At 11:04 PM 1/4/01 +0000, w8nf@mailroom.com wrote:
>Most of us who've done commercial amps have had to make capacitors.
Interesting stuff, Dave.
Several times in the past, I've had to make capacitors for specific RF
applications. Hmmmm, change that to "attempted to make".
One time I needed some small value like 23 pF across a P-P tank running
around 150 watts out with about 2kv on the anodes. Tried several things
with no success. Best try was a piece of teflon filled PC board material.
It lasted at least several seconds.
Most recent failure was in a matching network. I needed a value that I
didn't have handy in my collection of high voltage high current TX caps. It
was something on the order of 135 pF.
Using a piece of glass (common window type) for the dielectric and copper
tape for the electrodes, it was easy to cut to exact capacity required.
When run at several hundred watts (in a series configuration on 1.8 MHz) it
did not run very hot. But as I ran the power level up, the glass punched
through at a place where the copper tape had a sharp edge. The result was a
spot in the glass that was actually an alloy of glass and copper and was
quite conductitve! The thickness of the glass should have been about 4
times what was needed to standoff the voltage, but failed anyway.
Have you (or anyone else) had any luck with glass or other common
materials? I have had real bad luck with Teflon in the past and while
Kapton might work OK, it's not readily available in the thickness required.
73,
Larry - W7IUV
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