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9929RE: [loopantennas] Re: Variable Dielectric Capacitor?

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  • William Young
    May 16, 2013
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      No, I haven't tried a variable dielectric capacitor, but I have tried what I called a "floating rotor" variable capacitor with a metal plate rotating into and out of the space between two fixed capacitor plates.  The resulting change in capacitance for 180 degrees of rotation is small but very useful for fine tuning a receiver.  See QEX May/June 2002, page 31.  Bill Young WD5HOH 
       

      To: loopantennas@yahoogroups.com
      From: mars@...
      Date: Thu, 16 May 2013 12:45:09 +0000
      Subject: [loopantennas] Re: Variable Dielectric Capacitor?

       


      Hi,

      did anybody actually tried this back in 2005 or later? Are there any results? This thread is pretty old and seems to lack any conclusion :)

      I was planning to try to do exactly this (variable dielectric capacitor) and found this thread while doing my research.

      >
      > it occurred to me that single sided copper clad board might be suitable for
      > the capacitor plates as long as the substrate material was not interior to
      > the capacitor plates and subject to the fields. i don't think it should
      > contribute to loss in that postion.

      Exactly my thinking, it might make one very cheap variable capacitor that can withstand both high voltage and high current.

      My plan is to take two copper clad boards, separate them using plastic spacers (4 or 5mm) and slide polystyrene board between the copper planes.

      According to the resources (links below) I found online, polystyrene has tangent loss of about 2 - 5 x 10^-4 and relative permitivity of about 2.5. That is actually not bad at all as Mica has "similar" loss properties.

      http://www.rfcafe.com/references/electrical/dielectric-constants-strengths.htm
      http://www.kayelaby.npl.co.uk/general_physics/2_6/2_6_5.html
      http://www.eccosorb.eu/sites/default/files/files/dielectric-chart.pdf

      Polystyrene also supposedly has much higher dielectric strength (197kV/cm) than air (10-30kV/cm) so arcing does not have to be an issue (especially for QRP or low power) as well.

      73!

      Martin OK7MS


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