Re: air variable capacitor

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Ray von Postel by way of Terry Fritz <twftesla-at-qwest-dot-net>" <vonpostel-at-prodigy-dot-net>

Tesla list wrote:

> Original poster: "jesse hampton by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <zetoyoc-at-yahoo-dot-com>
>
> Hello,
>
> I have a large (18" X 6" X 6" aprox.) air variable
> capacitor.  My question is, how would this perform in
> TC operation as the main cap?  By the way this is not
> my coil it's going into but a vttc I know very little
> about. Could this variable cap handle the job? or do
> you need a bit more info and if so what?
>
> Thanks.
>
> Jesse
>
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Jesse:
It depends on the spacing as well as the capacitance.

If this was used or intended for a "Ham' transmitter it probably has a
maximum of
1/2 inch spacing between the plates, that is, from a rotor plate to a
stator plate.  Normally,  in
v.t. circuits you would expect to use it for not more than 4000 volts d.c.

Most air dielectric variable capacitors are measured in picoFarads.  50 pFd
to 100 pFd is typical
if they have large spacing.  General Radio Co. made laboratory variable air
dielectric capacitors
having a 600 volt 1100 pFd. rating. The multi-section variable capacitors
used in broadcast radios
are generally  around 450 pFd.

The capacitors used in a TC normally are rated at much higher voltages and
instead of
having a capacitance measured in pFd. (1 x 10^-12 Farad) are measured in
mFd. (1 x 10^-6 mFd.) .
The ratio is 1,000,000 to 1.  Therefore, a variable condenser as you
described would not work for
energy storage in a TC.  However in a VT circuit might be used as a
"trimmer" capacitor in the
grid circuit for frequency and or feedback adjustment where the voltages
are lower.

TCs of any type operate in the medium, low, and even the ultra low portions
of the radio
spectrum.  As you go lower in frequency the required capacitance go up.
Variable capacitors at
those frequencies become physically impractical if they use air dielectric.
Even
fixed air dielectric capacitors become physically unmanageable particularly
when they are required
to store energy.  There is/was a picture on the web  of one of the early
Marconi stations which
had its capacitor in the form of large iron sheets hung in a barn.  This
gives you some idea of
the lengths they had to do to get large capacitance high voltage condensers.

Hope this helps

Ray