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I can readily understand why the strength of the signal received by my mobile phone should vary while I am travelling by train. What I do not understand is why the same thing happens when I am at home. Why does my signal vary so much?
鈥� Most people have seen the ripples spreading out from a pebble that has been thrown into a still pond. Fewer will have noticed what happens if the ripples hit a wall at the edge of the water: they are reflected back into the incoming ripples, creating a criss-cross pattern of peaks and troughs.
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Radio waves behave in a similar manner when reflecting off solid objects. This causes peaks and troughs in signal strength, depending on the phone鈥檚 location inside your house.
Cellphones use radio frequencies with quite short wavelengths, so the distance between peaks and troughs can be as little as 75 centimetres. As cellphones transmit and receive on different frequencies, the troughs for speaking may not be in the same places as those for receiving, hence the oft-announced 鈥淚 can hear you loud and clear鈥� but can you hear me?鈥�
The search for a good spot in which to use your phone is often hindered by the fact that the signal strength display is relatively slow to update. And even when you have found a hotspot, just moving the phone to your ear may lose the signal.
Your hand, body and head also affect signals, so it鈥檚 a wonder cellphones work at all.
Joe Reddaway, Wrexham, Clwyd, UK
鈥� In an urban environment, your phone can talk to any one of several base stations or 鈥渃ell sites鈥�, each transmitting and receiving on multiple frequencies or 鈥渃hannels鈥�. Each cell site has only a limited range 鈥� partly due to buildings blocking the signals and partly to maximise the number of channels available by allowing nearby cell sites to use the same frequencies.
When it has channels available, a cell site will transmit an 鈥淚 am available for business鈥� signal. When all its channels are busy, this signal disappears and that cell site becomes effectively invisible to your phone, which then has to connect to the next nearest cell site. If that is busy too, it will perhaps bounce you to a third one further away, with a weak but still usable signal.
This can even happen during a call. The network knows if your phone has an alternative cell site within range. If your local cell site has become saturated with calls, it will free up some channels to handle those calls that only it can see, and the network will then hand you off to another site.
In rural areas, where each cell site has a range of perhaps 20 kilometres, there may be no alternative site within range. In that case your signal does not change strength. Instead, when your local cell site is saturated with calls, you get the 鈥淣o network coverage鈥� message until someone hangs up and a channel becomes free. So you either get a good-strength signal or nothing at all.
鈥淚n cities your phone can get a signal from more than one base station or 鈥榗ell site鈥�. Not so in rural areas鈥�
Rod Buck, Sheffield, UK