I know what a vacume tube is

and a capacitor

but never really heard of a vacume cap

what is it??

I've seen it in a tuner unit i was considering


thanks

A VACUUM capacitor is exactly what the name implies, the VACUUM
is the dialectric instead of paper or air.
It has bellows that is air tight, or should I say vacuum tight.

On Sun, 13 Feb 2005 04:03:16 GMT, "John Franklin"
wrote:

A VACUUM capacitor is exactly what the name implies, the VACUUM
is the dialectric instead of paper or air.
It has bellows that is air tight, or should I say vacuum tight.


The other responder described an "adjustable" vacuum capacitor. The
more general vacuum capacitor has no bellows. The plates of a vacuum
capacitor are concentric or coaxial conducting tubes, which is not
that important. The important things are that they are sealed in
relatively heavy glass envelopes with electrodes at each end and are
capable of handling high RF currents at high DC potentials with
extremely low leakage.


Bob, W9DMK, Dahlgren, VA
Replace "nobody" with my callsign for e-mail
http://www.qsl.net/w9dmk
http://zaffora/f2o.org/W9DMK/W9dmk.html

W9DMK (Robert Lay) wrote:
On Sun, 13 Feb 2005 04:03:16 GMT, "John Franklin"
wrote:


A VACUUM capacitor is exactly what the name implies, the VACUUM
is the dialectric instead of paper or air.
It has bellows that is air tight, or should I say vacuum tight.



The other responder described an "adjustable" vacuum capacitor. The
more general vacuum capacitor has no bellows. The plates of a vacuum
capacitor are concentric or coaxial conducting tubes, which is not
that important. The important things are that they are sealed in
relatively heavy glass envelopes with electrodes at each end and are
capable of handling high RF currents at high DC potentials with
extremely low leakage.


Bob, W9DMK, Dahlgren, VA
Replace "nobody" with my callsign for e-mail
http://www.qsl.net/w9dmk
http://zaffora/f2o.org/W9DMK/W9dmk.html

It should be noted that it is very unhealthy to get across one of these
baby's when they are live.

Dave WD9BDZ

W9DMK (Robert Lay) wrote:
The other responder described an "adjustable" vacuum capacitor. The
more general vacuum capacitor has no bellows. The plates of a vacuum
capacitor are concentric or coaxial conducting tubes, which is not
that important. The important things are that they are sealed in
relatively heavy glass envelopes with electrodes at each end and are
capable of handling high RF currents at high DC potentials with
extremely low leakage.

Can a doorknob cap rated at 20 KVDC be used to
resonate a small loop?
--
73, Cecil http://www.qsl.net/w5dxp


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Yes, but the ceramic is only 1/2"-3/4" in diameter, and thus power limited. I've seen at least one used that way which
ended up in two pieces with lots of plastic shrapnel produced.

--
Crazy George
the ATTGlobal.net address is a SPAM trap. Use the att dot biz version.

"Cecil Moore" wrote in message ...
W9DMK (Robert Lay) wrote:
The other responder described an "adjustable" vacuum capacitor. The
more general vacuum capacitor has no bellows. The plates of a vacuum
capacitor are concentric or coaxial conducting tubes, which is not
that important. The important things are that they are sealed in
relatively heavy glass envelopes with electrodes at each end and are
capable of handling high RF currents at high DC potentials with
extremely low leakage.

Can a doorknob cap rated at 20 KVDC be used to
resonate a small loop?
--
73, Cecil http://www.qsl.net/w5dxp


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Cecil Moore wrote:

Can a doorknob cap rated at 20 KVDC be used to
resonate a small loop?

Sure, but I think you'd end up with a pretty poor antenna.

I don't know for sure, but guess that doorknobs are typically made from
Z5U or similar ceramic. If so, the capacitance is a strong function of
both temperature and voltage. Therefore, the loop tuning will change
with both those factors. I'd also guess that the Q wouldn't be too hot,
so you'd also incur a lot more loss than you would if you used an air
dielectric part.

Roy Lewallen, W7EL

"Roy Lewallen" wrote in message
...
Cecil Moore wrote:

Can a doorknob cap rated at 20 KVDC be used to
resonate a small loop?

Sure, but I think you'd end up with a pretty poor antenna.

I don't know for sure, but guess that doorknobs are typically made from
Z5U or similar ceramic. If so, the capacitance is a strong function of
both temperature and voltage. Therefore, the loop tuning will change with
both those factors. I'd also guess that the Q wouldn't be too hot, so
you'd also incur a lot more loss than you would if you used an air
dielectric part.

Roy Lewallen, W7EL

It's not just the voltage but the RF current too. If this is a compact loop,
the RF current can be high and just because a capacitor can withstand a high
voltage doesn't mean it an also pass a high RF current. The spec's I've seen
call out the max voltage and a max RF current for transmitting capacitors.


--
John Passaneau W3JXP
State College Pa.