So what would happen to television reception if mobile base stations were allowed to operate in the television band in the UK? Clearly, mobile base stations could not operate on the same frequencies as TV transmitters without causing widespread loss of TV reception, and in many instances TV signals would prevent use of the mobile services. This is known as co-channel interference. If mobile base stations were to use only channels not in use for TV, then co-channel interference could in principle be avoided, but the problem of overloading amplifiers and receivers would still exist. But now, filtering would be vastly more difficult to implement: designing or manufacturing a filter that passes TV signals while reducing mobile base station signals that use interleaved channels would be practically impossible using conventional filtering components. There’s also the problem that many different combinations of channels are in use for TV across the country, so filters would either have
Television transmissions are designed to be received by high gain aerials mounted at roof height, while mobile networks are intended for reception by devices with low gain aerials at ground level, and even indoors. These requirements result in quite different networks: TV has about 80 main transmitters and a little over 1,000 relays, while mobile services use about 23,000 base station sites (source: Wikipedia). As television and mobile base station signals do not normally originate from the same site, it is quite likely that a television aerial is going to receive very strong mobile signals from a nearby base station while receiving much weaker signals from a relatively distant TV transmitter. Preventing strong mobile base station signals from disrupting TV reception can be achieved by using components that reduce base station signal levels, but do not significantly affect the levels of TV signals. This typically involves the use of aerials that are carefully designed to have reduced gain above 694 MHz, or low pass filters that also reduce signals above 694 MHz - or both!
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