AbstractThe proximity feed microstrip antenna (PMA) fed via a gap enables switching of the polarization with a simple operation of opening or shorting the end of the feed. However, since the PMA is coupled electromagnetically with the feed line printed on the same surface as the antenna, the effect of the feed line on the antenna characteristics is expected. In this paper, the full‐wave analysis developed by Itoh et al. is used for the analysis of the antenna system including the feed line. In this method, Galerkin's method is used in the Fourier transform domain. The effects of the feed line on the resonant frequencies and the radiation patterns of the PMA are investigated. The application of the PMA to a circularly polarized antenna has been tried and actual generations of circular polarizations have been confirmed. The effect of the feed line on the circular polarization characteristics (axial ratio) has been studied.