Are you sure that there is pseudo-Brewster enhancement over fresh water?
John KK9A
Subject: Re: [TowerTalk] Vertical in pond
From: Dan Zimmerman N3OX
Date: Tue, 17 Mar 2009 10:44:22 -0400
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
There should be less field cancellation at very low angles for a vertical
that looks out over fresh water... look up "Pseudo Brewster Angle" in your
favorite antenna book.
If I change the earth dielectric constant in EZNEC from 13 to 80 (soil to
water) underneath a 1/4 wave vertical on 40m, I pick up about 7dB at 1
degree elevation and 5dB at 5 degrees elevation.
Also, a lake is nice and flat without any scatterers or obstructions. I
would imagine that the PBA effect plus lack of scattering obstacles
intercepting low angle radiation could give you a pretty serious boost.
Of course, some of this would happen even if you were backed away from the
lake a couple wavelengths. You'd have to look at the geometry in detail to
see what you might expect advancing from 100m away from the lake to right up
on the lake, and you have to qualify what stations you're interested in and
what the takeoff angles to them are. High angle enhancement is there, but
it's only to the tune of a dB or two up at 30-40 degrees.
A test for whether or not it's a PBA and reduced ground clutter effect would
be that you only get enhancement for paths that go over the lake. Back
toward land should have no effect.
Improved earth conductivity would give you a boost in all directions. I'd
think by the time you got to "lots" of radials, though, you just wouldn't
have more than a dB or two to pick up even by copper plating the ground,
even with liberal interpretations of "lots"
An important thing for the original poster: the lake has to be very large
for this to work, many tens of wavelengths for the very low angle
radiation, probably at least a couple for any useful DX angles. A "pond"
probably wouldn't hack it. It only works if the ground reflections that
correspond to the desired elevation angle paths happen out on the water.
Add that to the fact that it's a pain to install and maintain an antenna
that's in a pond, and it's not worth it. If you can mount your vertical on
the edge of a freshwater lake, it's probably useful, but you wouldn't
necessarily want to mount it in the middle of one, unless you had a really
conveniently located small island (less than a wavelength or so in diameter)
73
Dan
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