"Reginald P. Smithers III" wrote in message
...

VHF transmission range is a function of transmitter power, receiver
sensitivity, and distance to the horizon, since VHF signals propagate
under normal conditions as a line-of-sight phenomenon.

The way to calculate the the theoretical line-of-sight horizon distance
is:
distance in miles = The square root of (1.5 X the height of the antenna
from the water in feet)



It's also a function of the proper matching of antenna length, usually
expressed in quarter wavelength values, for the lowest SWR value which
yields maximum power transfer of the radio's power. The shorter the
antenna, the more difficult it is to match it to the transmitting frequency.
Many short antennas have additional windings of wire to make the antenna
appear "longer" in order to get lower SWR readings and protect the radio's
output stages, but don't help with maximizing the effective radiated power.

Very few people adjust the antenna length to get maximum range. In many
cases the 25 watts you think you are transmitting with is in reality much
lower and can vary widely installation to installation.

Eisboch