Satellites are radio links and receive signals from the ground and other satellites which they must re-transmit. The signals from ground stations are comparatively weak and require high power amplification for onward transmission. As the satellite’s receive and transmit antennas must be close together, the possibility of RF instability prohibits on-frequency repetition. The up-link, from ground station to satellite, must therefore be converted to another before re-transmission. The up-link frequency is normally higher than the down-link and the frequency converter is referred to as a ‘down-converter’.
The frequency bands used for communications purposes are listed in Table 22.1.The broadcasting of television may be via either communications satellites or Direct Broadcasting by Satellite (DBS) satellites. The positions of non-DBS satellites relative to the UK are shown in Figure 22.2 and Table 22.2 lists the European channels. Figure 22.3 shows the world allocations of DBS satellites and Tables 22.3 and 22.4 list the channel frequencies and national allocations. The frequency plan for the Astra satellite is in Table 22.5.