[Amps] PTFE dielectricum variable cap

[pc5m, Carel]

First of all, thanks a lot for all contributions so far ! Seems to be a fun
project to take on : - ) 

 

One thing I did in the meantime was to calculate/check  the field strength
in the airgaps which will occur between stator-ptfe sheet-rotor combination
(5KV, 1mm[40mil] PTFE, twice 0.1 mm[4mil] air gap). If I did the math
correct it turns out that the E-field will be around 10KV/mm ! And as above
3KV/mm  air will breakdown this is not good...eating up the PTFE/corona/etc

 

Funny thing is that basically how small you make this inevitable airgap
(e.g.  0.001 mm / 0.04 mil ) still the E-field will remain too high. Seems
to be logically as you have basically 3 capacitors in serie: 2 big "air"
one's and one relative small PTFE one with the resulting capacitance.
Decreasing the air gap ->   air capacitor linear increases -> RF voltage
across it will linear decrease-> resulting RF voltage divided by distance
stays the same.. 

 

As I don't want to mess with oil and PTFE plating is too expensive (not in
the oil business as well.) it seems this is a real problem (THE problem with
this kind of contraptions?). 

 

To be continued.

 

Thanks again, Carel.

 

 

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 ...