Radiation and depolarization of multibeam offset reflectors fed by multimode pyramidal horns

The depolarization contents of the radiating currents on the surface of a multibeam offset parabola are minimized, for different beams, using multimode pyramidal horn feeds. Small percentages of the TE/sub 11/ and TE/sub 20/ modes are used, respectively, for vertical and horizontal polarizations, in combination with the fundamental TE/sub 10/ mode. The mixture ratio increases for feeds with higher pointing angles to the reflector surface. Additionally, the beams that are away from the symmetry plane, suffer changes in the polarization bearing angles, which increase for further beams. Far radiated fields are computed from the depolarization minimized reflector currents, using both the orthogonal components of the linear polarization, for a 3M offset reflector with a diameter-to-focal-length ratio of unity, at 4 GHz. Minimized depolarization levels of around -40 dB for vertical polarization, and of around -34 dB for horizontal polarization are found. The linearly and circularly polarized diagonal horn feeds yield symmetric pencil beams. Their polarization, however, is quite high and difficult to control for linear polarization. For circular polarization, to the contrary, the depolarization is quite low.<
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