AbstractThe constant impedance characteristics of self‐complementary antennas are theoretically established; however, the wideband nature of their radiation patterns is not in general assured. A number of works have been published in which widebanding of radiation patterns is attempted by creating axially symmetric self‐complementary monopole‐notch array antennas on the edge of a conductor. However, for the most part these studies are based on experimental procedures. In this paper the radiation characteristics of the self‐complementary monopole‐notch antenna are analyzed theoretically and the array structure with wideband constant gain characteristics is discussed. A monopole‐notch antenna created on the edge of an ideal semiinfinite conducting plane is first analyzed and found to possess wideband radiation characteristics. A more practical monopolenotch antenna created on a finite conducting plate is then analyzed by the induced e.m.f. method. The results show that the gain is higher for the structure with a longer edge in the main beam direction. It is also found that an antenna with constant impedance and constant gain characteristic can be obtained by setting up a monopole‐notch array on a triangular conducting plate. The validity of the theoretical analysis is experimentally confirmed and the limitation of the induced e.m.f. method