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@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
_________________
Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
|