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RE: [loopantennas] Re: Variable Dielectric Capacitor?

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  • William Young
    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
    Message 1 of 12 , 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


    • kevin asato
      Should work. By theory a capacitor is nothing more than 2 opposite potential surfaces separated by a dielectric (air, fiberglass, some sort of plastic, etc).
      Message 2 of 12 , May 17, 2013
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        Should work. By theory a capacitor is nothing more than 2 opposite potential surfaces separated by a dielectric (air, fiberglass, some sort of plastic, etc). Using square plates will produce very non linear capacitance values as capacitance is affected by the area of the opposing plates. PITA to try to calculate, too! One thing to be aware with what you are planning is that you may have to account for air acting as a dielectric when you insert and remove the polystyrene between the two plates. May not be a large contribution to the overall capacitance but needs to be looked at. Not trying to talk you out of trying this. Let us know how it turns out!

        >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.

        I took a basic electricity course seemingly eons ago where we took two sheets of aluminum foil and slightly larger pieces of wax paper. The wax paper was inserted between the foil and the 2nd sheet was laid on top of the foil of what would be the inner surface of the capacitor. The entire assembly was rolled/folded up into a tubular capacitor type form and jumper wires were connected, one to each sheet of foil. The capacitor was then connected to a low current assembly with a known value resistor and an NE-2 neon bulb. We could then see the neon bulb flash at a rate corresponding to the value of the capacitors made by the class. Since this class was geared towards elementary school children, we did not try to calculate/approximate the dielectric value of wax paper, though in retrospect, it would be a good exercise for high school physics.

        73,
        kevin
        kc6pob


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