Dave,
The type of capacitor to be recommended for matching loops is the Philips
386
polypropylene series.
If you are buying new capacitors, this would be a good approach. However,
there are lots of transmitting micas on the surplus market, and since they
are not hot sellers, the prices may be interesting. Many of the
bakelite-cased caps have voltage and current ratings (often at several
frequencies) printed on the case. Most of the older ceramic or glass-cased
micas with aluminum end caps have their ratings displayed as well. Some of
these things are sold as "door stops" at flea markets.
The only caveat is to inspect the capacitor carefully. If there is any sign
of it having overheated, reject it. A good RF impedance bridge can be used
to screen out caps that have changed value, or have a high series resistance
at RF. Most of the disappointments in this regard will be from really older
mica caps, such as the "sulphur micas" from the 1920's and 30's.
And, of course, don't drop one on your foot. It will cause you to have a bad
Farad-day.
John Andrews, W1TAG
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