[TowerTalk] Top loading HF-6V for 160

David Gilbert xdavid at cis-broadband.com
Sat Oct 20 18:12:36 EDT 2007

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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