Improvement of elevation directivity for ESPAR antennas with finite ground plane

Electrically steerable passive array radiator (ESPAR) antennas are investigated for low-cost and small analog adaptive beamforming. ESPAR antennas are constructed by dipole elements without a ground plane or monopole elements with a ground plane. A passive radiator located around an active radiator is loaded a reactor implemented with a varactor diode. We investigate the shape and size of the ground plane that lowers the elevation angle of the maximum radiation. It is a circular ground plane with a skirt that is on the edge of the circle. In this paper, the radiation characteristics of the monopole antenna that is on the circular ground plane with and without a skirt are calculated and measured when the radius of the circular plane is about 0.5 wavelength and the length of the skirt is about 0.25 wavelength, the maximum gain in the horizontal direction is obtained. For the 7-element ESPAR antennas, maximum radiation is obtained in the horizontal direction.