Topband: Shunt feed question

[N4ZR]

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 at 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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