What I wonder about is having small air gaps at spots where the teflon sheet
and the rotor/stator plates are bearing against the teflon. If the
dielectric is uniform there is no problem but it seems to me if there was an
air gap , yojwould get the voltage appearing across the air and the teflon
just is acting like an electrode.
I know for a fact that many years ago a capacitor manufactor supplied us
with a capacitor with plates coated with several mils of teflon and in fact
the capacitor broke done at lower voltage than it did before the teflon
coating (same plate and spacing as before the coating so just the air gap
reduced)
I am not sure of how the electric field intensity is modified when there is
an "air bubble" surrounded by surfaces fully in contact. Don't have access
to EM software to look at this anymore.
Just a thought -- Hank K7HP
----- Original Message -----
From: "la7sl Nils Petter" <la7sl@norgespost.no>
To: <amps@contesting.com>
Sent: Monday, March 15, 2010 2:12 PM
Subject: Re: [Amps] PTFE dielectricum variable cap
>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 ...
>>
>>
>>
>> _______________________________________________
>> Amps mailing list
>> Amps@contesting.com
>> http://lists.contesting.com/mailman/listinfo/amps
>>
>
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