Topband: Shunt feed question

[Herbert Schoenbohm]

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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