[TowerTalk] very short verticals over chickenwire

[Dave Hachadorian]

An idea that occurred to me recently was to try modeling a
very short vertical (10' on 80 meters) over a locally perfect,
but limited radius, ground (40 foot radius of chickenwire).

What I had in mind was that the downward radiation from this
short vertical, at all but the very lowest radiation angles,
would impinge on the chickenwire and be reflected upward with
little or no loss. In other words, the chickenwire would act as
a perfect reflecting plane for all but the very lowest radiation
angles.

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.

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.

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?

Dave Hachadorian, K6LL
Big Bear, CA
K6LL@juno.com

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