I'm seeing some overheating that can only be in one of two possible
components. One is a TenTec 238 antenna tuner, the other is the
loading coils in the antenna that I'm feeding with it. I've also
observed the same symptoms using a Ten Tec 229B tuner with the same
antenna. The tuners are nearly identical. Here's the setup.
Antenna is an 80/40 dipole with no balun, fed with some vintage 75
ohm KW twin lead, with both ends of the twin lead tied together and
fed as a long wire on 160m. The tuner has a good counterpoise on it
(roughly 140 feet of 6 ft high wrought iron fence that runs around
the front of my house). This thing tunes nicely, and indicates a nice
low VSWR at the beginning of transmission, but the SWR rises, and the
lights start dimming when I key it (indicating more power supply
current), about 15 seconds into the transmission. I'm seeing this now
with a Ten Tec Herc II at only 500 w, but I've also seen it with my
Titan.
Any ideas what's going on? The fixed capacitors in the tuner high
voltage types, but with significant temperature coefficient. Are they
heating and changing value?
The other possibility is that the loading coils are arcing or
drifting as they heat. They are pretty near each end of the antenna.
Jim K9YC
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