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 <dx...@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