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Resonant cavity antennas (RCAs) are usually used in applications that require high-gain antennas. High-gain is achieved by using highly-reflective frequency selective surfaces (FSSs) as the superstrate and adjusting the height of the RCA. In this paper, different types of FSSs are employed as RCA superstrates. The formulation provided in implies that the RCA directivity only depends on the reflection coefficient magnitude of the FSS superstrate. In this paper, it is shown that the reflection phase plays an important role, too. The importance of the reflection phase lies in the fact that the resonant height of the RCA is directly related to it. It is illustrated that for two RCAs having different FSS superstrates with identical reflection magnitude, the one whose resonant height is larger exhibits higher gain. Some other points, useful for RCA designs, are also indicated and briefly discussed.