficient of different modes depend on their proximity to the resonance mode and the location of the feed. When only one mode must be excited, the feed configuration or its location must be optimized to eliminate the excitation of other modes.