Quasi-Optical Polarization Diplexing of Microwaves

To achieve frequency reuse by employing orthogonal polarizations in a radio communication system, it is essential to avoid cross polarization in the feed patterns that illuminate the antennas; this relies upon the diplexing of two orthogonal polarizations with high isolation. Waveguide-type polarization couplers perform diplexing well where a carrier with an associated bandwidth of about 10 percent is involved. However, it is difficult to provide a low-loss (0.1 dB) waveguide diplexer to separate effectively and simultaneously the two polarizations in each of two widely separated common carrier bands, such as 18 and 30 GHz. The difficulty stems from the vulnerability of an oversized 1665 waveguide to higher-order modes in the higher-frequency band. Contamination by only 1 percent of the power in higher-order modes may cause unacceptable cross-polarized radiation in the feed. The necessity for overcoming this problem has led to the suggestion of using a closely spaced wire grid as a quasi-optical polarization diplexer. The purpose of this paper is to describe feasibility studies of this quasi-optical approach. Not only should the insertion loss be small and the feed pattern distortion slight in the principal polarization, but minimization of the cross-polarized radiation should also be achieved. Our measurements have shown that the wires must be oriented in a preferred direction to minimize the cross-polarized radiation. This property is explained theoretically by utilizing a magnetic current sheet for transmission through the grid and an electric current sheet for reflection from the grid.