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Re: Paper Capacitors
Quoting Dan Gowin <gowin-at-epic-1.nwscc.sea06.navy.mil>:
> I've seen some listings from Richard Hull and others in
> regards to using the correct glass for Salt water caps. My cap
> from College days, was made from an old Pyrex medicine jar with
> a narrow mouth. But, recently, I've also made some caps from
> Wine coolers.
> Besides differing dimensions and glass thickness, the K of the
> glass is about the same. (I've measured the caps with an LCR
> meter and checked my results with cap calculations using
> assumed K factors.).
> At the same time I've heard that Borosylicate and other
> types of glass have different K characteristics. What type of
> glass or bottle should a coiler use?
It is not the initial "K", but the end "Q" that makes the
difference. Pyrex makes a pretty decent salt water cap in my
experience, the "Q" being higher than normal window glass, even
though the "K" is about the same. What you need to do if you are
looking at glass caps in Tesla tank circuit service is to compare
the K value of the glass to the dissipation factor of the glass
at your frequency of operation; the lower the dissipation factor,
the higher the Q factor. If you have access to a large selection
of glass types then I would select RF dissipation factor over "K"
without a second thought.
The bottom line here is not how large the cap gets (a factor of
"k") it is how hot the cap gets (a factor of "Q" dependant upon
the RF dissipation factor at the system operation frequency).
Even though glass caps are very heat tolerant, heating in itself
is a warning flag that the losses are unacceptable.
Richard Quick
... If all else fails... Throw another megavolt across it!
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