LOW POWER 433MHz wireless links - How far will the signal go?
"Radio waves" is the name we give to electromagnetic radiation with wavelentgths that range from tens of kilometres to just a few millimetres long. Other forms of electromagnetic radiation include light, infra-red and ultraviolet.
If a radio transmitter was sited on a spacecraft in deep space, far away from any other object, and if that transmitter was fitted with an ideal antenna system, then the radio waves would radiate out from the trasnmitter in all directions, just as light spreads out from a light bulb. We can imagine that the radio wave expands to produce a spherical wavefront and that a redeiving antenna captures part of the surface of that sphere. This ideal cases is referred to as "Free-space propagation".
The surface area of a sphere has the formula 4PR2, where R is the distance from the transmitter to the receiver. So when a portion of the radio wave interacts with the receiving antenna, the power of the received signal is proportional to 1/R2 . If we double the distance R, the received signal power is one quarter of what it was before. If we increase R ten times, then the received signal is 1/100th of its original power.
Clearly, the strength of a radio signal quickly falls away to almost nothing, no matter how powerful the transdmitter. It would also seem clear that while the received power falls to almost nothing, it never reaches zero. Since amplification of weak signals by modern components is easy and cheap, what limit is there on the range of a transmitter? In fact, the real world supplies a numbner of limits:
Clearly, the more transmitter power, the greater the potential range. Engineers prefer to operate with reasonable levels of transmitter power to ensure reliable reception, however fortituous circumstances are for reception.
In the 433.9MHz band transmitter power is restricted to 25mW (milliwatts) .
Turn on a radio and tune away from a station: the rough hiss heard is noise, commonly referred to as "static". Noise is caused by natural phenomena (the Sun, distant stars, tropical stroms) or human activity (ignition systems, light dimmers, electric motros). There is also internal noise in electrical circuits.
At 433.9 MHz the greatest source of noise would be the receiver itself, although electrical activity around the receiver (computers etc) would also be contributers.
Other radio signals on the same channel can overwhelm the received signal.
The European experience teaches us that 433.9MHz quickly fills up with competing radio links. Interference from the primary users of the band military height-finder radar and the radio amateur service is also to be expected.
Like light, radio waves radiate out from the source in a straight line ("radius" means a straight line originating in the centre of a circle, hence "radiation" propagates along a radius). Solid objects can block the signal, although some materials are transparent to radio waves, just as other materials are transparent to light.
On the other hand, the phenomenon of diffraction can allow a radio signal to be received on the other side of an apparently blocking object, so long as it presents a "knife-edge" surface to the wave.
Some materials allow radio waves to travel though them, but absorb some energy. Building materials (gypsum panels, dry timber, brick), surprisingly, are relativeley transparent to radio, but live wood absorbs most of the energy of a radio wave, making operation of, for example, a military backpack radio problematic in thick bush or rainforest. Concrete, especially with steel reinforcement, is transparent but absorbs a good deal of radio energy.
Metal completely blocks or reflects radio waves, although some wavelengths can escape thorugh small gaps.
In practice, an urban area with dense construction (such as a typical European city) will allow radio waves to propagate, but with rapid absorption of the signal energy. More open, suburban areas, will allow better passage of radio waves. Radio equipment based on hills, or the tops of tall buildings standing well apart from other buildings (as is the case with many areas of Sydney) will allow radio waves to propagate over larger distances.
If a transmitter and receiver are close to the ground, then a reflected wave can cause harmful inteference at the receiver. The signal will not be received, even though well within range. One the other hand, another reflected wave can cause reinforcement of the signal. In practice, it can be very diffciult to predict where these interfering and reinforcing waves appear, making the siting of radio equipment as much an art as a science.
Predicting the range of a radio transmitter is a very demanding task. While radio-physicists produce complex mathematical analyses, radio engineers have to interpret these studies in the light of real-world experience. Increasingly computer modelling is used, replacing the older graphical (nomograph) methods.
It would not be realistic to expect that the installers of a low-power licence-free radio link to have these tools available, or even to be aware of them. the European experience is that the installers may not even have a background in radio engineering. There is a tendency to rely on manufacturers' publications, or else rules-of-thumb promulgated in technical magzines or passed along by more experienced individuals.
Some estimates of the range of a 25mW transmitter may be made from both theory and comparable cases.
References - magazine articles
Humphris J, Wireless monitoring System; Practical Electronics: Vol 28 No 2 & 3 1999
_, Emetteur de Puissance AM Experimental A Module MIPOT et Champmetre 433,92; MHzElectroniqu Pratique: Nu 218 October 1997
Doni F, Ricevitore Miniature per Radiocomando; Elettronica In: Anno III N 23 October 1997
_, 418/433 MHz short-range communication; Elektor: May 1998
_, 418/433 MHz control system; Elektor: September 1998
_, 418/433 MHz fieldstrength meter; Elektor: October 1998
Hickman I, Low Power Radio Links; Electronics World: February 1993
Simms R, How far will it go?; Electronics World: September 1998
An P, Computer RS232 wireless link; Electronics World: June 1996
An P, Analogue input without wires; Electronics World: January 1998