AbstractOne of the self‐complementary antennas is axially symmetric and three‐dimensional. Since this antenna has an axially‐symmetrical antenna, made of a finite‐sized plate, it becomes a hard‐to‐control multibeam antenna. to obtain a single beam we remove one of the unipole arrays from a self‐complementary unipole‐slot array antenna made of two planes intersecting orthogonally. the unipole‐slot array antenna to be studied then becomes a quasi‐self‐complementary antenna. First, we derive analytically that the unipole‐slot antenna has a constant impedance of 2.30 and show that this antenna has a single beam. Next, to develop a broadband high gain antenna, we analyze a unipole‐slot array antenna using a network technique and derive the relation between the gain and the structural parameters of an array antenna with a common scaling ratio. It is shown numerically that this quasi‐self‐complementary antenna maintains a constant impedance characteristic. These theoretical results are confirmed experimentally with an antenna made of finite conductors.