K7ITM wrote:
Also to consider is air pressure. The breakdown voltage decreases with
decreasing air pressure, and to a slight extent with increasing
temperature.

"Reference Data for Radio Engineers" has a graph of breakdown for a
needle gap and for smooth surfaces. For the needle gap (pretty much a
worst case) at sea level (760mm Hg), it's a bit over 25kV peak at 1
inch spacing, and just over 3kV at 0.1 inches. For comparison, a
couple smooth 10" diameter balls spaced with a 1" gap between them will
exhibit a breakdown greater than 80kV. Plates for an air-variable high
voltage cap should have smooth, rounded edges. Looks like the
handbook's table assumes sharp corners, and/or gets its safety factor
from an assumption of rounded edges. It's usually not much of a
problem, but the higher dielectric constant (rel. to air) of the
ceramic or other insulation in a capacitor can create locally higher
fields that cause trouble.

Cheers,
Tom

(Great quote about hammers, Mike!)

Yep - the hammer quote was great! (finally, a sensible use for
computers hi hi)

But lets look at this a bit laterally - I cant argue the figures given,
just a lowly tech. BUT consider this - you should approach it from the
viewpoint of what variable capacitors you can get hold of!

What power you are running, and thus voltage etc, is the deciding
factor - in the case of 1 (or 2 even) 6146's, its wont need all that
huge a capacitor - most surplus transmitting caps would do it no
trubble. And the traditional amateur method is, after all:-

Do the calculations, and then build it with what you can lay your hands
on !

73 de VK3BFA Andrew