Myth: Double power will double the distance of my transmitter.
It is a common misconception that in order to achieve twice the distance in a radio link, you can simply double the power. The reality is that in order to double the distance, 4 times the Tx power is needed. This rule comes from a simple geometric formula: the propagation of radio waves expands in the shape of a sphere, and the surface of a sphere (and thus power density) changes proportionally to the square of the radius (in this case the radius = distance).
To put it simply, if with a 1 Watt transmitter you can reach 1 mile, in order to reach 2 miles you will need a 4 Watt transmitter. Of course the distance depends not only on the power of a transmitter but also–and very much so–on the antenna gain and height, the quality of the receiver and on the obstacles in the terrain.
A very useful way to calculate radio links parameters is using dB’s. In this case, mathematics show that doubling the power is the equivalent to increasing the power by 3 dB. It can also be demonstrated that doubling the distance is the equivalent of an increase by 6dB.
Seen below is a table of equivalence between Watts and dBm.
|1 mWatt = 0 dBm|
|1 Watt = 30 dBm||2 Watt = 33 dBm||4 Watt = 36 dBm|
|10 Watt = 40 dBm||20 Watt = 43 dBm||40 Watt = 46 dBm|