[HomeBrew] HomeBrew Digest, Vol 109, Issue 1

[Michael Barlow via HomeBrew]

Hi Sonny,
 If I understand your proposed construction plan correctly, I see no reason for connecting the rotor segments together---The stators, yes. The rotors, no.
 You effectively are connecting a number of pairs of series connected caps in parallel, and I believe that the rotor to rotor connections would be superfluous.
 However, I do not understand whether you intend to use the acrylic plastic coating as a dielectric. If you do, I suspect that the increased dielectric loss (vs air dielectric), may be greater than the rotor contact losses might be, in a good conventional capacitor.
 Have you considered using a standard two gang variable cap, and ignoring the rotor connection---use only the stator connections, and support the capacitor frame by some type of very low loss insulation--steatite, etc.?? This scheme would cause the rotor connection to be unimportant--similar to a butterfly cap.
 I have a number of butterfly caps around here, but their max capacitance is almost certainly too small to be useful, for your purpose, as they were intended to be used at VHF & UHF. 
             GL with your project & 73 de Ron
   
 Message: 4
 Date: Thu, 23 Oct 2014 14:31:02 -0400
 From: N1KHB--- via HomeBrew <homebrew at mailman.qth.net>
 To: homebrew at mailman.qth.net
 Subject: [HomeBrew] Variable capacitor
 Message-ID: <9a401.6bab4b4f.417aa365 at aol.com>
 Content-Type: text/plain; charset="US-ASCII"
 
 Hi all,
    One simple (I hope) question - do the
 rotor plates on a  variable 
 capacitor actually NEED to be electrically connected to each
 other  in order to 
 vary capacitance value as they move in and out of the stator
 plates,  or can 
 they be electrically isolated from each other? I'm thinking
 in the split  
 stator types of capacitor here where the stator sections
 form the two 
 electrical  ends of the capacitor.
     Common manufacturing techniques have
 them  electrically connected to 
 each other, but may be as much convenience in the use 
 of various metal parts 
 being assembled rather than necessity. I should  know
 the answer to this, 
 but don't. Thanks!
  
 Sonny N1KHB
 
  Message: 7
 Date: Fri, 24 Oct 2014 16:59:21 -0400
 From: N1KHB--- via HomeBrew <homebrew at mailman.qth.net>
 To: homebrew at mailman.qth.net
 Subject: [HomeBrew] More variable capacitor
 Message-ID: <754a3.2f63c9ec.417c17a4 at aol.com>
 Content-Type: text/plain; charset="US-ASCII"
 
  
 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.
  
 Best,
 Sonny N1KHB