that channel antenna were first amplified in a 6GHz tunnel diode amplifier performance targets are reached. This margin provides in part for (TDA) and then frequency translated by 2225MHz for further under achievement of equipment performance and operating targets broadband amplification in a 4GHz TDA and a low level travelling and in part for propagation loss, due to absorption in the tropos- wave tube (TWT). This part of the system had fourfold redundancy phere, which is additional to the free space value. to achieve high reliability and long life. The signals were then split The principal cause of tropospheric loss at frequencies in use or in by a filter dividing network into 12 channels, each 36MHz wide and prospect of use for satellite communication is absorption in rain. 40MHz spacing between centre frequencies. Each channel had its Molecular resonance in water vapour begins to be significant as the own redundant high level TWT amplifier rated at 6 watts single frequency rises above 20GHz. The extent of the loss due to rain carrier saturated output power. The gain of the amplifiers for chan- varies greatly, with frequency and climate but also with the angle of nels 1 to 8 was controllable in eight 3.6dB steps; for channels 9 to elevation of the satellite as seen from the earth station, and Figure 12 there were four steps. The outputs of all of these amplifiers, after 51.8 can give no more than a very general indication of how these bandpass filtration to remove out of band distortion products, could losses vary with frequency under given weather conditions. be connected to one of two horn antennas covering the whole visible The margin of satellite carrier power that must be allowed, to disc of the Earth. Alternatively, any or all of channels 1, 3, 5 and 7 protect link performance against the effects of rain, depends signi- could be connected by telecommand to spot beam antenna number ficantly on the percentage of the time during which deterioration of 1 (half power beamwidth 4.5°) and similarly channels 2, 