This paper investigates a method to improve the aperture phase distribution of a Fabry–Perot cavity antenna (FPCA), using convoluted frequency selective surfaces (FSS) to improve uniformity of electromagnetic (EM) wave phase inside the cavity, thus improving peak directivity. To test the effectiveness of the phase correcting FSS to improve peak directivity, four FPCAs were designed to operate at 2 GHz. The first FPCA is a conventional FPCA with a Perspex partially reflective surface (PRS), no walls, a patch antenna emitter, and a copper base reflector. The second FPCA design is the same as the conventional FPCA with a copper base reflector, however its cavity is fully enclosed with copper walls. The third FPCA also has a copper base reflector and a fully enclosed cavity, but the copper walls are replaced with phase correcting FSS walls. The fourth FPCA replaced the copper base reflector with the same phase correcting FSS used for the walls, but they gradually decrease in size to produce an increasing reflection coefficient phase change on EM rays inside the cavity. This had the effect of countering the phase non-uniformity across a conventional FPCA’s aperture and increased its peak directivity to 20.48 dBi, which is 8.42 dBi greater than the peak directivity of the conventional FPCA.