[Amps] PTFE dielectricum variable cap

[ToddRoberts2001 at aol.com]

In a message dated 3/15/2010 6:42:36 P.M. Eastern Daylight Time,  
TexasRF at aol.com writes:

Teflon is rated for 1500 kv per inch. .040 thickness is good for 60,000  
volts!
You can find the info at google "teflon voltage". 
A typical  application is for a plate bypass at VHF or UHF where 5 to 10 
mil material holds  off 4 KV.
73,
Gerald K5GW
 
Hi ,
       There are different grades of PTFE.  Standard PTFE is rated for only 
500 volts/mil and that 
is only for the first 2 or 3 mils thickness. When you increase the  
thickness of PTFE to 20 mils or 
more the ratings could go down to 300 volts/mil or even less. So a 20 mil  
thickness of PTFE may have 
a breakdown rating of 6KV or less. There are special grades of modified  
Teflon out there but any Teflon
you might buy in the regular market I would consider standard Teflon.
 
       You also have to take into account  irregularities in the Teflon 
sheet like pinholes,  air bubbles,
etc. Also is the 5KV RF voltage RMS or peak? The peak voltage could be far  
higher.
So to be on the safe side I would
derate standard Teflon to 200 volts/mil and give a generous safety  factor 
for the
voltage rating needed when using a thickness over 20 mils.
 I would only trust a 40 mils thickness of Teflon to handle 6KV  without
any surprises. To be on the really safe side I would increase that to 60 or 
 80 mils thickness.
 
       There are builders of transmitting  loop antennas who thought they 
had enough thickness
of Teflon in their home-made tuning capacitors only to have them flash  
over.
 
73 Todd WD4NGG