> 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'
> gauge wire from the top of the tower over (and maybe slightly down)
> fir tree a ways from the tower. This should be long enough to ensure
> 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,
> from matching cap to shack.
> I was looking at the prices in the Surplus Sales of NE catalog, and
> price of vacuum varibles took my breath away. But I am concerned with
> weatherizing/condensation problems attendant with using an
> Also, ballpark figures for a range of capacitance needed for this
> (I am "antenna-modeling and mathmatically-challenged", shall we say).
Steve, I'd suggest that you pick up one of the antenna analysis
programs, to insure that the electrical length of the antenna presents
an inductive reactance at the feedpoint that is within the matching
range of your variable capacitor. It may take a large capacitor to
present a low capacitive reactance that would cancel a low inductive
reactance at 1.8 mhz, assuming you want to be able to tune up to 2.0
mhz, where much less C would do. Barring analysis, a little cut-n-try
can do with a simple wire antenna. Unfortunately, shunt fed towers are
rather difficult to model accurately.
I'd suggest an inverted-L configuration for your antenna, as
considerably more than 90 feet is going to be required to set the
antenna resonance below 1.8 mhz, unless you have a lot of top loading
present. Rather than shunt feed, perhaps a separate wire for the
radiator going up alongside the tower then out to some convenient
tie-off tree to get the quarter wave-plus required length.
My quickly installed (and un-modeled prior to installation) 160 meter
wire is 140 feet long. It starts at the end of my porch deck (about 6
ft high) and slopes up at about 60 degrees to the 65 ft level on a
tower sidearm. From there, out about 30 feet to a mast in a far corner
of the back yard. The extra length over a quarter wave also assists in
getting the highest current portion of the antenna off my porch deck
level and slightly higher up in the air and away from the house. This
"extra" length, to allow tuning with a single variable capacitor, also
reduced my telephone RFI from severe to nil.
Although a tad noisy on receive, it does work. I can usually manage
EU/AF qso's from the midwest "black hole" without great effort in any
160 contest with a simple ground system, consisting of an attachment to
my multi-rod entrance panel ground about 5 feet from the antenna
feedpoint, and a single 120 foot counterpoise wire stretched out under
the antenna at a height of about 4 feet. Much better on DX than the
full wave horizontal loop (30 ft height) that I had up last winter, and
better overall than a 160 meter inverted V with apex at 70' tried out
this fall. I plan to add a few more counterpoise wires, although won't
be approximating a full elevated radial system by any means.
My tuning unit consists of a three section broadcast type variable
capacitor in series with the wire at the feedpoint, about 1100 pf
maximum. With the basic wire resonant below 1.8 mhz, I can tune out
the reactance anywhere between 1.8 and 2.0+ mhz. The resistive
component varies between 40 and 90 ohms between 1.8 and 2.0 mhz.
A vacuum capacitor isn't really required here, as the voltage levels
are fairly low. I have no arcing problems with up to 150 watts out
with the receiving-type capacitor. A 1000 to 1500 pf vacuum variable
isn't cheap, as you noted!
Something folks should be concerned with is high voltage generated on
that long wire during high wind or storm conditions. I have been able
to draw arcs to ground from 3/8 to 1/2 inch long from my 160 meter wire
on windy days when the environment is highly charged. A simple 1 mh rf
choke from feed end of the wire to ground eliminates that voltage
buildup. I suggest incorporating a means like this to bleed off static
charge even if you use a vacuum variable. It takes a LOT of volts to
jump half an inch! Much more than the frequently found 10kv rating of
the VVC's. You don't want that expensive unit arcing internally.
Note that the above won't be a concern if you do shunt feed a grounded
Have fun.. 160 is a great band for antenna experiments!
73! - Mark
Mark Shaum K9TR
Central Illinois Grid EN50
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