Cross polarisation of electromagnetic waves transmitted through waveguide dichroic plates

Dichroic or frequency-sensitive surfaces modify the cross-polar performance of feeds transmitting through them. In the paper we investigate the influence of two flat waveguide dichroic plates 1 cm thick and 15 cm in diameter, placed 4 cm in front of a simple, linearly-polarised, choked waveguide feed. Measured cross-polar levels, mainly in the 45° planes, are compared with the levels predicted by a modal analysis in the frequency range 19¿26 GHz. The results agree well. With these particular plates the cross-polar levels over most of the frequency range are lower when the guides are arranged on a close-packed equilateral triangular lattice than when on the more-symmetrical square lattice. Ten wave guide modes are included in the analysis, and their relative importance is discussed. Two cumulative radiation patterns are illustrated, comparing the development of the farfield radiation patterns for the two lattices as the contributions of successive guide modes are summed.