 by controlling spillover using a high-performance corrugated horn and by using a beam waveguide feed system. 3. Beam waveguide feed systems place the low-noise receivers and high-power transmitters in a convenient, stationary location on the ground. The beam waveguide feed system (Rudge, 1986), as shown in Figure 9.16, consists of at least four reflectors, whose shape and orientation are chosen SO that the transmitter and receiver can be stationary while the antenna is free to move in two planes. The free-space beam suffers very little loss. The dual-polarized transmit and receive signals need to be separated by a beam-forming network placed behind the main feed horn. For 4/6-GHz operation, this also incorporates circular polarizers. The narrow beam from the large antenna necessitates the incorporation of some form of tracking into the antenna because even a geostationary satellite drifts periodically. There are a number of schemes available, including monopulse, conical scan, and hill climbing. The favorite is a monopulse scheme using additional modes in the feed horn to electro- magnetically abstract the tracking data. Chapter 9 Antennas The first generation of medium Earth station antennas were axisymmetric Cassegrain reflector antennas, sometimes shaped. However, the advent of tighter pattern specifications has led to the widespread use of single- or dual-offset reflector antennas (see the Offset Reflectors section). These can meet low-sidelobe specifications by removing blockage effects from the aperture. Very-high-efficiency designs have been produced by shaping the reflectors to optimize the use of the aperture (Bergman et al., 1988; Cha, 1983; Bjontagaard & Pettersen, 1983). For these high-efficiency designs, the r.m.s. surface error on the main reflector needs to be less than 0.5 mm for operation in the 11- to 14-GHz band. The feed is a high-performance corrugated horn. The offset reflector configuration lends itself to deployment, and portable designs have been produced where the offset reflector folds