Surface admittance of circular microstrip antenna

The cavity model is a simple and efficient analytical method of a microstrip antenna (MSA) of any configuration, where the separation of variables is possible in the wave equation expressed in that particular coordinate system. In this method, the antenna is treated as a resonant cavity bounded above and below by conducting plates and on the side by an admittance wall. The electromagnetic fields within the cavity are expanded in terms of the eigenfunctions and determined from the impedance boundary condition at the side aperture. The accuracy of the surface admittance at the aperture affects the resonant frequency of the MSA. The surface conductance of a rectangular MSA was obtained from the radiated power leaving on the edge and the susceptance from the capacitance of an open microstrip circuit. Shen (1979) determined the surface admittance of circular MSA from the radiated power and the fringe field on the edge of the antenna. The surface admittance of a circular MSA is calculated by spectral domain analysis. The calculated input impedance of this antenna agrees well with the measured result.