Assorted combined replies...
>I haven't seen water filled variables but have seen oil filled
>es 73 de Pete WA5JCI
Yep. I've seen them too. I think used for airborne applications to
avoid the de-rating that would otherwise occur at high altitudes.
>That might work, at least until something corroded and the metal salts
made the water conductive.
>What would you do when your tank circuit ( ha ) went off like "Old
>Maybe an umbrella would come in handy.
Well, that's why the plates need to made from gold or platinum. :-)
>A terrific idea! Why didn't I think of that? :-)
It has its limitations!!!
>Maybe you haven't forgotten anything. The main
>concern, of course is that the water remain pure. When
>you start passing currents through the metal emersed
>in it, you might cause things to ionize or oxidize.
>Platinum would seem to be the best to experiment with.
>73 de Paul, K7CW
What's wrong with gold? I didn't think pure water would have any
reaction with it?
/\ (from Rich)
>However, water absorbs carbon-dioxide from the air - which forms
>carbonic-acid, which is conductive.
Yes, which is why the thing needs to be sealed before that happens!
>/\ so try it, Marv.
I'm not Tom Edison. Some things are more easily conceptualized
>Hi Marv, I enjoyed reading your recent posting in the AMPS list. That's
>interesting idea you had about using water as a dielectric in an
>capacitor. But there must be some overriding reason why using pure
>water, even though it is an insulator, will not work in that
>probably has a terrible dissipation factor or loss factor.
I don't know the numbers at HF but, the dissipation is certainly high
at 2.5 GHz... which is why the market for microwave ovens is so
> Also water is a terribly unstable medium under changes in temperature.
It expands when hot
>and look what happens to it when it freezes!
Yep. I thought this was a perfect setup for someone from VE land to
advise us which were the best glaciers from where to mine... plate
separators . :-)
Here in Southern CA, it is not a big concern. I have to go visit my
snow... it doesn't come visit me. :-)
> Its expansion and contraction
>under changes in temperature would likely create a very unstable and
>capacitance, if it worked at all.
A small expansion chamber should take care of that.
>Also as Rich pointed out water has a lot of
>gasses dissolved in it. That is a fascinating study in itself. That's
>fish are able to breathe under water! It is very difficult to maintain
>gas-free water unless it is first degassed (usually by boiling or using
>vacuum pump) and then maintained in a sealed container.
Yep. That's why you need to start with pure water and seal it before
it collects... smog.
>I had a similar idea about immersing an air-variable capacitor in a
container of mineral oil.
>Mineral oil does have an extremely good dissipation factor - about the
>as good mica! But I think the fact that it is a liquid and will expand
>contract under changes in temperature would also result in a very
>and shifting capacitance.
As long as the plates don't move, it shouldn't vary in value.
> The fact that apparently no one has ever
>manufactured an oil-filled variable capacitor must be due to a good
Those are made. I've seen 'em. Just pricey and rare.
>73 Yours Truly, Todd Roberts WD4NGG .
>Or, if not water, some non-conductive liquid. Don't have a dielectric
>chart handy, but there must be something suitable.
Most of the other ultra hi-dielectric (>10) fluids are nasty things
(look in a CRC Handbook)... and rather polar so, therefore possess poor
>Transformer oil (or nearly any mineral oil) would work quite well. The
>dielectric constant typically is just over 2.0, and the voltage
>characteristics are excellent. A good application would be for the tune
>cap on 160 or 80 meters : a 150 or 200 pf, 1000V air variable would
>a 300 or 400 pf HV variable cap.
Silicone based fluids are just a skosh better.
73 & Thanks for all the responses!
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