A microstrip antenna having a dual polarization and a smaller size than feed horn polarizer of F.R.R.S(Faraday Rotation Rotary Switch) is designed, in which GaAs MESFET switches are inserted for selective reception of RHCP or LHCP. For an accurate analysis of the resonance frequency, input impedance and radiation pattern of the circularly polarized microstrip antenna, finite difference time domain (FDTD) method is used. When the GaAs MESFET switch in the feeder is ON-stats, the truncated patch antenna has a gain of about 16.6dB including amplifier gain, while the switch is OFF-state, this has a isolation level of -24dB.