Topband: Shunt feed question

[Lee STRAHAN]

  Marty,
  You may be seeing errors in your readings due to broadcast pickup. This is common when modern R+Jx measurement tools are connected to large antenna structures. Others have described ways around this problem before here on the topband chat.
Lee  K7TJR  OR


Thanks for the response Herb. I can obtain a good match using the 65 ft tap point, but my question is why my analyzer is measuring a change in the feedpoint resistance (real component of R + jX). 

Regards,
Marty

> 
> 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