[Amps] PTFE dielectricum variable cap

[ToddRoberts2001 at aol.com]

In a message dated 3/15/2010 4:36:46 P.M. Eastern Daylight Time,  
pc5m at 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