Frequency for Satellite

SELECTION OF FREQUENCY FOR SATELLITE TV AND COMMUNICATIONS

You may recall that there is an 'ATMOSPHERIC WINDOW between 1 GHz and 10 GHz, where the atmospheric attennuation is a minimum. The atmospheric attennuation adds to the space attennuation resulting in increased path or link attennuation. Selecting the frequency of operation of satellite transponders to be within this window helps in reducing ground equipment cost and complexity. You have also seen that higher the frequency of operation, higher can be the gain of the antennas, with reasonable size; on the other hand, higher is the spreading loss. Also higher the attennuation due to atmosphere; above about 20 GHz this attennuation increases rapidly. Taking all these into consideration the CCIR has allocated frequencies for satellite communication and broadcasting uses. Satellite transmissions also need to coexist without interference with a large infrastructure of terrestrial communications in the frequencies above 1 GHz. The most extensively used frequency band for satellite communication is the C band 3.7 to 6.4 GHz. Usually a 500 MHz bandwidth is used in the transponder, that is 500 MHz bandwidth for the transmitter at around 4 GHz frequency. Usually 3.7 to 4.2 GHz is used in the satellite transmitter and 5.9 to 6.4 GHz is used in the satellite receiver. Hence the ground transmitters will operate in the band 5.9 to 6.4 GHz and the ground receivers operate in the band 3.7 to 4.2 GHz. Each transponder has a bandwidth of 36 to 40 MHz, Therefore in the 500 MHz bandwidth 12 transponders can conveniently operate. Using polarisation discrimination that is possible, vertical and horizontal polarised waves provide discrimination of pick up one against the other (could be '20 30 dB), you can thus have a maximum of 24 transponders coexisting in a single satellite, and all using the same antenna by multiplexing. The frequencies of the transponders are offset by 20 MHz between the two polarisation, and 40 MHz between same polarisation. Thus if the transmitter of first transponder is 3720 MHz, with V polarisation, the next transmitter will have a frequency for its RF carrier as 3740 MHz and it will be Horizontally polarised; and the next transponder will have 3760 MHz and Vertical polarisation again, followed by 3780 MHz transponder with H polarisation and so on.

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