Have you put a capacitance meter on the vacuum variable and actually
measured it?
David Robbins K1TTT
e-mail: mailto:k1ttt@arrl.net
web: http://www.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://dxc.k1ttt.net
> -----Original Message-----
> From: Kim Elmore [mailto:cw_de_n5op@sbcglobal.net]
> Sent: Wednesday, December 31, 2008 01:17
> To: towertalk@contesting.com
> Subject: [TowerTalk] Maddening Problems Redux
>
> 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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