ns is large, it will usually be carried by one of the other all parts of the world, for re-broadcasting. A transponder is reserved satellites serving the region, called a Major Path Satellite. for this purpose in each Primary Satellite and frequency modulation by an analogue video signal, which may contain a digitalised sound All current INTELSAT satellite series have transponders opera- signal, time division multiplexed amongst the synchronising pulses, ting in both the 6GHz and 4GHz and the 14GHz and 11-12GHz is usual. bands. High gain spot beam dish antennas, steerable from the Wide band low noise first stage amplifiers, typically parametric ground, are used for the transponders operating in the higher fre- amplifiers, are used to compliment the high receiving gain of the big quency band pair. For the lower band pair, some transponders use earth station antennas, and these amplifiers are usually thermo-elec- dual polar horn antennas covering the whole Earth visible from the trically cooled (Peltier effect) at 4GHz. High power is often required satellite whilst other transponders use higher gain reconfigurable from the output stages of earth station transmitters, involving hun- dual polar multi feed reflector antennas to cover smaller areas, the dreds of watts output and sometimes several kilowatts, more espe- footprints being shaped to fit the geographical configuration of cially when the station will be transmitting television signals to a important service areas (see Figure 51.10). Both travelling wave global coverage transponder or where large numbers of telephone tube and solid state power amplifiers are used with single carrier channels are to be transmitted on several carriers. These high power saturated output power ratings up to 20 watts, and transponder amplifier stages may consist of wide band travelling wave tubes, or bandwidths ranging between 36MHz and 112MHz. groups of klystrons operating in parallel through frequency selec- Earth station antennas having access to INTELSAT satellites for tive com