A 20 GHz 2 X 2 subarray radiator of microstrip-fed cavity-backed circularly polarized horn is presented. Since horn antennas incorporating waveguide types of circular polarizers have big size of radiator height, microstrip feeding and polarization is proposed and cavity-backed introduced to improve radiation performance. Two approaches of single and 2 X 2 subarray of horn are compared to be a basic radiation element of an array antenna. (c) 2006 Wiley Periodicals, Inc.