A novel antenna that utilizes waveguide properties of a cylinder with a longitudinal slot is presented in this paper. It has been shown theoretically and experimentally that if the length of the slot is about the wavelength in free space, the field distribution in the antenna aperture is very close to the optimal and consequently the antenna can reach its maximum efficiency. It has been shown that the single slot antenna can be used as a basic building block for high performance directional panel and base station antennas and also in active antenna arrays.