One of the questions we get asked most frequently is:
What’s the range of your two way radios?
The problem with this question is that there are simply so many factors which can cause issues with radio communications signals. These include atmospheric conditions, obstructions such as buildings, condition of the two way radio battery and many other things.
Radio to Radio (Back to Back) – A Simple Answer
A simple answer is that portable radios are to be used for close range communications only. Some manufacturer’s state 5km, 10km etc. The reality is that would only be possible in ideal conditions with no obstacles and is unrealistic in the real world. So the best answer is when using two way radios in back to back (radio to radio) mode, you should expect no more than a mile or so in open land and no more than a couple of hundred meters in built up areas when outside; and no more than probably 100m when indoors.
Caveat with Licence Free radios: Licence Free two way radios (in the EU) transmit on 0.5w. Therefore regardless of line of sight, they may not produce enough power to give the radio signal enough energy to travel very far. Licensed two way radios, on the other hand, typically transmit between 4w and 5w. This is plenty of power to produce enough energy for the transmitted signal to be able to reach it’s maximum potential distance.
A More Detailed Answer – Line-Of-Sight
The fact is, there is a way to technically answer this question. But this is a technical answer and might be a little baffling for those who are not technically minded. Technical answers such as this rely on constants such as absolutely perfect and ideal circumstances and atmospheric conditions – which just do not exist in the real world.
Nevertheless, we can calculate the maximum theoretical range of a radio signal using a line-of-sight calculation which considers the curvature of the earth and presuming the earth to be a perfect sphere – which of course it is not. And this also assumes your radio signal is transmitted with sufficient power that is maintains enough energy to travel the distance.
The equation is:
- horizon (km) = 3.57 x √ height (m)
- horizon (mi) = 1.23 x √ height (feet)
The typically the height of an adult holding a portable radio when in use is 1.5 – 1.7m high. Let’s assume a typical height of 1.5m to be conservative.
- √1.5m = 1.22474487139 x 3.57 = 4.37 km
- √ 4.92126ft = 2.21837778568 x 1.23 = 2.72 miles
Using this theory, at 1.5m high, the point of the horizon is 2.72 miles (approx) which is the point where radio signals will unlikely be able to reach.
However, assuming that two users are both 2.71 miles (approx) either side of the horizon, in theory, given they are transmitting the signal with enough energy (watts), the very maximum distance you can achieve from a portable two way radio used back to back is 2.71 miles x 2 which is 5.42 miles. However, this is quite unrealistic. As mentioned above, atmospheric conditions, ionospheric absorption, and potential obstructions, will prevent this from likely ever being achievable.
For those even more technical, I agree that there could be a case to argue about diffraction and skip propagation etc. But for the purposes of this article, those points are not covered here.
Source Used: https://en.wikipedia.org/wiki/Line-of-sight_propagation#Radio_horizon
Point to Point Line of Sight
This article has not covered things such as Near Line of Sight, or the Fresnel Zone. Here’s a digram from L-com and an explanation.