Scott M wrote:
Hi,

I am going to build a variable capacitor for a gamma match in the
usual fashion where there is a rod inside a tube by an insulating
tube. I need it to handle legal limit, so how would I go about
*safely* calculating/estimating the power handling capability of what
I have built?

Thanks,
Scott, WU2X

Being safe while you do the calculations, or calculating the safe power
handling limit? For the former, which you asked, I suggest sitting in a
chair with proper ergonomic design..grin

But, I suspect what you're really asking is how to go about calculating
the power handling limits of the coaxial structure...
Consider two factors:

1) High voltage breakdown. What's the breakdown strength of your
insulator? If the two tubes are fairly large in diameter, you can
probably approximate the field as the voltage divided by the difference
in radii. Air breakdown is 30kV/cm in a uniform field. If the ratio of
outer to inner diameters is large (so it looks more like coaxial cable),
you might want to use a different formula for fields.

Don't neglect breakdown along the surface of the insulator. If the
"creepage distance" is less than 3 times the free air breakdown
distance, then you've got something to worry about.


2) Thermal limits. How much current is being carried by the inner
conductor? What's the path to somewhere the heat can be dissipated?


For both of these you'll need to model the circuit so you know the
current and voltage across your capacitor.