[TowerTalk] very short verticals over chickenwire

[Tom Rauch]

> I did some modeling using EZNEC, with a 10' tall vertical on 80M, with 10'
> tophat, loading coil just below tophat, 40' radius perfect ground, very
> poor ground beyond 40'. Sure enough, the modeling showed that as I made
> the vertical taller, the gain fell off and the radiation angle went up. As
> I made the ground screen longer, the gain went up and the angle came down,
> but not at a rapid rate.

The loading coil can be anywhere when you have a large hat. 
Current is uniform on either end of the "coil". The hat will cause 
uniform current in the radiator, if the hat is large enough regardless 
of where you place the inductor.
 
> For the 10' vertical with 40 foot chickenwire, max gain was 4.3 dBi
> at 14 degrees elevation, which I think is pretty darn good for such
> a reasonable investment.

That seems high unless you modeled a lossless system.
 
> All of this assumes 100% efficiency in the feed system, which may
> turn out to be a significant challenge, since the radiation
> resistance of such a short vertical is about 2 ohms, when tuned to
> resonance with the tophat and top loading coil. So what would be the
> most efficient way to feed something like this at the 1500 watt
> level? Anyone out there have any ideas or experience in this area?

Put a large high-Q coil at the base to resonate the system. Tap up 
on the coil to find the point where the impedance is 50 ohms. The 
highest practical Q of an inductor is about 800. That's what you 
can expect with an optimized edge wound inductor like BC stations 
use, or a copper tubing coil of proper form factor.

What you are building is a mobile antenna except on a larger 
"vehicle". Watch out, the modelling can fool you when messing 
with stuff like this. What ground calculation mode were you using? 
Mininec, high accuracy, or what?


73, Tom W8JI
w8ji@contesting.com

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