FWIW, I had a shunt feed on my 97-foot Rohn 25 with stacked tribanders
and a shorty forty on it. I spent a long time trying to find a 50-ohm
tap point on the tower and was never successful, so I changed to an
Omega match, which was very easy to adjust once the local broadcast
station on 1550 KHz switched to night-time power (6 watts!) and stopped
scrambling my antenna analyzer.
73, Pete N4ZR
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On 10/15/2019 3:19 PM, Herbert Schoenbohm wrote:
Best to use a 3 or 4 wire cage feed and you will find the match easier.
You should tap the tower at 50 feet and work down till you find the sweet
spot. A 500 to 750 vac variable will take care of any measure inductive
component.
Herb, KV4FZ
On Tue, Oct 15, 2019 at 3:10 PM Marty Ray <dxcc1@comcast.net> wrote:
I am shunt feeding a 70 ft Trylon tower with a Tennadyne T12.10-30HD LPDA
at 70 ft and a full size 40m rotatable dipole at 79 ft, (the top of the
mast is ~85 ft). Both antennas have relays that electrically bond them to
the tower when the shunt feed is in use.
I have tried two shunt tap points, one at 65 feet and another at 45 feet.
Using a Rig Expert AA-55 Zoom, the Rs measured a little over 100 ohms on
the 65 foot version and 49 ohms on the 45 foot version. In both cases,
adding the shunt capacitor caused Rs to drop by approximately 50 percent,
(to around 60 ohms and 23 ohms respectively).
I expected Rs to not change much, if any. I tried a vacuum variable, an
air variable and a silver mica. Same result.
Has anyone seen this happen before?
Regards,
Marty N9SE
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