Homebrew, High Voltage Variable Capacitor for Magnetic Loop Antenna

I built a prototype today of a high voltage variable capacitor for the 40m Magnetic Loop antenna I am building. My original plan was to make it a fixed capacitor out of a length of coax for a single 40m frequency. I played with cutting the coax to the right length and finally decided against this method.Instead I started looking at a “trombone” style of cylindrical capacitor made from copper pipe. After making some calculations, I settled on the following for the prototype:

  1. Inner cylinder made from 6″ of 1/2″ copper pipe: O.D. = .0.625 in
  2. Outer cylinder made from 6″ of 3/4″ copper pipe: I.D. = 0.811 in
  3. Dielectric made from 4mil Polyethylene clear plastic sheeting from Home Depot. You will need a strip about 7 inches wide and 6 feet long.
  4. A couple of sheet metal screws and some scotch tape

The nice thing about this capacitor is that PE sheet has a dielectric strength of 3kV per mil. This will easily allow operation at the 4.2kV generated in a magnetic loop when operated at 100W.

Here is what it looks like now:

 

So as a prototype definitely a success!

I have a scheme in mind to make this precision adjustable using a threaded rod so I will likely do that in the final version. On the 40m loop antenna, I expect to use this almost fully engaged and only need to tune about an 1″ of play to cover the whole band.

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6 responses to “Homebrew, High Voltage Variable Capacitor for Magnetic Loop Antenna

  1. Damien Wilson August 12, 2017 at 1:37 pm

    Hi Nice job, im building a 2m Dia: Mag Loop at the minute, i have built my own Variable Capacitor, i dont have it all put together just yet, i can send you some pics of the build so far if you are interested,i would also like to hear how your loop works on 40m, i have a hexbeam for 6-20m but space limits me for anything worthwhile on 40, Thanks for the great artical, & all the best to you.
    73s
    Damien
    2i0fnn.

    Like

  2. Dr. David Kirkby November 11, 2017 at 1:55 pm

    Your 4mil polyethylene clear plastic sheeting does not increase the working voltage, but decreases it! The reason is there will be air-gaps, and the electric field across those airgaps will be higher than if the polyethylene was not present!

    The reactance of a capacitor is Xc= 1/(2 pi f C). So the bigger capacitance, the lower the reactance. The dielectric, which will have a relative permittivity greater than that of air, will create a higher value of capacitance than the same thickness of air. When multiple capacitors are in series, the voltage across them distributes itself such that most is across the higher reactance – just as with resistors in series. The air gaps have the highest reactance, so the voltage across the air gaps is higher.

    Only if you can completely remove the air will you be able to increase the working voltage. For that you will need a liquid.

    Like

    • rjbono November 11, 2017 at 4:49 pm

      Thanks for the comments David. I am not suggesting that this be used with power above 100W mainly due to exposure to the fields. I can say that this capacitor works reliably at 100W. No shifts in tuning frequency or evidence of arcing at all.

      Like

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