Topband: Shunt feed question

[Gene Smar]

Marty:

     Did you mean it literally when you wrote you added the shunt capacitor,
that is, did you put the cap from the shunt wire/coax center conductor
junction to ground?  The cap in a "shunt-feed" arrangement goes in series
between the coax center conductor and the shunt wire up the side of the
tower.  Its purpose is to cancel the series inductive reactance of the shunt
wire, just like in a gamma match.

     Maybe a bit more detail of the feed arrangement would help us help you
trouble-shoot.  Good luck.


73 de
Gene Smar  AD3F


-----Original Message-----
From: Topband [mailto:topband-bounces at contesting.com] On Behalf Of Marty Ray
Sent: Monday, October 14, 2019 7:45 PM
To: topband at contesting.com
Subject: Topband: Shunt feed question

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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