Jeff wrote:
"Denny" wrote in message
ps.com...

In my search for knowledge I recently went through a big thrash with
antenna tuners... Because of cost I built my own variable caps... To
keep the size down I experimented with various insulators between the
plates, from air up to glass...

Being that the tuners on 40 and 20 were end matching a half wave
antenna element the circulating tank current was considerable... On
20 meters where the capacitor plates are 1.5" square for resonance the
glass would simply shatter at about 800W for 5 seconds... Plexiglass
would boil internally in roughly 20 seconds leaving this interesting
rippled piece with 'steam' bubbles in the center of the plexi...
Ceramic tiles seemed to work OK, but I didn't find any thin ones
locally and the thick pieces resulted in plates being awkwardly
large...

On 80 meters the glass plates are 4" X 6" and the glass runs barely
above ambient at 2KW...

denny / k8do



An very odd result considering that glass dielectric capacitors are sold
specifically for high rf current use by people such as AVX and the like. I
guess it depends on what type of glass you used. You shouldn't have used
your old lead crystal glasses (;-))



Lots of kinds of "glass" out there.

fused silica is pretty low loss
borosilicate glass (e.g. Pyrex, Kimax, etc.) is medium
windowglass (soda lime) is all over the place.