[Lowfer] Question on building homebrew variable caps...

[BOB RIESE]

well glass at extremes will crack fer sure

I use a built up glass cap with my Spark Transmitter on uhh 900 Kcs
you can see the foil sheets stress under key down but with a 3/8 inch
spacing
around the edge of the glass it has held up over the years. This is a
tight stack
and the biggest problem was the voltage at syntony, going around the
edges. 
Glass isnt going to be a vaccine variable but the original idea was to
build a
low loss cap. By varying the dielectric and keeping the cap plates fixed
the condenser
can be made quite solid. I have never tested the idea,, as far as the
current it will handle 
or/voltage etc

Bob

On Mon, 11 Nov 2002 18:34:55 -0500 "Bill Ashlock" <ashlockw@hotmail.com>
writes:
>Ralph,
>
>> > consider a set of fixed cap plates separated by glass plates
>> > now sliding the glass in and out between the plates will make
>> > the cap vary. Advantage would be a cap with good mechanical
>>
>>Glass is a very lossy dielectric at RF.  Ask the guys who build 
>Tesla
>>Coils.  I have popped more glass plate caps than I want to think 
>about.
>>Much better is PVC plastic.
>
>Are you sure that glass is lossy at 185KHz? Or is it just the way that 
>these 
>glass plate capacitors are constructed - allowing uneven heat 
>generated from 
>the metal plates to crack the glass. PVC isn't that great either. Ask 
>the 
>guys that build the really high-Q loading coils.
>
>Bill A
>
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