A new configuration of a self-polarizing Fabry-Perot (FP) antenna is presented to generate circular polarization with high gain levels using a simple linearly polarized feed. It consists of an FP resonator combined with a polarization-twisting ground plane. An analytical model is proposed to facilitate the antenna design, and the corresponding results are shown to be in very close agreement with full-wave simulations. The experimental prototype built in -band exhibits a combined bandwidth (3 dB axial ratio, 3 dB gain drop, and 10 dB impedance matching) of 3% with a maximum realized gain of 18.0 dB. The antenna is completely shielded with an aperture size of and a height of only . Such antennas are attractive candidates for high-power space applications at low frequencies ( -to -bands) where standard horns are very bulky.