Topband: Submerging variable caps in oil as substitute for vacuum variables

[HAROLD SMITH JR]

Subject:Re: Topband: Submerging variable caps in oil as substitute for vacuum variables


The author points out (correctly) the tempco issues with oil dielectric.

Still I am intrigued by the thought of a remote tuning capacitor via hydraulic tubing :-). The capacitor plates could be as simple as two concentric cylinder conductors with appropriate spacers. I betcha crud collecting on the top of the oil would set voltage limit.

Tim N3QE

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The omega match for my shunt fed tower uses two 300 uF roughly 3000-volt 
variables I bought from Henry radio a few years ago.  I think 
occasionally about running more than 100 watts on 160, but the cost of 
the vacuum variables is a real issue.  February QST has an article on 
using mineral oil immersion to increase the voltage capability of 
variables in a 600M amplifier.  The article does not include any hi-pot 
data, but indicates that the maximum capacitance went up a lot.

Opinions?

Tim,

I have an oil filled variable capacitor that I picked up surplus many years ago. The spacing is about 0.01 inch, capacitance 1000 pF and 
if I remember correctly it hi-potted to around 5kV. I still have it. 
I am using 2 X 500pF @ 7.5kV vacuum variables  in my Omega Shunt feed. I normally run 1500 watts out on 160. Top loaded 80 feet of Rohn 45G. 
Guyed with 6700 lb Phillystran. 

73 de Price W0RI and Club Station W0CKC