AbstractA balanced wideband antenna based on the self‐complementary principle is known as a log‐periodic antenna. Another type of self‐complementary antenna, the unbalanced type, has not been extensively studied, even though it is useful in many areas, including mobile communication. This paper describes an unbalanced wideband antenna with an extremely simple structure based on the principle of the three‐dimensional self‐complementary antenna. A method for deriving a unidirectional and constant‐impedance antenna from a three‐dimensional self‐complementary antenna having several symmetrical radiation lobes is proposed, and a theoretical and experimental analysis of the radiation characteristics of a monopole‐slot pair with a finite ground plane is given. It is shown that the radiation characteristics remain almost unchanged when the ground plane is larger than one wavelength in four directions from the center of the antenna pair. It is also shown that the gain in the maximum radiation direction of an element pair is much higher on a semi‐infinite ground plane than on an infinite ground plane. A wide‐band antenna consisting of element‐pairs spaced at equal intervals and with a length taper is described. the antenna exhibits a substantial array effect on the gain. the gain is higher than that of a monopole‐notch array an