iciency). By shaping the surfaces of a dual reflector antenna, it is possible to increase efficiency and produce a more uniform illumination across the aperture. A well- known method to produce a high-efficiency Cassegrain symmetric-reflector antenna is from Galindo (1964) and Williams (1965). It is a geometric optics technique in which the shape of the subreflector is altered to redistribute the energy more uniformly over the aperture. Then the shape of the main reflector is modified to refocus the energy and create a uniform phase across the aperture. After this process the reflector surfaces are no longer parabolic and hyperbolic. The method works well for large reflectors. For small- or medium-size reflectors, geometric optics are not adequate, and physical optics, including diffraction, must be used, at least on the subreflector. Offset Reflectors In recent years the growth in communications systems has led to a tightening of radiation pattern specifications and the consequent need to produce reflectors with low far-out Chapter 9 Antennas Fig. 9.12 Offset reflector antennas. sidelobes. Symmetric reflectors cannot be made to have low sidelobes because of the inher- ent limitations caused by scattering from the feed and feed supports. This blockage loss can be entirely eliminated with the offset reflector (see Figure 9.12), which consists of a portion of a parabola chosen SO that the feed is outside the area subtended by the aperture of the reflector. The projected aperture is circular, though the edge of the reflector is elliptical. The removal of the blockage loss also means that smaller reflector antennas can be made efficient, which has led to their widespread use as DBS receiving antennas. In addition to the unblocked aperture, the offset reflector has other advantages (Rudge, 1986; Rahmat-Samii, 1986). The reaction of the reflector on the primary feed can be reduced to a very low order SO that the feed VSWR is essentially independent of the reflector. Com- pared to a symmetric paraboloid, the offset conf