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This is actually a really simple kind of cap to make, in fact it is quicker to build than a salt water cap, if you've got everything lying around of course.

Just take two layers of some kind of polymer, with aluminum foil inbetween, and another layer of  foil on top.

Do mind,that when you're rolling it, that the foil and polymer stay aligned (no shifting or swirling)

The capacitance, is hard to determine, since there are a couple of varias to keep in mind :

  • how much air is left between the layers when rolling
  • the total size of the foil in contact with the polymer
  • the thickness of the polymer
  • the dielectric constant of the polymer, or other material used for the cap.
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    It weighs about 7 or 8 kilograms and has a capacitance of about 17 nF.

    But still the best way to get to know the capacitance is to hook up your multi-meter and measure it.

    I also made one from Mylar, but it seemed to be a problem to keep it running for longer than 1 minute, since 2 of them punctured through, so maybe later when I've got time for my coil again, I will make another one and try something different with it to prevent all of that.