Re: Rules-of-thumb for RF variable cap plate spacing?
- From: "Andrew VK3BFA" <ablight@xxxxxxxxxxxxxxxx>
- Date: 7 Aug 2005 07:04:08 -0700
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
.
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