 large reflectors. For small or medium size reflectors geometric optics is not adequate and physi- cal optics including diffraction must be used at least on the subre- flector. 17.4.3 Offset reflectors In recent years the growth in communication systems has led to a tightening in the radiation pattern specifications and the consequent need to produce reflectors with low far-out sidelobes. Symmetric reflectors cannot be made to have low sidelobes because of the inherent limitations caused by scattering from the feed and feed supports. This blockage loss can be entirely eliminated with the offset reflector, Figure 17.10, 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 Figure 17.11 Types of feed horns for reflector antennas: (a) rectangular or square; (b) small conical; (c) small the edge of the reflector will be elliptical. The removal of the blockage loss also means that smaller reflector antennas can be conical with chokes; (d) conical corrugated; (e) array feed Specific antenna types 17/11 reflector antenna. High performance feeds are necessary to achieve 17.4.4.4 Conical corrugated horns high performance antennas. The diameter of the feed in wave- lengths will be determined by the angle subtended by the reflector These are the leading choice for a feed for dual reflector and at the feed. A prime focus reflector with an F/D between 0.25 and medium size offset reflectors, Figure 17.11(b). They have excellent 0.5 will have a subtended half angle of between 90 degrees and 53 radiation pattern symmetry and radiate very low crosspolarisation degrees. Application of the general rule that beamwidth is approxi- over a broad range of frequencies. mately equal to the inverse of the normalised aperture diameter A corrugated horn propagates a mixture of TE 11 and TM 11 modes shows that this means a feed with an aperture diameter of between called a hybrid HE mode. The corrugations are approximately ab