There are two major factors when considering coverage: the phone itself and the nearest mast.
A phone's antenna is crucial. The bigger it is, the better it is at picking up a signal. The ongoing battle to fit more gadgetry into mobiles of a smaller size means there is often little room left for the single most important part. Not all phones are equal and I have seen a 50% difference in the signal two adjacent phones pick up purely because of the quality of the antenna. Unfortunately there is little way to tell this before you get the phone home but a general rule of thumb is that a small phone with lots of features (5mp+ camera, satnav, wifi) will not have much room for a decent antenna. If you are in a marginal area, get a simple business orientated phone (such as the Nokia E series).
Mobile phones are just complex radios. If anything is blocking the signal from the mast they are connected to they will not work. Over time a number of things can change including new buildings, trees and masts from other companies conflicting with existing signals. This means that just because you had a good signal a few months ago, you won’t necessarily have one now.
Most masts do not operate at full power. For example, where I live the two closest Orange masts are showing as working at 27.2 dBW out of a licensed maximum of 32 dBW and 28 out of 32, both of which are normal. There are lots of reasons for running under power. Mainly that it saves electricity but also because it will reduce interference with other masts in the area. Mast tuning is a black art and is very difficult to get right because of constantly changing environmental conditions.
If you are suffering changeable reception in the same area you may be suffering from 'ping-ponging' where the phone is switching between two equidistant masts, especially when moving about the local area. Another aspect is saturation. Masts only have a finite capacity and so the more users connecting to that mast, the less of the signal you get to use. Too many people and you won't get a signal which is why it is always difficult to make a call during big events unless the operators lay on temporary 'masts on wheels' to provide additional capacity. This is usually the case only for voice calls. Text messages were invented for technicians and they use the 'carrier' wave which is why they will often get through when a voice call will not.
Different companies use different wavelengths. At 900 MHz Vodafone and O2 use a longer wavelength than Orange, T-Mobile and Virgin at 1800 Mhz. 3 operate in the 2100 Mhz range and use masts from Orange where there is no 3G coverage. This means that Vodafone and O2's masts can transmit further but they are less likely to work in small valleys surrounded by big hills (such as a coombe). It is also why Orange can boast to have more masts than anyone else. It is a marketing gimmick - the reason is that they have a lower power than Vodafone and O2 and so you need more masts just to get the same area of coverage.
To find out where your nearest mast is or to identify known masts go to http://www.sitefinder.ofcom.org.uk/ and enter either the postcode or the nearest town. Bear in mind that most operators do not want you to know where their masts are so they put some pretty heavy constraints on the use of the information they supply to Ofcom, the result of which makes finding unknown masts difficult. The trick is to zoom out no more than 500m in order to find the masts and zoom in to 100m to get the detail. If you zoom out more than 500m you won't see a thing - just as the operators would like it!
If you want to test out coverage from various suppliers, you'll either need an unlocked phone and some PAYG Sim cards from the various operators or friends who use the companies you are interested in. Bear in mind what I said about the antenna though as it is by far the most underrated consideration when buying a phone.
Originally published at OneMobileRing.com