A new self-polarizing Fabry-Perot (FP) cavity antenna is presented in this paper. The idea is to use a polarizing Frequency Selective Surface (FSS) over a corrugated ground plane in order to convert a linearly-polarized input wave into a circularly-polarized one, along with sufficient gain and good impedance matching performance. The proposed solution is optimized for a finite, shielded antenna, with a square aperture size of 1.62λ0 using full wave simulations for operation in S-band around 2.5 GHz. Simulation results show a maximum gain of 13.8 dB with a minimum AR level of around 0.6 dB. The proposed configuration is compact, shielded and suitable for narrow band applications.