Ever wondered about the claims some manufacturers put on the box? 10, 20, 30, 40 kilometers between two handhelds? Did you ever achieve it?
Probably not, and I’ll try to explain why by using a simple example.
Before we go any further we’ll going to assume a few things.
- Both persons holding the radios are 1.90 meters (about 6″2) tall
- Both radios are identical in every respect
- We use the 2 meter amateur band (VHF) and 5 Watts output
- There are no obstacles between the two radios
- Nothing above ground affects the signal in a negative nor positive way
- There are no special band conditions
Under these circumstances, what is the maximum range?
a) up to 6 Km
b) up to 10 Km
c) up to 20 Km
The answer is: up to 6 Km, give or take a few meters (the exact number is 5.7 Km). The reason why the signal can’t travel any further has to do with the radius of the earth. Beyond 5.7 Km your signal will literally hit the ground.
You can use this formula:
- Take the square root of the antenna height. In this case the antenna height is identical to the length of the person who is holding the radio, e.g. 1.90 meters. √1.90 = 1.378404875.
- Multiply this number by 4.12.
- Now you have your maximum range: 1.378404875 * 4.12 = 5.7 Kilometers.
Hopefully you now also understand why more output power can’t change the final outcome.
In real life chances are slim that you will actually be able to keep in touch over such a distance. There’s not much chance you run into the ideal circumstances and assumptions I described above. A few factors are:
- Free space loss. The signal will diminish as it spreads out.
- Absorption loss. Signals might pass through a medium which is not totally transparent to radio signals. Even a glass pane will do that.
- Diffraction. This occurs when an object blocks the path. The signal can diffract around the object, but can’t do so without losses.
- Terrain, composition of the ground. The terrain over which signals travel can have a negative effect on the signal, even without hills or trees.
In reality you should be happy when achieving a range of 1 or 2 Km.
Ways to improve range
- Antenna height. The higher, the better.
- Radiation pattern. Antennas should not waste energy by radiating it in unwanted / useless directions.
- Free space loss can be partly compensated by increasing power output. Doubling the output power will not double the range, the theoretical improvement is about 35-40%.
- The use of repeaters at high elevations, which will compensate for the radius of the earth.
This article is intentionally kept as simple as possible and doesn’t cover all aspects. If you want to read more, go here.