Polarization properties of offset dual-reflector antennas

For an offset dual-reflector antenna design of zero geometrical optics cross polarization, the diffraction-induced cross polarizations calculated from the differential phase shift between the fundamental TEM/sub 00/ and higher order TEM/sub 01/ Gaussian beam modes, which represent principal and cross-polarization components, respectively, are in agreement with measured data. Since this residual cross polarization in the main reflector aperture is in approximate phase quadrature with the principal polarization, the far-field cross polarization is in phase with the principal polarization. Thus, both principal polarizations exhibit the same slight rotation for a given far-field direction. An important consequence is the preservation of orthogonality between two orthogonal linear polarizations from the feed.<>