In article ,
Tim Shoppa wrote:

In the real world, most non-miniature variable capacitors were made
with spacers and plates that weren't soldered all together, and
generally didn't use star washers or anything except at the connecting
posts. But these are generally either all-brass, all-aluminum, all-
steel, etc., and not a mismash of different materials. You don't
explicitly say so, Dave, but are you implying that it's the dissimilar
materials that are the problem, or that it's the use of aluminum?

It's a combination of both, I think.

A cap of this sort would work quite nicely indeed if it were made out
of aluminum, and welded together. I believe that such capacitors are
available even today (perhaps on special order). Aluminum's
conductivity is certainly high enough, and the thin layer of aluminum
oxide on the surface would have no effect on the cap's usability.

The biggest problem with a hybrid aluminum/brass capacitor is that
it's hard to get a reliable, low-resistance bond between the two
metals. It's certainly not impossible - there are some fluxes and
solders which can bond to aluminum - but it's not necessarily easy for
the hobbyist to make this work reliably.

Non-welded aluminum air-variable capacitors seem to work find in most
applications... but in most applications, the currents flowing through
the cap are relatively low, the circuit impedances are relatively
high, and thus the resistive losses in the plate/spacer connections
are negligible. In magloop transmitting antennas, the radiation
resistance is small (often just a fraction of an ohm) and the
resistive losses in the tuning capacitor can significantly effect the
efficiency of the antenna.

My
personal taste would be to make it out of all-brass.

After the 50's, for the miniature variable capacitors, they seem to be
made out of material that has been silver-soldered together without
any explicit spacer components. (Maybe brazed or spot-welded, in some
cases.) This seems to be either an improvement made for VHF/UHF work
(remember when 50MHz was UHF?) or an admission that using spacers
doesn't make economic sense when components are so tiny.

The highest-quality seem to use silver-plated brass, soldered or
welded together. This would assure a good electrical connection, and
I believe that this combination of materials and methods also helps
reduce the tendency of the capacitor to drift in value as a result of
temperature changes, or exhibit microphonic effects. People who build
low-drift VFOs seem to favor this style of air-variable capacitor.

--
Dave Platt AE6EO
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