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This letter describes a square patch antenna with a switchable circular polarization (CP) sense. The proposed antenna has four L-shaped slots on the ground plane, and the CP radiation can be generated by current perturbation due to the slotted ground plane. Because the CP sense of the proposed antenna is altered by the current path that is redirected with switched p-i-n diodes on the slots, the CP direction of the proposed antenna can be simply switched between the right-handed and left-handed CPs. As the slot and bias circuits are not placed on the patch side, the proposed antenna has a simple structure and can radiate a CP wave without altering the main beam direction. The experimental results show that antennas have excellent switchable radiation performance at 2.4 GHz.