Carel
I am pleased that you checked into this and did the calculations. As you
see, the smaller the air gap, the worst off it can become. Its almost
better to have a big air gap betwen a big slab of PTFE or use only air.
This effect, by the way, is also a real pain when terminating HV cables,
where the outer shield in a coax cable peels away from the inner
insulation. It is often called the 'triple point' where you have a thin
point of air next to insulation, with a conductor adjacent. This is
usually where cables will eventually breakdown if the stress is high
enough. A solution there is to use a semiconducting tape or goo that
fills the area. Another solution is to use RTV compounds to pot the
small gap, sealing it off from atmosphere. This needs to be done in
vacuum to remove air pockets.
But this doesn't help in your quest for a capacitor that is variable.
I recommend sticking with air only, or if adding a dielectric, then seal
it off completely.
73
John
K5PRO
---------------------
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..
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