A compact slot antenna backed by a shallow cylindrical cavity for high‐gain unidirectional radiation in the 5.8 GHz ISM is presented. The antenna has a 96% efficiency, 8 dBi gain, 12 dB front‐to‐back ratio, and a −10 dB impedance bandwidth of 7.5%. A prototype of the antenna was fabricated and measured and the measurement results were verified against the simulated ones.