Topband: Shunt Fed Tower Question

[Chuck Dietz]

I would not use any inductance unless the tower naturally resonates above
1800 kHz. If I were going to use a cage for the gamma connection, I would
make a small cage that would only be on one side of the tower. I find that
the shield of old coax spaced a couple of feet from the tower works fine. I
would start with a variable capacitor in series with the coax center
conductor feed to the gamma connection to the tower. Find the best dip,
replace it with the closest fixed cap and use the variable cap from the
gamma connection to ground to find the value that resonates it perfectly.
(The dips will be sharp.)

Good luck Herb!

Chuck, W5PR

On Thu, Aug 2, 2018 at 8:04 AM Herbert Schoenbohm <
herbert.schoenbohm at gmail.com> wrote:

> I recently installed a 2 element 40-meter beam and a 4 element 20-meter
> beam on top of a 90-foot tower which I use for TX on Topband.  The tower is
> cage fed with a 3-wire cage spaced24 inches around the tower,  I am able to
> get 1.2 to 1 on 1845 by putting about 1200 pf and a 500 pf fixed HV cap to
> ground.  The coax feed goes to the wires with about 40 mh in series.  My
> taps to the tower are at 60' which may be too high up on the tower
> considering all the toploading I have now.  My question is:  How far down
> should I move the tower taps to make the feed appear inductive rather than
> the capacitive value currently what I believe I have?
>
> Any suggestions would be appreciated.
>
> Thanks,
>
> Herb Schoenbohm, KV4FZ
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