On Fri, 16 Sep 2005 16:43:10 -0400, Harry Krause
wrote:

With my VHF tranmitting at 25 watts, I can easily reach out from one
side of Chesapeake Bay to the other in most places.

At the 1 watt setting, who knows?

For the sake of discussions, let's say on a clear day from my VHF
antenna, which is approximately 14' above the waterline, a 25 watt
transmission reaches someone 12 miles away with a similar antenna height.

Is there a way to figure how far a 1 watt setting will reach with the
same equipment?

No.

The problems are ERP (effective radiated power), environmental
conditions and physical conditions. ERP is based on power to the
antenna and then its actual dB gain. So, if you have a 1W transmitter
with no antenna attached, your ERP is probably going to be near zero.
Put on a duck stick and you might get +3DbM. Get a long duck and it
might be +6DbM. Energy falls off with the square of the distance. So
doubling the power won't get you twice as far. All other things being
constant.

Environmental conditions can mess up all communications below 350MHz.
Yes, it can mess up those above but higher frequencies are more line
of sight. HF is dependent on the ionisphere for reflectance around
the diameter of the Earth. VHF does not do this very well. UHF
doesn't either.

You might easily get four NM on one day and 2 NM on another.

Overall, it good that the makers include different power settings.

N6OIJ

Gary Gaugler, Ph.D.
Microtechnics, Inc.
Granite Bay, CA 95746
916.791.8191
gary@microtechnics dot com