I. Introduction
Fabry–Pérot resonator antennas (FPRAs) have been investigated for many years because of their capability to provide high directivity with a simple configuration. FPRAs usually consist of a primary radiator, such as a print-patch antenna or a waveguide horn, embedded inside a cavity between a partially reflective surface (PRS) and a metal ground plane [1]. The electromagnetic field, which is caused by multiple reflections inside the cavity, spreads from the center to the edges of the antenna. As a result, a higher directivity pattern in the broadside direction is produced than the directivity pattern produced by the primary feed. Different PRS types, i.e., multilayer superstrates [2]–[5], electromagnetic band-gap (EBG) structures [6]–[8], nonuniform metallic lattice [9], and metamaterial-based superstrates [10], [11], have been proposed in an attempt to enhance the antenna performance. Apart from the PRSs, specific ground planes, such as the curve ground plane [12], [13] and the metamaterial ground plane [14], have been proposed to enhance the performance of FPRAs in terms of wideband operation [12], beam-switch capability [13], and radar cross-section (RCS) reduction [14].