Date: Tue, 16 Mar 2010 23:51:14 -0600
From: John Lyles <jtml@losalamos.com>
Subject: [Amps] Teflon dielectric variable capacitor
Carel
I am pleased that you checked into this and did the calculations. As you
see, the smaller the air gap, the worst off it can become. Its almost
better to have a big air gap betwen a big slab of PTFE or use only air.
This effect, by the way, is also a real pain when terminating HV cables,
where the outer shield in a coax cable peels away from the inner
insulation. It is often called the 'triple point' where you have a thin
point of air next to insulation, with a conductor adjacent. This is
usually where cables will eventually breakdown if the stress is high
enough. A solution there is to use a semiconducting tape or goo that
fills the area. Another solution is to use RTV compounds to pot the
small gap, sealing it off from atmosphere. This needs to be done in
vacuum to remove air pockets.
But this doesn't help in your quest for a capacitor that is variable.
I recommend sticking with air only, or if adding a dielectric, then seal
it off completely.
73
John
K5PRO
## here's another thought. Back in 1977, a buddy across town tried
gamma matching his 50' tower on 160m. It worked, but arced with
> 80 watts ! I had a new can of this stuff ..called 'GE HV acrylic
plastic spray' . I doused his air variable at the base of the tower with
it,
and once dry.. no more arcing.. even with 120 w. I took my amp over
there 2 x weeks later.. and stuffed 1100w into it.... no arcing !
## another friend had a bad load cap in his FL-2100b amp. The
plates would touch and rub... only in one narrow range. I doused
it with the GE spray bomb... and he never had a problem with it
again. And that's with the plates still rubbing together !
## No LCR meter's back then.. so no clue how the CLEAR
HV spray affected the min /max C. Does GE still make this
stuff? I used it on yagi's, tubular gamma match's, all sorts
of DC HV stuff, and all sorts of HV RF stuff. I'd love to get
a couple more cans of this stuff. Regular garden variety
Acrylic plastic, in spray bombs.. is NOT the same as the GE
HV version.
## Having several LCR meter's, hi pot tester, etc, it would be
a fun experiment, to see if the min /max C is affected.. and by how
much.. on everything from narrow spaced broadcast variables [what's
used in the 2100-B] to wider spaced TX types. I'd love to know what
the before and after Hi-pot test results are.
## If GE doesn't make it, does anybody else ?? I swore by it..at the
time... and never saw it since. NO AIR between plastic spray and
the metal plates. I quickly doused the stator, rotor, shaft, everything
BUT the rotor moving contact... that was taped off.
later........ Jim VE7RF
capacitor
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