Range on PMR446
PMR446 is a UHF radio service, UHF standing for Ultra High frequency, on UHF the WAVELENGTH of the wave is shorter than VHF radio waves since the frequency is higher. Because of the shorter wavelenght, UHF radio waves have their own unique characteristics.
Line of Sight
One of the most important characteristics of UHF PMR446 is that the range is largely line of sight.
This means if one aerial is in sight of another, communications can be made with ease. UHF radio waves may still refract off structures and materials which can allow some non-line of sight communication, as well as penetrating through the materials themselves but it is generally more restrictive in range than a line of sight link.
Since PMR446 is largely line of sight, the saying of “height is might” applies tremendously.
If someone is talking on PMR446 from a high place such as on top of a hill, mountain or high structure/building the range is only limited by the curvature of the earth, and like the line of sight rule, anything the radio antenna is in view of can be communicated with to a large extent. Most cases of long distance transmissions have been recorded when apparatus has been set up at a high elevation place.
Buildings and obstructions
Because UHF is line of sight buildings and other obstructions can be an issue for long distance transmissions.
However buildings may sometimes be of an advantage too, this is because UHF radio waves can diffract (bounce) like a mirror from these structures allowing a path to be made.
This is why despite not being fully line of sight you may still receive transmissions from others in a close range in an obstructive area such as a city, however diffraction comes at a cost, the more the radio wave is diffracted from objects the weaker it becomes.
A large flat area such as the countryside provides a potentially increased range with no obstructions to diffract or potentially block the signal, a range of 10 to 20 miles can be expected on a standard 0.5 watt radio in open countryside if conditions are ideal.
Power is the amount of RF energy measured in Watts a radio will output, the more power outputted the higher the potential intensity of the signal can be.
Officially, PMR446 radios are limited to 500mW (0.5 Watts), in practise this rule is scarcely ever followed and many users choose to use higher power.
Power is not the main factor that determines a radios range on UHF, that is location and height are far more important, even a 0.5 watt radio located on the top of a large hill can outperform a 50 watt radio with an antenna indoors low down.
This means in laymans terms, more power does not equate to more range, an efficient antenna as well as an ideal location is far more important. As stated before, the average PMR446 type approved radio will output around 0.5 watts, the average handheld UHF radio converted for use on 446 unofficially will output approximately 5 watts, whilst the average mobile radio will output roughly 25 watts.
Another potentially range-increasing phenomenon exists known as tropospheric ducting.
The troposphere is a section of the lower atmosphere in which when high pressure is formed with cool moist air below and warm air above, the radio waves may defract and become trapped in a “duct” travelling much further than usual before the duct fades.
Tropospheric propagation is largely dependent on the weather. The chance of tropospheric propagation increases when there is high pressure.
It is important not to confuse tropospheric ducting with ionospheric propogation or “Skip”. Unlike radios that operate on a much lower frequency (CB radios, HF amateur radio) UHF radio signals simply pass through the ionosphere (upper section of the atmosphere) that is responsible for long distance transmissions on high frequency radios because the UHF radio waves on PMR446 are too short of a wavelength to interact and bounce off it.