Influence of dichroic plate thickness on polarisation of transmitted electromagnetic waves

A computer model, described in previous papers, is used to calculate the crosspolarised component of the field transmitted through waveguide dichroic plates, with thickness in the range 1.0 mm to 2.0 cm. A linearly polarised feed operating over the frequency range 19.0 to 26.0 GHz is placed 4.0 cm from the plate centre. The crosspolar levels vary more rapidly with frequency for the thicker plates and, on average, are marginally lower when the waveguides are arranged in an equilateral triangular lattice, rather than on a square lattice. With both lattices the levels fall below ¿30 dB in all planes over narrow ranges of frequency. The locations of these minima depend on the plate thickness and the lattice geometry, and are determined by the TE11 and TE*11 modes. They occur when both modes are in phase at the guide apertures on the output side of the plate.