We often get asked this question. And we always say: “it depends”. As you can see on our forum (http://openmicros.org/), different people obtain very different results.
This afternoon, we spent a very informative afternoon with db Technology in Cottenham (http://www.dbtechnology.co.uk/), who have labs for testing radio equipment. As we were testing our radio modules (and obtaining some excellent results), we talked about the key factors that influence range. To these I added a bit more background, to give a fuller picture.
The most important thing to understand about low power radio is that there are a huge number of factors that influence the way in which radio waves propagate from a transmitter to a receiver. And how both are matched Here are some of the most important:
The frequency of the radio wave. Low frequency radio waves tend to travel further than high, very high or ultra-high frequencies. One medium wave transmitter (500 – 1500 kHz) can serve a whole country, where several VHF (FM) transmitters (87 – 108 MHz) would be needed to cover the same area. Short wave radio signals (3-30MHz) travel right round the world as the signals can be reflected by the stratosphere. A UHF TV transmitter (470 – 862 MHz) covers an area of around 50 miles around it. WiFi, at 2.4GHz has a limited range, just around your house. Ciseco radio units operate in the UHF band, at 868MHz in Europe and 915MHz outside.
The power of the transmission: obviously the more powerful the signal, the further it will travel. Ciseco radios maximum power is 10 mW.
The polarity of the radio wave. Radio waves are electro-magnetic waves. The electrical component of the wave is at 90 degrees with respect to the magnetic component. A transmitting aerial has a polarity in the sense that it places the electrical component either horizontally (horizontal polarisation) or vertically (vertical polarisation). For best results, the receiving aerial should adopt the same polarisation as the transmitting aerial.
If you use whip aerials on Ciseco XRFs for instance, you want to arrange for them to be both either vertical or horizontal to improve signal strength at the receiver.
And by virtue of the radiation pattern of each whip antenna and if they are both horizontal align them in the same direction, e.g. both north-south or both east-west; not one north-south and the other east-west (the worst combination). When they are vertical they are already aligned correctly (up-down).
The height of the transmitter. The higher the transmitter, the further it can “see” over the curvature of the earth. This is especially important for transmitters at the higher end of the frequency spectrum, as higher frequencies tend to travel in more of a straight line than lower frequencies.
Obstructions between transmitter and receiver. These can be of different type. Some, such as the foil lining on the walls in modern buildings, simply stop UHF radio waves dead (Faraday cage). Others, such as trees absorb some of the energy of the radio wave. Line of sight is short hand for the absence of obstructions between transmitter and receiver.
Reflections. Radio waves can be reflected off different surfaces. Short wave radio (3 – 30MHz) can even bounce off layers in the atmosphere, allowing signals to travel right round the world. UHF signals however, tend to travel straight through such layers, but can bounce off other, more solid objects, thus providing an alternative path between transmitting and receiving aerial. The most important surface in this respect is the ground plane. Some emitted radio waves will hit the ground and some of these can bounce up to reach the receiving aerial. The ground conditions play an important part too. In wet conditions, where the ground tends to be more conductive, more radio waves are reflected and fewer absorbed, thus resulting in a better range. Some of the signals that hit a building or similar structure could also bounce off and reach a receiving aerial.
In addition tot he above, there are many many more factors, including the type of aerial in use, the way the aerial is directed, the way it is matched to the transmitter and receiver circuitry, and so on. But that may be a subject for another time.