There are THREE variants of losses to a vertical system related to the
ground.
1) resistance loss from using the ground as 1/2 of the circuit for a
traditional, base-fed vertical. Consider the dirt a dummy load with
worms.
2) ground penetration and excitation by the intense fields at the
lower end of any vertical. Consider it diathermy on the ground.
3) reflection loss off the ground by any vertically polarized signal.
Consider it a polarized mirror that reflects horizontal signals and
lets vertical signals through to be partly absorbed by what's
underneath. Horizontally polarized signals are not affected anywhere
near as badly as vertical. Dipoles get to keep the 3db that bounces,
verticals lose it
The Sigma eliminates 1) by just not using the ground as 1/2 the
circuit. Feeds the top half against the bottom half.
2) and 3) are improved greatly by a dense ground screen underneath,
the farther out the better, as much a loss mitigation for the Sigma as
for any vertical antenna in proximity to the ground. For 2) the ground
screen acts as a shield. For 3) the ground screen removes the
polarization from the mirror.
The most sensitive variant, with the largest possible loss if not
handled correctly, is 1) far and away. Some physical situations will
not let you put down any kind of a ground screen. At least a vertical
dipole variant will allow you to eliminate 1), even if 2) and 3) are
significant and beyond help in your situation.
If you also want to run high power on such a solution, some vertical
solutions have ohmic losses in the antenna (due to design and/or
aging) and have been known to literally melt down (been there, done
that...). The Sigma is efficient, and won't melt down. It's not the
only one.
In some situations, support is an issue and blow-down wind speed must
be high. Here, again the Sigma will perform. It's not the only one.
Whether you consider these worth the price is your call. Some clearly
think it beyond justification. Interesting to hear remarks in that
vein from someone who has 300 foot towers, and a 4-square on 160, not
to mention the REST of the farm. Just goes to show that the adding up
of these concerns bearing on design and purchase is a matter of
personal preference. In my case, my 80m 4-square verticals MUST be
self-supporting, because guys of any kind in that part of the woods
will NOT be tolerated. Ropes must be vertical and a color that blends
in with the visual vertical clutter of the woods. Infer what you will,
it changes one's choices.
Know the WHOLE equation, run the numbers for your place, and good
luck.
73, Guy.
----- Original Message -----
From: "Kondra Tieff" <k_tieff@yahoo.no>
To: <towertalk@contesting.com>
Sent: Tuesday, January 22, 2002 12:07 PM
Subject: Re: [TowerTalk] Force 12 Sigma 80
Just a little side comment to this discussion;
Brad was considering the Sigma80, a center-fed vertical **dipole** for
80 meters, which does not need any form for counterpoise or radial
system. In order to match Force12's specs, I believe a shortened GP
would hardly do the job. A full size homebrew vertical dipole needs a
mounting point some 40 meters (120+ feet) above ground, with feeding
cable perpendicular to the dipole wire.
I have used a 5 band Force12 Sigma5 antenna since September. Easy
mount on the ground, no radials, partly surrounded by trees and our
house, and I am very pleased with its DX achievements.
73
Chris / LA8OM
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________________________________________________________________________
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