Whilst
clearing out some of the boxes in the workshop so I can get
back to work I discovered a vacuum variable (8" long 3"
diameter) I had kept from PhD days (1993-96). The original
use I had was a planned magnetic loop (using two joined lengths of
22mm copper pipe (about 6.2m long) just one join using silver solder,
to keep the loop resistance down) with a 2m diameter.
Bit
like this one (no photo right now)
The
capacitor would go across the gap in the
loop.
a
a
The feed point was using an FT240 torrid (-43 I
think) with 3 turns to an N type, the copper pipe would have slid
through this.
The feed point is at the top of the loop, which sits at
a slight angle to the vertical. The capacitor was to be in a length
of 4" soil pipe, supported on an old 1/2" thick white
chopping board.
The capacitor would be tuned remotely using
a spare (NEMA17) stepper motor and a reduction planetary drive from a
CNC project.
The
loop should have been usable 3.5MHz right up to 21MHz.
However ... capacitors with clear glass envelopes and copper
electrodes, if there is any air ingress you will see discolouration
of the copper about the electrodes, slowly at first and depending on
how humid it will completely darken and discolour.
Originally the insides were all shinny, now they are dark so I expect
the envelope has been damaged.
When I
was a PhD research student in the Chemistry department in
Cambridge University the gas handling line I used was maintained by
the department glass blowers and I
had accurate vacuum pumps. Given this I am pretty
sure I could at one point have 'fixed' the capacitor and that was the
plan, to glass blow on a small tube to attach to a line, then heat
the capacitor whilst pumping down the interior and then to seal it
again.
Now you
can pretty much assume the pressure inside is at atmospheric not
fractions of what it was originally probably near
total vacuum.
I know
that capacitors at atmospheric pressure (760 Torr, 1.01 Bar) have a
voltage rating about 12-15 times lower plate
spacing dependant than that of an otherwise
identical vacuum capacitor (at 10-7 Torr). How do I know this... well lets just say during
research if a capacitor failed I'd know about it damned quick. We
used a thyratron pulse across a flash lamp at 5Hz and used a bank
of vacuum storage capacitors. The voltage was up to 15KV. If
one ever failed then audibly as the experiment ran it was like a
fizzing as it shorted and the main flash lamp never fired. We used to
have a selection of new 'spares' that we'd swap in. I never asked
where they came from I think we had an Eastern European post doc in
one of the other groups and they had 'acquired them', I never asked
or where they went after we finished with them, several researchers
before me had often throw them away (such a waste). Used to go
through about 2 or 3 a year.
I might
just use it in an ATU where the high voltage doesn't matter so much
as the capacitance is fine and it requires multi turns
10-470pF.
Back to
cleaning up....
No it isn't for sale :-)
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