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[AMPS] Water dielectric?

To: <>
Subject: [AMPS] Water dielectric?
From: (Radio WC6W)
Date: Tue, 31 Jul 2001 07:33:22 -0700
Assorted combined replies...

>I haven't seen water filled variables but have seen oil filled

>es 73 de Pete WA5JCI

Hi Pete,
   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?  :-)

>Bill, W7TI

Hi Bill,
  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.
>Gold, not.

>73 de Paul, K7CW

Hi Paul,
  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
than... built.


>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
application. It 
>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.

>Interesting thought.

>Bill, W7TI

  Most of the other ultra hi-dielectric (>10) fluids are nasty things
(look in a CRC Handbook)... and rather polar so, therefore possess poor
dissipation characteristics.


>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.


Hi Phil,
  Silicone based fluids are just a skosh better.


73 & Thanks for all the responses!
   Marv  WC6W


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