> The original was less than 1/8" square. I was able to obtain a 47pf cap
which worked, but was about 10pf to high. Unfortunately, after soldering and
unsoldering it a couple times I killed it.
Mirage or other amplifier manufacturers have small multilayer caps that have
very high Q and very high self-resonant frequencies.
> I tried a 500v rated silver mica cap with the leads cut very short and it
worked, but heated up quickly.
Silver micas I have tested, even with very short leads, have Q's in the
several hundreds or very low thousands. Self-resonant frequencies are also
low (compared to real multilayer surface mount parts), so the capacitance
value of the cap is almost certainly not the effective value at upper VHF.
Single layer ceramics are even worse.
Multilayer ceramic chips or special multilayer micas are much better
components, having less ESR and significantly higher series resonant
frequencies. You don't always need a special cap, but you sure do need a
similar style capacitor to original unless you are willing to tweak other
components a bit.
You might try looking on Internet for engineering evaluation kits of proper
voltage rated surface mount capacitors. Murata and others offer such kits.
You'll find a much wider selection of multilayer ceramic chip capacitors
than the special metal micas for VHF/UHF work. Unlike common disc ceramics,
multilayer surface mount ceramics (with proper voltage and current ratings
for transmitting applications) work very well way up into upper UHF.
73 Tom
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