[HomeBrew] More variable capacitor

[KA4INM radio]

On 10/24/14 16:59, N1KHB--- via HomeBrew wrote:

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> Hi Dick et all,
>     I want to build a magnetic loop antenna which requires a  variable
> capacitor on the antenna in order to bring it into resonance. The  rotor-stator
> type is disliked for it's lossy frame connection to the rotor.  Split stator,
> and less popularly, butterfly capacitors are used instead. These  caps are
> expensive and hard to find. Some people then come up with their own  ideas -
> trombone capacitors with one pipe sliding inside of another, and other
> designs are on the web.
>      Then it dawned on me that CD's have a metal layer  embedded within, so
> the idea that it might be possible to build a variable  capacitor out of
> CD's which have that metal layer in them. The entire  mechanics are still only
> fragmented thoughts floating around in my head, but the  basic idea is that
> I could build the stator sections by stacking and spacing by  some
> appropriate distance with threaded rod with the lacquer layer stripped away  somehow
> for contact. Then if the rotors didn't need to be stripped too it  would
> make assembly that much easier. But if electrical contact is actually
> necessary in the rotor plates, I would need to do that much more work.
>     Hence the original question of whether a collection of  isolated plates
> inserted into the stator area would still function  to disallow varying
> degrees of charge between  the "blocked" stator  plate areas depending on rotor
> position. If I had a C meter, I'd be able to just  do a mockup to see what
> happens. That mockup would consist of two plates acting  as the two stator
> sections, with a single isolated plate being moved into  and out of the stator
> plates to see if the C value changes. So my thinking is  that if it does,
> then multiple isolated rotor plates would act similarly on more  stator
> plates. My intuition says that it should work, yet some say no. Mostly  just
> trying to save some lacquer removal work in the end, but it also became an
> interesting mental exercise. I should know the answer to this with my
> background, but it hasn't surfaced as yet.
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   There is not enough aluminum sputtered on to CD and DVDs to be 
conductive, you can see right through them if the "paint job" is removed.
There is also no way to connect a wire to the aluminum coating on them.
If you could you really need an extremely low inductance connection 
individual wires will not do.

   You would be better off looking into copper foil between sheets of 
glass, "compression capacitors" have been made that way in early radio.
Whatever you build must be effectively water proofed.

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-- 
   Ron  KA4INM - Youvan's corollary:
                 Every action results in unwanted side effects.