In an urban environment, your phone can talk to any one of several base stations or "cell sites", each transmitting and receiving on multiple frequencies or "channels". Each cell site has only a limited range - 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 "I am available for business" 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 "No network coverage" message until someone hangs up and a channel becomes free. So you either get a good-strength signal or nothing at all.

Rod Buck, Sheffield, UK

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.

Radio waves behave in a similar manner when reflecting off solid objects. This causes peaks and troughs in signal strength, depending on the phone's 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 "I can hear you loud and clear... but can you hear me?"

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's a wonder cellphones work at all.

Joe Reddaway, Wrexham, Clwyd, UK

As well as certain areas being shielded by brick and losing signal, the radio waves also reflect and interfere, causing nodes and antinodes where they add or subtract.  This is very similar to the effect of playing a high frequency tone through a loudspeaker and moving around the room.  You will hear the sound vary from loud to completely silent as you move your head (particularly if you cover one ear, and only play the sound through one speaker.)

You'll see the same effect if you use an internal TV aerial and someone walks around the room - their body reflecting the signal.  Most frustrating, as you can adjust the antenna for the best signal standing next to it, and then when you move away and sit down it goes all fuzzy (or pixellated nowadays thanks to digital TV!)

In Shannon and Wiener's information theory, every message between an emitter and a receiver will have some kind of noise. It could be raised from the devices for emission/reception or the coding/decoding process involved. In a certain way, it is a direct consequence of the Second Law of Thermodynamics.

If there are physical barriers to the transmission of the message, they will affect the quality of the message or even the amount of information emitted/received.

Variations in strength of the signal in your cell phone are simply a manifestation of noise. If sound (during a call for example) gets dizzy, delayed or have any echoes, these are symptoms of specific kind of interference in communication.

Despite being stationary, a mobile phone at home can be affected by dynamic environmental interferences on the signal mainly due to the effects of shadowing and fast fading losses. These two losses, on top of path loss, form the three main causes of signal quality variations in a wireless communication channel.

Path loss is simply the power law loss of propagation of electromagnetic wave in free space.  Shadowing is caused by reflections and diffractions of electromagnetic waves from both stationary and moving objects medium to large distances away from receiver. In another words, multiple pathway exists for radio waves to travel to the same location, constructively or destructively interfering with itself. Fast fading is similar to shadowing but instead is caused by objects in proximity to the mobile device. Shadowing and fast fading appear in the communication channel as random noises with fast fading generally being of a higher frequency disturbance, hence the name fast fading.

If the receiver is far away from the transmitter, variation of signal strength will be caused mainly by shadowing and fast fading, as path loss is relatively constant. If receiver is close to the transmitter, only fast fading has significant contribution to signal quality.

Imagine radio waves diffracting and bouncing off any surfaces that can be either stationary or moving, finally reaching your mobile phone. Any objects from moving cars to swaying trees in a forest can introduce significant distortions when their effects are combined together in a multiplicative and random manner. As the disturbances are random, your mobile phone will encounter varying degree of signal quality despite the mobile phone and base station both being stationary. This is similar to on some areas with poor analogue TV reception, passing cars can introduce ghosting or shaking. If you are otherwise on a train, Variation of Doppler shift due to different train speed in additional to aforementioned losses all contribute to the signal degrade.

SIGNAL FAILURE

NS 7 November 2009

If Jonathan Wallace first considers what happens when the train goes into a longish tunnel, the signal generally disappears until he can start to see “the light at the end of the tunnel”.

Most houses are built of material resembling tunnel structures, the surrounding rock etc corresponding to “bricks and mortar” etc, so in a house without openings, he would lose virtually all of that signal.

Now make openings for windows, doors etc; and light can come into the house, with some dark patches etc, corresponding to generally lower and higher signal strengths on his mobile phone.

To a first approximation, each of those openings amounts to a secondary radiator or aerial or antenna, acting almost independently and interacting like a multi-element phased array aerial.

The interactions give rise to “standing waves”, where the carrier transmission repeated from each secondary radiator interact to give rise to additive and subtractive conditions, similar to multiple ripples on a pond.

Assuming that the frequency used is 1,000 MHz, this corresponds to a wavelength of about 33 centimetres. The additive and subtractive points will be about one-quarter of that wavelength apart, about 8 cms (about 3.25 inches). He can check this by moving his mobile phone slowly over the 33 cm distance, whilst switched to speaker mode, observing the various signal strengths. Conversely it is a simple way of confirming the wavelength and frequency in use.

It is similar to what he may have observed with other portable devices such as TV sets, particularly those with indoor aerials.

It does also happen with the sunlight particularly coming in through the same windows; but its wavelength is so short that the effect is not noticeable.

Closing metallic (venetian) blinds on a sunny day may also produce a substantial reduction in the general signal strength. A large vehicle parked outside can have a similar effect.

Alex Dow

There are many possible reasons for this. In part it depends on whether the questioner means the signal varies as they move about the home, or even when still.

There are substantial barriers to the signal in the average home, ranging from brick walls to refrigerators, and the signal will naturally vary as it travels through or around them. If the home is near a road, railway or airport there may be reflections and shadows cast as large metal objects move around. If the questioner's home is nearly equaly distant from 2 or more phone towers the signal will suddenly dramatically improve when it switches from one tower that is blocked by a fridge or lorry to another that is not.