The circular polarization characteristics of stacked microstrip antennas with a single coaxial feed are analyzed by the method of moments. Green's functions for the patch currents are formulated in the spectral domain. Galerkin's method is employed to determine the input impedance, far-field patterns, impedance bandwidth, beamwidth, and axial-ratio bandwidth. The possibility of improving the axial-ratio bandwidth of the microstrip antenna is demonstrated. Numerical data are obtained to compare with the experimental data available in the literature. (C) 1993 John Wiley & Sons, Inc.