As everyone now knows, I've pretty much overhauled my 160 m L-network
match box that I use for feeding my shunt-fed tower after finding
that I improperly used a PL-259 on the high-impedance output side of
the network. Note that I swapped the transmission line with a known
good line and I still had the problem. And, as an added sanity check,
I tried the original line on a different antenna and all is well. So,
the transmission line feeding the match box has checked ok using
another known-good antenna (a 40CD2 and a KLM KT34-A).
After all of this, it still acts hokey. I ran the vacuum variable
pretty much through its entire range and absolutely nothing changes.
I admit ignorance about the internal workings of vacuum variables. I
know that they're essentially coaxial and have a bellows, that they
have fine threads that can be stripped, that their full range tends
to run over about 10 turns, that they are evacuated (hence the name),
and that's about it. The one I have in there is a used Jennings
15-500 pF good for 15 kV, and I used to get a match with it about 3-6
turns out from "fully meshed" (the clockwise stop). The place I
bought it from noted that it has cosmetic dents, but checks out
electrically. It's been in use for about a year, now, though not
heavy use. I've never run it hard against the stop and the threads
that move the internals are not striped. I don't typically use high
power levels. When I do, about 800-900 W is all I can make on 160 m.
I have a Comet 10-1000 pF good for about 7 kV that I could put in
there for testing, but before I do that, a question: aside from
losing the seal integrity, is it even feasible for a vacuum variable
to fail spontaneously? If so, what sorts of failure modes occur?
This one looks open with an ohm meter, but I don't have a capacitance
meter good for such small capacitances so I don't know if it's become
open internally.
Everything else seems to check out: the shunt is intact, checks good
with an ohm meter, and I've verified that the transmission line is
good. All the internal match box connections are good and intact. The
Teflon SO-239 at the low-impedance side is good.
Any more ideas?
Happy New Year,
Kim Elmore N5OP
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