Impedance variations in controlled reactance parasitic antennas

Harrington (1978) proposed a circular array of reactive loaded parasitic elements surrounding a half-wave dipole antenna. The radiation characteristics are electronically controlled by changing the values of the reactances. Recently this approach to adaptive beamforming applied to a monopole array on a finite ground plane has been of significant interest. While some attempts have been made to optimize the structures for maximum gain, often the input impedance of the antenna is not considered. Given that some applications of smart antennas relate to target tracking and positioning, the effect of changing parasitic element reactance on both gain and antenna impedance needs to be assessed simultaneously if the signal direction is to be determined from maximum signal input.