ont-end transistor. This was improved dramatically in recent years, especially for small DBS terminals where the economies of scale have supported considerable research to reduce the noise temperature. The ratio of the gain to noise temperature, the G/T ratio, is a useful measure of the influence of the noise components. Typical values are 40.7dBK¹ for an INTELSAT A, 30 metre diameter antenna operating at 4/6GHz (Pratt, 1986) Large earth station antennas are expensive to construct and to maintain SO that there is a premium in obtaining the maximum efficiency from the system. The axi-symmetric Cassegrain antenna (see Section 17.4.2.) is the favourite choice for a number of reasons: FEED PHASE CENTER 1. The gain can be increased over the standard parabola- hyper- bola combination by shaping the reflectors. Up to an extra 1dB is possible. Low antenna noise temperatures can be achieved by controlling spillover using a high performance corrugated horn and by Figure 17.14 Beam waveguide feed system for large using a beam waveguide feed system. earth stations 3. Beam waveguide feed systems place the low noise receivers and high power transmitters in a convenient, stationary, loca- tion on the ground. 17.5.2 Earth station antennas The beam waveguide feed system (Rudge, 1986), Figure 17.14, Earth station antennas are at the earth end of satellite links. High consists of at least four reflectors, whose shape and orientation is gain is needed to receive the weak signals from the satellite, or to chosen SO that the transmitter and receiver can be stationary whilst transmit strong signals to the satellite. The antennas can be divided the antenna is free to move in two planes. The free-space beam into three types: suffers very little loss. The dual polarised transmit and receive signals need to be separated by a beam-forming network placed Large antennas required for transmit and receive on the INTEL- behind the main feed horn. For 4/6GHz operation, this will also SAT type global networks with gains of 60 to 65dBi (15 to 30 in