Free-space Ranges
The formula for electromagnetic radiation dispersion and information is:
Where D=Distance, P=Power, and S=Speed
What this means is that double the communication distance requires four times the power. It also means double power allows double communication speed (bit rate). Double power is approx. 3dB (10 log(2) to be exact) increase. Of course in the real world there are all sorts of other phenomena which enter in, such as Fresnel canceling, path loss, background noise, etc.
Read more about this topic: Antenna Measurement