Satellite TV Technical Information

As far as Sat TV reception only is concerned, the earth station is much simpler. The antenna need not be to very stringent specification with very tight sidelobe control, surface accuracy etc. This is because the earth station does not transmit any signal to the satellite, with the attendant, essential, International standards for earth stations to be observed. Neither is there a need for any duplexer, as no transmitter is involved in the ES. So also you can eliminate the transmit reject filter. The simpler antenna system could be connected straight away to the LNB and a coaxial cable used to connect its output to a video receiver.

The video receiver accepts the IF signal, between 950 and 1450 MHz, converts it, usually, to a lower IF and demodulates it, to provide separate Video output and Audio output. These can be used to record the programme on to a VCR or drive a modulator to provide a TV signal on a desired TV channel, which a normal domestic TV set can receive, like a normal broadcast signal, when the signal is connected to the TV RF input; where you normally connect the TV antenna. Video receivers also have an internal modulator giving an RF signal suitable for TV reception on a designated channel, generally channel 4. Some video receiver manufacturers offer a choice of modulated output channel say channel 3 or channel 4.

A TV broadcasting satellite may transmit different TV programmes on different transponders using different polarisation. The TVRO or Sat TV receiving system must ensure that its antenna is designed/set to receive the required polarisation. When you desire to receive a programme from a transponder operating with Vertical or V polarization, your earth station must also be set to receive V polarised waves. When the programme to be received is transmitted with H polarisation, your system must be set to receive H polarisation signals. If this is not done your system would not receive the TV broadcasts the earth station Video receiver is tuned to, even when everything else is OK This is because the signal power levels on the earth's surface due to the satellite radiations are controlled to tight specifications to prevent interference with other/neighbouring satellites signals, terrestrial systems and spillover to neighbouring countries etc. The field intensity permitted is about 103 dB W/m' at the edge of the beam in the footprint of the satellite. This is adequate to give a C/N ratio of 14 dB considered necessary for good TV reception with reasonable earth station systems. A wrong polarisation setting in the antenna results in a loss of upto 20 to 30 dB; the loss is called Polarisation Loss. This loss results in the input RF signal from the satellite being reduced to far below the receiving system sensitivity.

MAIN CONSIDERATIONS IN SAT TV RECEPTION

*When you set up an earth station to acquire a signal from a satellite, you must ensure the following:

*The system must have the minimum g/T required to receive the satellite signals.

*The antenna mainlobe is correctly aligned to the required satellite. (The look angle coincides with the direction linking the antenna to the satellite, from that location.)

*The antenna polarisation is set to receive the polarisation of satellite signal.

*The receiver system has the correct frequency and IF bandwidth, required to accept the satellite signals.

*The demodulation system in the receiver system is matched to the modulation employed in the satellite signal. (NTSC, PAL, SECAM, CCIR standards concerned etc.)

*The audio subcarrier or subcarriers are matched to those used in the satellite signals. The system output gives the required TV channel signal.

*Necessary stabilised reliable power supply is ensured. All units have been properly Installed, correctly connected and perform adequately.

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