r- amplifiers have largely replaced TWTs in power amplifiers at The communication chain 51/13 The basic high gain antenna is an offset fed reflector, generating Frequency modulation is most commonly used for signals which an approximately circular beam. The beam edge gain of such an are radiated in analogue form and a relatively high index of modu- antenna is given approximately by Equation 51.1 where D is the half lation is typical. Ideally, the index of modulation, and therefore the power beamwidth in degrees. bandwidth occupied before demodulation, and the carrier power level are chosen so that: Gain = 41 - log dBi (51.1) 1. The threshold of the demodulator under clear sky conditions However, it is so important to obtain the highest feasible gain from (that is, in the absence of signal absorption in the troposphere) a satellite antenna that it is usual to adopt a more complex antenna will be exceeded by a few decibels (related to the required rain design, producing a pattern of illumination on the Earth's surface and implementation margins) and the necessary post demodu- (called the 'footprint') which matches closely the geographical area lator signal to noise ratio (S/N) will be attained for the specified which is to be served. Thus, for example, INTELSAT V has six proportion of the time. antennas for access to and from the transponders and two of them, 2. The power and bandwidth available from the transponder for optimised to generate beams from a mid-Atlantic orbital location the carrier, and any other carriers that the transponder may be which serve areas on both sides of the Atlantic Ocean where large relaying, will be occupied when the transponder is fully loaded. amounts of traffic originate, take the form of front fed reflectors with an array of 88 carefully phased feed horns at their offset foci. The choice involves consideration of many of the characteristics (See Figure 51.10.) The corresponding feed horn arrays on INTEL- of the satellite network, the most important of which is probably the SAT VI a