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Old 01-15-2010, 10:18 AM
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RKG RKG is offline
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As a very broad generality, talk distance on a VHF or UHF FM system is affected by three factors:

Attenuation due simply to distance, which is known as free space propigation. There are some software packages that will calculate this for you. The limiting factor is when the power of the transmitter is diminished simply by distance through a medium to the point where the signal strength at the receiver is below the receiver's minimum sensitivity point.

Attenuation due to the curvature of the earch. This is a macro form of intervening terrain. As a broad rule of thumb, the distance to the radio horizon of a transmitter or receiver, in miles, is 1.34 times the height of the antenna, in feet. The talk distance is the sum of the radio horizons of the transmitter and the receiver.

Location specific terrain: mountains, buildings, and the like. This is obvious and location specific, and there are no general rules.
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