Compact Fabry-Perot Cavities in x-band with PEC and PMC Boundary Conditions

Resonator antennas based on Fabry-Perot (FP) cavities are low-profile directive antennas widely studied for moderate or high-gain applications. In contrast to previous studies, we investigate here their capabilities when employed as primary sources of Focal-Array Fed Reflector antennas (FAFR) for space applications in X-band. In addition to standard specifications in terms of bandwidth, return loss and radiation efficiency, one of the main requirements not addressed in prior works consists of the optimization of their aperture efficiency. To this end, we first investigate numerically the radiation performance of pure metallic (PEC) compact FP antennas whose size is in the order of a few wavelengths. Their key features are discussed and compared with the well-known characteristics of electrically large FP antennas. In particular, it is shown that there exists an optimum value for the grid reflectivity in order to maximize the antenna aperture efficiency. Then, a new TEM-mode FP antenna with PMC walls (closely-coupled printed dipoles on a grounded substrate) in H-plane is proposed. This configuration enables one to enhance significantly the aperture efficiency of the antenna as well as to produce an axis-symmetric beam, which is not possible with compact FP cavities with PEC walls.