Topband: Tophat? Does it have to be at the top?
w8ji at contesting.com
Thu Jun 10 08:12:10 EDT 2004
> My plan is to erect a 40-foot vertical using 1 1/2 inch
> aluminium masting then to use the top set of four
guy-wires declined at
> 45 degrees with a skirt wire connecting the bottoms
> Is there a way of calculating the optimum length for this
> At the risk of showing my ignorance - how do you arrive at
> capacitance of this arrangement?
The best thing to do is model it George. This is the type of
thing models work well for, as opposed to radial system
design (hi hi).
Using a perimeter wire around the hat and increasing the
density of the hat (more wires) increases the effects of the
hat with minimal downward slope. Some combination of
inductance at the attachment point also could help keep the
hat small while maintaining resonance.
The flatter the hat the better, of course. VK3ZL uses a
push-up mast and a flat hat with a loading coil lower on the
mast, a bunch of radials, and does quite well up here at
this distance. His 40ft (or so) tall vertical antenna almost
always beats his dipole at 100 feet.
I have some measurements of mobile hats, but no big antennas
like you have planned, on my website. Since I used a FS
meter in a controlled path at a short distance the data is
probably pretty good. K0BG made some mobile hat
measurements that roughly agree even though the data was
collected using an S meter at a longer groundwave distance.
The mobile antenna measurements are up on my website,
although they don't mean anything for this application. They
just show a hat is very effective at allowing a height
reduction without an accompanying large FS reduction.
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