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The input impedance of a circular microstrip ring antenna excited by a coaxial probe is studied using cavity Green's-function technique. The model of an annular cavity with the boundary admittances at the edges is considered in the analysis. The boundary admittance is obtained by considering the radiated and reactive power flows through the apertures at the edges. Input impedance is formulated from the conservation of energy principle, taking into account the losses in feed pin, conducting patch and dielectric substrate. The expression for input impedance includes the feed location, the feed diameter, the ring dimensions, thickness and dielectric constant of the substrate. TM120 mode is considered for antenna applications. Experimental results of input reflection coefficients show good agreement with the theory.