Effect of edge region on crosspolar performance of dichroic plate

The effects of the edge regions of a waveguide dichroic plate on its crosspolar performance are examined. Crosspolar levels were measured from a feed transmitting through a dichroic plate 20 cm in diameter consisting of stacks of circular waveguides of diameter 6.5 mm and length 1 cm arranged on equilateral triangular lattices, in which the materials, extent and arrangement of the ground plane surrounding the area occupied by the guides were varied. Results indicate that the edge region is responsible for variations of the feed/plate radiation pattern due to edge illumination and surface current excitation, but that crosspolar levels as high as -23 dB can be reduced below -33 dB by proper edge design.