Conrad, and all,
keep in mind that the voltage rating of the capacitor is just one part
of the whole thing. The other is its current rating. Once you have found
out what's the highest RF voltage the capacitor will ever see, simply
use that value in Ohm's Law, along with the reactance your capacitor has
at the operating frequency, to get the current flowing to it. Then look
for a capacitor that can handle both the voltage and the current, with
some safety headroom.
You will quickly find that for most capacitors no current rating is
given. Usually these are ABSOLUTELY unsuitable for RF applications at
anything close to their rated voltage! In these applications you need
"transmit-type" capacitors, which are designed to handle high current.
I have the same problem right now, only that I have it at least 18
times. I'm looking for capacitors suitable for a set of legal-limit low
pass filters. That requires capacitors of roughly 100 to 2000pF, each of
them able to withstand AT LEAST 500V and 10A. Preferably some more, to
have a better safety factor for high SWR situations. Can anybody point
me to good sources of such capacitors? Everything I have found is either
unsuitable, or unspecified and thus risky to use, or extremely expensive.
There are lots of hams out there trying their luck and using capacitors
that lack a current rating. And also there are lots of reports out there
of these capacitors blowing up in use... Not all do blow up, but I think
it's a good idea to use capacitors rated to handle the job, rather than
run risks. Specially with solid state, a blown capacitor can blow up a
$200 LDMOSFET.
Manfred
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