Offset Multireflector Antennas with Perfect Pattern Symmetry and Polarization Discrimination

A suitable feed for the antennas considered here is realized by properly corrugating the walls of a circular horn. 1-4 The spherical wave radiated by the horn then has circular symmetry and, by placing the feed at the focus of a paraboloid, an antenna with circular symmetry in the far field is obtained, provided the paraboloid is centered around the feed axis. Furthermore, the polarization of the plane wave reflected by the paraboloid then coincides with that of the feed excitation. However, in the centered configuration the reflected wave is in part blocked by the horn.* To avoid this, the horn axis can be offset as in Fig. 1, but unfortunately this causes asymmetry in the pattern after reflection, resulting in an undesired cross-polarized component. 5,6 The same behavior occurs if, instead of a paraboloid, an arbitrary reflector system with a single axis of revolution is used. In Fig. 1, the asymmetry of the reflected wave increases with the angle of incidence a of the ray corresponding to the horn axis. This particular ray will be called principal ray. Although a single offset reflection always causes some asymmetry, it