On Mon, 30 Nov 2015 17:51:13 -0500, Michael Black
wrote:

Nobody has yet come up with a replacement for high voltage variable
capacitors, so they still have to be made.

I've done fairly well with window glass covered with aluminum foil.
Also HDPE (cutting board) dielectric. These are commonly used in
Tesla coils. Something like this:
http://www.deepfriedneon.com/tesla_f_calccap.html
I don't have any built right now, but they do work. The problem is
mechanical stability. When tuning a magnetic loop with one of these,
the adjustment is rather critical. My last experiment was to build
the tuning capacitor into a plastic box and compress it with a spring.
Adjusting the plates required considerable force, and some of the foil
was torn, but it was fairly mechanically stable. The idea was not so
much to have a variable glass capacitor, but rather adjusting the
capacitance in steps by adding or removing glass plates.

More tinkering is needed, which probably won't happen until summer.

Incidentally, plugging into the above calculator, two 3 x 3 inch
plates, with 0.125" glass dielectric, yields about 120 pF capacitance.
The same dimensions but using an air dielectric yields only 16 pF.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558