In a message dated 3/15/2010 4:36:46 P.M. Eastern Daylight Time,
pc5m@xs4all.nl writes:
Hi, I would like to get the opinion of this group on constructing a
homemade
variable 500 pF/1nF capacitor for my new 160mtrs amp. To get sizes down I
am
considering using 1mm (40 mill) thick teflon sheet material as dielectric
(10 times or so capacitance increase when comparing with air construction).
So a sandwich of several stator-PTFE-rotor-PTFE-stator-etc, etc. Capacitor
stress will be around 5KV and 20 Amps of RF current.
Did google a bit,but no hits.should this be attainable ? I think losses of
PTFE are well enough at 2 MHz to cause any heating or change in Eps
R/capacitance and 40 mill will provide enough rigidness and stability ?
Thanks, Carel
Hi Carel,
Teflon is indeed one of the best dielectrics available but you will
need to
consider several things if building a high-voltage RF capacitor with it.
Just guessing I would say 40 mils would be running on the edge for 5KV -
I would use 60-80 mil thickness for extra safety.
Perhaps it would be more practical to use a lower voltage rated
commercial
type air-variable capacitor and put sheets of Teflon in-between the plates
to raise
the voltage rating versus trying to build your own variable capacitor from
scratch.
It can be an awful lot of work trying to build your own variable capacitor
by cutting
out all your own plates, fixing them to a shaft, aligning all the plates,
polishing the
edges of the plates, spacing the plates to a high standard, etc.
73 Todd WD4NGG
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