Electronically Steerable Parasitic Array Radiator Antenna (ESPAR) consists of a central active element surrounded by several reactance-load controlled parasitic elements and a ground plane. The ESPAR antenna can realize beam scanning. For the advantages of simple structure, easy control and etc, the ESPAR antenna has been applied widely in the vehicle, airborne and other mobile communication system. In this paper, the one-port driven passive ESPAR antenna and the improved ESPAR antenna are studied. The main...