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Tech Note: Radio Power vs. Distance

Myth: Doubling the 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 transmit 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

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