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.


4 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.


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