CROSS POLARIZATION OF ELECTROMAGNETIC-WAVES TRANSMITTED THROUGH WAVE-GUIDE DICHROIC PLATES

Dichroic or frequency-sensitive surfaces modify the crosspolar performance of feeds transmitting through them. The influence of two flat waveguide dichroic plates 1 cm thick and 15 cm in diam, placed 4 cm in front of a simple, linearly-polarized, choked waveguide feed is investigated. Measured crosspolar levels, mainly in the 45 degree planes, are compared with levels predicted by a modal analysis in the frequency range 19-26 GHz. With these plates, the crosspolar 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 waveguide modes are included in the analysis, and their relative importance is discussed. Two cumulative radiation patterns are illustrated, comparing the development of the far-field radiation patterns for the two lattices as the contributions of successive guide modes are summed.