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The differential Fabry-Perot resonator (FPR) antenna is investigated for the first time. It has advantages of suppressing the cross polarization and enabling integration with differential circuits. Its basic structure consists of two vertical parallel metal plates, which are connected by a horizontal ground plane. The differential FPR antenna is fed by a pair of differential L-probes protruding from the ground plane. It is found that the basic differential FPR antenna has considerable sidelobes in the E-plane. To suppress the sidelobes, a modified differential FPR antenna is proposed which has a pair of ridges at each side-opening of the parallel plates. In addition, a second modified differential FPR antenna is also proposed to reduce undesirable backward radiation. The second modified antenna has a pair of ridges placed at the top of each metal plate. It is found that all of the basic and modified antennas have low cross polarizations. The two modified differential FPR antennas were fabricated and measured, and the measurements agree reasonably well with HFSS simulations. The first and second modified antennas have calibrated measured gains of 14.2 dBi and 15.4 dBi, respectively, with measured 10-dB impedance bandwidths of ~ 2.4%.