All I have is a Fluke 79 multimeter. It will read capacitance, but
the lowest it goes is a nF range. At max capacitance, this vacuum
variable will be at about 0.5 nF. I've found that this meter is
pretty sensitive to stray capacitance and I've had no luck measuring
capacitances this small with it.
I'll wok some more with it, but I tried this as soon as I got the
capacitors and I was never confident in any readings I got.
There may be better instrumentation that I can borrow locally and if
I can find it, I'll give it a go.
Kim Elmore N5OP
At 03:54 PM 12/31/2008, K1TTT wrote:
>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
> >
> > _______________________________________________
> >
> >
> >
> > _______________________________________________
> > TowerTalk mailing list
> > TowerTalk@contesting.com
> > http://lists.contesting.com/mailman/listinfo/towertalk
>
>
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