TranspondersA transponder is the series of interconnected units which forms a singlecommunications channel between the receive and transmit antennas in acommunications satellite. Some of the units utilized by a transponder in agiven channel may be common to a number of transponders. Thus, althoughreference may be made to a specific transponder, this must be thought ofas an equipment channel rather than a single item of equipment.Before describing in detail the various units of a transponder, theoverall frequency arrangement of a typical C-band communicationssatellite will be examined briefly. The bandwidth allocated for C-bandservice is 500 MHz, and this is divided into subbands, one for eachThe Space Segment 213transponder. A typical transponder bandwidth is 36 MHz, and allowingfor a 4-MHz guardband between transponders, 12 such transponders canbe accommodated in the 500-MHz bandwidth. By making use of polar-ization isolation, this number can be doubled. Polarization isolationrefers to the fact that carriers, which may be on the same frequency butwith opposite senses of polarization, can be isolated from one anotherby receiving antennas matched to the incoming polarization. With linearpolarization, vertically and horizontally polarized carriers can be sep-arated in this way, and with circular polarization, left-hand circularand right-hand circular polarizations can be separated. Because thecarriers with opposite senses of polarization may overlap in frequency,this technique is referred to as frequency reuse. Figure 7.12 shows partof the frequency and polarization plan for a C-band communicationssatellite.214 Chapter SevenFigure 7.11 Satellite control system. (Courtesy of Telesat Canada, 1983.)Frequency reuse also may be achieved with spot-beam antennas, andthese may be combined with polarization reuse to provide an effectivebandwidth of 2000 MHz from the actual bandwidth of 500 MHz.For one of the polarization groups, Fig. 7.13 shows the channelingscheme for the 12 transponders in more detail. The incoming, or uplink,frequency range is 5.925 to 6.425 GHz. The carriers may be receivedon one or more antennas, all having the same polarization. The inputfilter passes the full 500-MHz band to the common receiver whilerejecting out-of-band noise and interference such as might be causedby image signals. There will be many modulated carriers within this500-MHz passband, and all of these are amplified and frequency-converted in the common receiver. The frequency conversion shiftsthe carriers to the downlink frequency band, which is also 500 MHzwide, extending from 3.7 to 4.2 GHz. At this point the signals arechannelized into frequency bands which represent the individualtransponder bandwidths.A transponder may handle one modulated carrier, such as a TV signal,or it may handle a number of separate carriers simultaneously, eachmodulated by its own telephony or other baseband channel. explain