On Tue, 25 Sep 2007 08:32:14 -0700, Frogwatch
wrote:


If distance goes as square root of the height, a 3' antenna mounted on
the same mount as the 8' antenna will reduce my distance by only about
40%.

I may be wrong about this but I think the shorter antennaes are less
"gain". This means they send less power in a horizontal plane.
Sailboats use low gain antennaes because they heel but generally have
them up very high.

Hmmm - it's a bit more complicated that that.

The long/short of it is that antenna gain is based on the hallowed
theoritical half-wave dipole in free space - eerything is compared to
that.

It really doesn't matter if it's horizontal or vertical.

Bottom line, I suspect the real answer is that the 3' antenna probably
loses more than 50% of the range due to "gain" and distance to
horizon.

Not really. Of course it depends on the rating of the antenna in
terms of power, but a 3' antenna should be as capable of handling 25
watts as an 8' antenna.

Gain is a tricky term to use. The antenna, at it's designed frequency
will provide "gain" of X dB over space at a 1:1 SWR (Standing Wave
Ratio). So if you have 9 dB of "gain" at 1:1 that doesn't mean that
you have tripled your Effective Radiated Power - more like you have
increased the ERP by 40% or thereabouts for reasons beyond the point
of this discussion.

Where the loss comes in is away from the center frequency as the SWR
rises. At 1:1 you are getting the power efficiently transmitted. The
further you move up and/or down the frequency range, the SWR rises
which introduces loss. That loss, in dB, is subtracted from the
"gain" resulting in decreased ERP.

There are also other factors to consider such as ground system,
height, type of antenna, ability to retune, etc.

Properly installed, there is no honest advantage to a 8' antenna over
a 3' antenna for small boats.

For larger boats, that's a whole different ball of wax. :)