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.
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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