ptable to the domestic market. form of unfurlable mesh or panel reflector. The trend towards smaller footprints on the earth can be met either by using a larger reflector or by using a higher frequency, both of which involve 17.5.3 Satellite antennas higher costs. To date most spot beam communication satellite have used two prime focus offset reflectors, one for transmit and one for receive, producing footprints on the earth's surface which are ellip- tical because of the curvature of the earth. 17.5.3.1 Telemetry, tracking and command (TT&C) The ideal TT&C antenna would give omnidirectional coverage SO that the orientation of the satellite would be irrelevant. Wire an- 17.5.3.3 Multiple beams tennas are used for VHF and UHF coverage but the spacecraft is a few wavelengths across at these frequencies and therefore consid- It was early recognised that by using a single reflector and an array erable interaction between the antenna and the satellite distorts the of feeds it was possible to produce multiple beams on the earth, radiation pattern. An alternative approach is to use a low gain horn Figure 17.15. This has the advantage that most of the antenna antenna to provide full earth coverage. This is particularly useful for sub-system is re-used with the penalty of having to design and make spin stabilised spacecraft. The earth subtends 17 degrees from a the array of feed horns and the beam forming network behind the geostationary satellite which can be met with a small conical horn. array. The array feed elements must be compact SO that they occupy Antennas used in communication systems 17/15 Figure 17.16 European contoured beam the minimum space in the focal plane of the offset reflector. At the theoretical design process is quite extensive and involves a syn- same time the crosspolarisation must be low. This tends to mean that thesis process with the input of the required beam shape and the corrugated horns cannot be used and small diameter dual-mode output of the contours of the reflector surface. A single offs