[TowerTalk] Shunt Feed Mystery

[K9MA]

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 qrz.com.

Any ideas?

73,

Scott K9MA


-- 
Scott  K9MA

k9ma at sdellington.us