Raghu,

Several watts of RF will travel considerable distance if it is in line-of-sight of an antenna. That's what NASA depends on, clear line-of-sight, and of course, large antenna arrays.

Keep in mind that RF energy is absorbed by trees and reflected by buildings and other structures. I have radio telemetry systems in the field that routinely communicate up to distances of 30 miles using 5 watts at 450MHZ.