corporate circular polarisers. metres diameter). The narrow beam from the large antenna necessitates the incor- 2. Medium sized antennas for cable head (TVRO) or data receive poration of some form of tracking into the antenna because even a only terminals (3-7 metres diameter) geostationary satellite drifts periodically. There are a number of 3. Small antennas for direct broadcast reception (0.5-2 metres schemes available, including monopulse, conical scan and hill diameter). climbing. The favourite is a monopulse scheme using additional modes in the feed horn to electromagnetically abstract the tracking Types 1 and 2 have to satisfy stringent specifications imposed by data. regulatory bodies. When the recommended spacing of satellites in The first generation of medium earth station antennas were axi- the geostationary arc was 3 degrees, the pattern envelope was symmetric Cassegrain reflector antennas, sometimes shaped. How- specified by 32 - 35 logO. This could be met with a symmetric ever the advent of tighter pattern specifications has led to the reflector antenna. With the new spacing of 2 degrees, the pattern widespread use of single or dual offset reflector antennas (see spec has been improved to 29 - 25 log 0, Figure 17.13. This can Section 17.4.3). These can meet the low sidelobe specifications by best be met with low sidelobe, offset reflector designs. removing blockage effects from the aperture. Very high efficiency The minimum receivable signal level is set by inherent noise in designs have been produced by shaping the reflectors to optimise the system. Earth stations are required to detect small signals SO the the use of the aperture (Bergman, 1988; Cha, 1983; Bjontagaard, control of the noise parameters is important. The noise appearing at 1983). For these high efficiency designs, the r.m.s. surface error on the output terminals of an earth station used as a receiver has three the main reflector needs to be less than 0.5mm for operation in the components; the noise received by the main beam of the re