I have never had much success modeling shunt fed towers but I am a sorry
excuse for a model builder. I can sometimes get them to be what I expect
and then a small change makes it depart dramatically from what I know to be
realistic.
I have an 80ft Rohn45 tower that I could (before my skirts were lost in a
wind storm) load AND talk very nicely on 80. The 2 skirts were shorted at
about 60 ft. and as referenced below only had an air variable cap in
series. I used a small gearhead motor intended for an ancient satellite
polar rotator to turn the cap remotely. I didn't own an antenna tuner at
the time so all my antenna's were resonant (or at least made the
transmitter happy).
It talked great on 80 DX w/ minimal VSWR (no idea what the actual Z was),
It tuned fine on 40 but wouldn't talk or receive any further than you could
throw it,
It tuned with a barely acceptable VSWR on 20 but talked OK depending on
conditions.
Other bands it would tune fine but not talk OR didn't tune and didn't talk
either.
I never tried it on 160.
All this working over a ground system of about 30 ground radials about 60ft
long.
Unless someone knows how to build a good NEC4 model for short spaced
conductors OR has a copy of the old Mullaney Engineering Unipole software.
Pick a spot about 2/3s the way up the tower and try it. Don't expect much
in the way of local coverage in any case.
If you want some light reading, google "Mullaney Engineering Unipole
Software" and read John's treatise on the shunt fed tower.
Later,
On Tue, Jun 23, 2015 at 1:27 PM, TexasRF--- via TowerTalk <
towertalk@contesting.com> wrote:
> My experience with shunt fed verticals has shown that a reasonably sized
> shunt feed device ends up with an impedance of much less than R=50 and
> includes a series L of a few hundred ohms at the feed point.
>
Kevin C. Kidd, CSRE/AMD
AM Ground Systems Company - WD4RAT
kkidd@kkbc.com -- 866-22-RADIO -- 866-227-2346
www.amgroundsystems.com
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