TopBand: Shunt matching tower for 160M

Steve Zettel
Tue, 2 Dec 1997 14:56:08 -0700

Just finalizing details prior to placing some parts orders for my Topband,
Contesting Tower. I'd like to tap the wisdom of the reflectors before I do
though, and perhaps avoid an error or two.

I plan to  shunt feed my 90' Rohn 45G tower on 160M. Since it will be
(soon!) populated by insulated-element F12 antennas, top-loading
contributed by the other antennas will be nil. I plan to run a 90' heavy
gauge wire from the top of the tower over (and maybe slightly down) to a
fir tree a ways from the tower. This should be long enough to ensure the
tower/wire combo is resonant somewhere below 1.8-2.0 MHZ (inductive).
Matching will be by shunt feeding with a vacuum-varible in series, hardline
from matching cap to shack.

I was looking at the prices in the Surplus Sales of NE catalog, and the
price of vacuum varibles took my breath away. But I am concerned with
weatherizing/condensation problems attendant with using an air-varible out
at the base. Also arcing (not running HP yet, but that is in the future

Also, ballpark figures for a range of capacitance needed for this
(I am "antenna-modeling and mathmatically-challenged", shall we say).

And finally (for now!), suggestions for the configuration of the shunt fed
leg. I have plenty of 3/4" hardline to use as a conductor, but my lowest
sidemounted and rotatable antenna on the tower is at 30', so I can't run up
past that (unless I make the shunt leg REALLY close to the tower face to
avoid interference with the antenna boom/side-arm assembly, not so good
from a shunt rod situation, from what I understand).

Thanks for you assistance,

Steve Zettel  KJ7CH
near Libby, MT  (no snow YET at 2100')

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