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The wall admittance of circular microstrip antennas with perturbation segments is evaluated by using Green's functions in the spectral domain. The wall susceptance can be regarded as the LC parallel circuit. In the neighborhood of the resonant frequency, the wall susceptance of the primary mode is inductive at the aperture of the perturbation segment and is capacitive at the aperture except the perturbation segment. The input impedances and axial ratios are calculated by the cavity model with these wall admittances and the geometry of the patch to radiate circular polarization is determined.