Some Far-Field Studies of an Offset Launcher

At millimeter wavelengths, normal waveguide losses become too large in many applications; for example, long lengths of w a v e g u i d e are required in satellite earth stations between the transceiver and the reflector antenna focus. To reduce these losses, one m a y use quasioptical beams t h a t employ reflectors or lenses for refocussing at various intervals, thereby confining the beam within a geometric tube with no (lossy) guiding walls. To couple the circuit components to these beams, it is desirable to provide a beam launcher t h a t has quasi-gaussian amplitude over the aperture, low loss, good polarization purity, and high return loss. Offset reflectors provide high return loss, i.e., they are well matched, because the radiation field of the illuminated aperture bypasses and, therefore, does not reenter the feed horn. 1 If the reflector is made large so t h a t the level of the feed-horn illumination at the edge of the reflector is low, spillover and diffraction losses are small. Also, since the feed horn does not block the aperture of the reflector, blockage losses are negligible, and the radiation patterns are unaffected. B u t cross polarization can be serious with offset reflectors, as pointed out in Ref. 2; however, by choosing a small offset angle the cross polarization can be held to acceptable levels throughout the beam. Here, the far-field properties of an offset paraboloidal reflector fFig. 1) are investigated. Section II describes the radiation charac1319