in determining the range of a 2-way radio system is HEIGHT, both in terms of ground elevation and height above average terrain. Suffice it to say that selecting the best possible antenna location is very important. The second most important consideration is the choice of antenna and cable system.
It's all about dB's. The more dB's you have, the better your system works. Antennas are available in unity gain (zero dB), 3 dB (2X power), 6 dB (4X power), and 10 dB (10X power). This means that not only is talk-out power from the station is effectively multiplied by the applicable factor, but the RECEIVED signal from mobiles and hand held radios is well!
