Because you're using a shunt capacitor. This in conjunction with the inductive
reactance forms an L-network. As I said before, you need a series capacitor.
Move the tap to get 50 +jX and then add -jX.
Wes N7WS
On 10/15/2019 1:30 PM, Marty Ray wrote:
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
On Oct 15, 2019, at 3:19 PM, Herbert Schoenbohm <herbert.schoenbohm@gmail.com>
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@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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