...there is a company in Canada that produces a 50ft helical center loaded
antenna for use on ships and for Public Safety AM LP stations. Arlington
County OEM, VA installed one of this self-supporting antennas over about 100
short radials and a 40ft 4" cooper ground rod.
The station operates at 10 watts on 1700AM 7X24.
We get QSL cards from up and down the coast, including Canada. The top hat
consist of six 102" CB whips and a two foot in diameter egg beater...
It is pretty amazing how well the shortened verticals perform.
----- Original Message -----
From: "David Gilbert" <firstname.lastname@example.org>
Sent: Saturday, October 20, 2007 6:12 PM
Subject: Re: [TowerTalk] Top loading HF-6V for 160
> I agree, Terry. If aesthetics aren't a big issue, I'd put as much
> surface area of metal up there as I could. In that vein, larger
> diameter wire is helpful as well.
> For what it may be worth, I have found EZNEC to be surprisingly useful
> for analyzing the effect of a top hat made from wires. Apparently EZNEC
> handles capacitive effects fairly well. I once put an 80m groundplane
> on the flat roof of my house, with the vertical section comprised of 45
> feet of guyed two inch aluminum tubing and a top hat of four sloping
> wires (aluminum guy wire from RS stretched out by nylon twine at the
> ends). Getting the thing erect by myself was quite a chore (hold the
> chuckles) and I only wanted to to it once, so trimming the top hat
> radial wires was something I wanted to avoid. I modeled it ahead of
> time with EZNEC and to my amazement the thing tuned right where I needed
> it to be. I've read here about others who had similar success modeling
> top hats, and I'm sure one or more of us would be willing to help with a
> ball park analysis.
> Dave AB7E
> Terry Conboy wrote:
>> If the top loading wires have to slope down due to a lack of tall
>> supports, it makes sense to use as many wires as possible, so that
>> each one can be shorter (for the same resonant frequency) and thus
>> cancel a smaller portion of the current in the vertical
>> radiator. This assumes that the low support points remain at the
>> same distance from the vertical radiator and that the ends of the top
>> loading wires are supported by insulating extensions.
>> You can also create a spider-web by adding wires horizontally between
>> the ends and/or middles of the top loading wires. This increases the
>> capacitance and allows you to further reduce the extent of the
>> capacity hat and the resulting height of the sloping portion.
>> 73, Terry N6RY
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