I tried to make a fixed padding (parallell) cap for 160 plate circuit ( PI )
using a stack of brass/ptfe/brass etc connecting together
every second layer. It was finally pressed together by means
of 3mm ptfe sheet.
The dielectric layers were 0,5 mm
It was supposed to be used in a 2x 4CX250A amp.
It tested just fine at highpot to 4 kV
The amp was run at 1800V.
The "smoke test" ended as just that...
fine up to 50% drive ...then smoke...then BANG !
Inspection showed a lot of very fine holes burned through the
PTFE-dielectricum all across sutfaces. May be I did not pay enough attention
to
polishing the brass sheet, and so created "hot spots"---
or maybe it was just a bad idea ??
----- Original Message -----
From: "pc5m, Carel" <pc5m@xs4all.nl>
To: <amps@contesting.com>
Sent: Monday, March 15, 2010 9:36 PM
Subject: [Amps] PTFE dielectricum variable cap
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
p.s. Yes, of course I do know that vacuum capacitors do exists ...
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