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[TowerTalk] Shunt Feed Mystery

To: "" <>
Subject: [TowerTalk] Shunt Feed Mystery
From: K9MA <>
Date: Tue, 10 Dec 2019 23:31:34 -0600
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Here's the mystery: After a long transmission on 160 at 1.5 kW, the SWR suddenly, but smoothly, rises to 1.4:1, but rises no further. A pause of a few seconds allows the SWR to drop back to 1:1, but it will rise again when transmission is resumed. Retuning the gamma capacitor will reduce the SWR, but not quite back to 1:1. Clearly something is heating up, but I can't think of an explanation. There are some N750 doorknob capacitors in the matching box, but if one of them were heating and drifting, why would it stabilize? Could a ferrite balun be heating enough to reach its Curie temperature? But since the element ends of those are already shorted to the mast or boom through shunt chokes, why would that affect the shunt feed.

The 70 foot tower has an A3 at 50 feet, a JK Mid-Tri at 29 feet, and a D40 at 75 feet. It is shunt fed it on 80 and 160 with separate gamma matches, switched to a single coax with a relay. This has worked reliably for over 25 years, with a TH7 where the Mid-Tri is now. The TH7 had grounded elements, while the Mid-Tri does not. There are bead-type baluns on the A3 and D40, and a toroidal balun on the Mid-Tri. All three antennas now have shunt chokes at the feedpoints ala N9NB/W5JAW. (None did before.) The coax shields are bonded to the tower at its base, but not to the booms or mast at the baluns. The feedlines all are fastened to tower legs or booms, except at the rotator loop. There is a 40 meter full wave supported by a sidearm at its feedpoint near the top of the tower, with a balun, but it's feedline is NOT on the tower, but pulled away at about a 45 degree angle. That's been like that since long before the problem appeared. There's a photo with the TH7 at

Any ideas?


Scott K9MA

Scott  K9MA


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