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Topband: Gamma matching crank-up tower

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Subject: Topband: Gamma matching crank-up tower
From: (
Date: Thu, 24 Oct 2002 09:36:47 -0700
Jim, N7US wrote:

"I now have an inverted vee hung at the top of my motorized US Tower 89'
crank-up (the one that is rated at 30 square feet in a 50 mph wind).  I
know I should have a modeling program, but I don't, so I'd appreciate any
suggestions on a starting point for a gamma match."
The first thing you must do is run all of the existing antenna coax
cables and rotor cable down all the way to the ground INSIDE the tower,
otherwise the structure will not work well as a 160m vertical.  Cables
that leave the tower at some height above ground really screw things up.

Just dangling the cables down the inside of the tower works well for
telescoping towers.  I did it that way when I was W5RTQ in Houston with
my telescoping tower and the cables were no problem when cranking the
tower up and down, and the tower got out like gangbusters on 160m.

With all of that aluminum up there, I suggest that you start with the
gamma wire attached to the tower at about 60 feet high and spaced aboput
2 feet from the tower.  I always use #8 AWG wire for the gamma wire (old
RG-58 or RG-59 coax can also be used with the center conductor and shield
tied together).

The required gamma capacitor will be in the range of 100 to 200 pF.

If you have an Autek or MFJ antenna analyzer, Tune the capacitor for the
lowest SWR at the target frequency.  If not close to 1:1, go up about 100
kHz and tune again for lowest SWR.  If the SWR at the higher frequency is
better than at the target frequency, that means you have to tap the gamma
wire higher onto the tower.  If the SWR is higher at the higher
frequency, then the tap is too high.

One ham in PA-land with a Force 12 C-31XR had arcing occur from the
beam's 20m reflector to the boom while he ran QRO on 160m.  I told him to
ground all parasitic elements to the boom and that solved the arcing
problem and the C-31XR performed the same as before.

Modeling towers MUST be done by modeling the actual structure, i.e.,
tower legs, rungs, etc.  I developed this method and it has proven to be
very accurate.  It's a rigorous method, but if you want, I can do it for
your structure to give you a better idea where the tap height should be
(which varies depending on the electrical height of the tower with all of
that top-loading).

Hope this helps.

73, de Earl, K6SE

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