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The six basics of sat TV broadcasting Most satellite TV viewers are blissfully ignorant of how it works, but if you want to get the most out of your system it helps to know a little more 98W. So it’s a little like signalling with a torch from the top of the Empire State Building in New York to someone wearing sunglasses on the London Eye. When the signals get to your home they are incredibly weak, so the antenna uses a large collecting area to gather up as much signal as possible (that’s the dish reflector) and a very sensitive detector (the LNB).
You don’t have to know how satellite broadcasting works to watch satellite TV, any more than you need to know the ins and outs of the internal combustion engine to drive a car. But understanding the background can help you choose the best system for your circumstances, get the most out of system – and diagnose what’s the matter when it goes wrong. So let’s leave our armchairs for a trip into space to get a close-up look at TV satellites and how they TV signals to us on Earth.
Frequency and bandwidth
03 Just like terrestrial TV and radio
In orbit Why in space?
01 Until satellite TV came along, TV was distributed to viewers from terrestrial transmitters and by cable. The problem with cable is you have to physically run a wire to each and every viewer – and that’s disruptive, expensive and wasteful. Broadcasting the signal as a radio wave is much better, but terrestrial transmitters don’t have a very wide reach, particularly in hilly areas where would-be viewers in the valleys are shielded by the hills. The solution is to broadcast from the top of a tall mast – but the geography can still get in the way – or to build a lot of small ‘repeater’ masts at great expense to reach a small number of people. A TV satellite is effectively an incredibly tall transmitter mast in orbit 36,000km above the Earth so the signals can reach into the deepest valleys with ease. The really clever bit (thank you, Arthur C Clarke) is that the satellites are in a special ‘geostationary orbit’, so they appear to stay still in the sky.
Orbiting at 36,000km, satellite TV can reach the places that terrestrial transmissions cannot see
transmissions, the TV satellites use different frequencies for different signals, and your receiver tunes to a particular frequency to access a particular service. However, satellite TV signals are at a much higher frequency than terrestrial TV or radio transmissions (even higher than ‘very high frequency’ VHF and ‘ultra high frequency’ UHF). Most satellite TV signals for Europe are in the range 10.70-12.75GHz (compared with 470-862MHz for UK terrestrial TV). Using a higher frequency avoids interference with terrestrial transmissions,
and it works well from orbit. Each signal is not just at one particular frequency, but takes up a small range of frequencies (the bandwidth) centred on the one frequency we use to identify it (and which the receiver tunes to). The wider the bandwidth, the greater the amount of information that can be carried in the signal. Each signal comes from one ‘transponder’ on the satellite – a single transmitter of which modern satellites typically have 20 to 40. Several TV and radio channels are usually compressed and multiplexed on each transponder’s signal. Polarity
04 Because each signal from the satellite takes up a chunk of the whole satellite frequency range, only a limited number of signals can be broadcast at once – the individual signals cannot overlap in frequency or they won’t be separable by the receiver. To get more signals out of the same frequency space the satellites broadcast
Satellite transponder
The need for a dish
02 As they are so high up, the satellites’ signals are dissipated over a wide area and have to travel a very long way to get to the viewers’ antennas, so they’re weakened considerably. What’s more, the satellite gets all its power from sunlight that charges onboard batteries, so the transmitters on board are not very powerful – even the powerful Astra 2A has transmitters of just 2 What Satellite & Digital TV
Terrestrial transmitter
Home with satellite only
Home with terrestrial and satellite TV
ABC guide to... bandwidth
guard band
frequency
frequency
frequency
using two ‘polarities’ – called horizontal and vertical to describe the way in which the electromagnetic fields that make up the signal are oscillating. The receiving system can distinguish between signals that have the same frequency but different polarities, and so twice as many signals can occupy the same frequency space – it’s a bit like putting houses on both sides of the road so you can have twice as many homes on one street. In practice, the signals are staggered in frequency to avoid crossover interference between the polarities. There are two detectors in the LNB, one each for horizontal and vertical signals, and the receiver selects which is used by switching the supply voltage to the LNB – 13V for vertical and 18V for horizontal. In older systems or on advanced motorised dishes, the polarity is selected with a control signal to a separate ‘polariser’, which can adjust to a signal of any polarity angle to accommodate different satellites. Frequency bands
05 The receiver tunes to an individual transponder’s signal from all that are received, but it cannot easily cover the whole range so the reception is divided into low and high frequency bands. The LNB switches between reception of the two frequency bands under the control of the receiver using a 22kHz tone superimposed on the power supply to the LNB – no tone for the low band and 22kHz for the high band. So, by supplying the LNB voltage at 13V and 18V and switching on or off the 22kHz tone, the receiver can select between four ‘sub-bands’ of received
Each transponder’s signal is separated from the next by a ‘guard band’ to prevent interference between them (top). Cramming more transponders into the frequency space causes clashes between the signals (middle) but with alternate transponders utilising the opposite polarity more transmissions can be fitted into the same frequency spectrum (bottom)
signals – low band horizontal polarity, low band vertical polarity, high band horizontal polarity, and high band vertical polarity – as well as tuning to the individual signal required within the selected sub-band. Going up
06 Of course, no signals could be broadcast from the satellite unless those same signals were sent up to the in-orbit transmitters from the ground. The path of
the signal from the ground to the satellite is called the Uplink (the path from the satellite to your dish is the Downlink). The uplink uses a different range of frequencies (14.0-14.5GHz) and requires a huge dish to beam the transmission at the satellite. Of course, the signals can be uplinked from close to the receiving dish, or a different country. The whole round trip from uplink transmitter to the home dish takes the signal about 0.25secs n Geoff Bains
horizontal, low band 18V 0kHz
A universal LNB divides the Ku-band satellite signal into four frequency/polarity sub-bands
vertical, low band 13V 0kHz horizontal, high band 18V 22kHz vertical, low band 13V 0kHz
Glossary Geostationary orbit
An object circling the Earth at about 36,000km above the Equator travels at the same speed as the Earth turns – so it appears to be stationary to an observer on the ground. Arthur C Clarke realised this would make it easy to target with cheap, fixed antennas, instead of expensive motorised systems.
Low Noise Block Downconverter (LNB)
The LNB picks up and amplifies a wide bandwidth (‘block’) of faint high-frequency satellite signal to convert it to a lower ‘intermediate frequency’ (IF) signal suitable for the short cable to a satellite receiver, adding as little interference as possible.
What Satellite & Digital TV 3