[Amps] Teflon dielectric vraiable caps.. another thought.

[Jim Thomson]

Date: Tue, 16 Mar 2010 23:51:14 -0600
From: John Lyles <jtml at 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