On 10/13/17 11:24 AM, Howard Hoyt wrote:
Hi Bob,
Two data points:
1) When I worked at a radio astronomy lab as a college student the
engineers there said silver plating the machined copper cavities ruined
their performance which I didn't understand at the time.
2) The broadcast equipment I maintain uses aluminum straps and tank
components in the plate cavities and running at quite high powers the
straps themselves seem to run cool, except for where they are tied to
the anodes of the 4CXxxxxx tubes. I believe they are intended to help
pull heat away from the anode seal in that usage.
If that study is to be believed, perhaps the silver-plating thing in ham
equipment is a vestige of earlier manufacturing beliefs and techniques,
plus corroded copper is difficult to clean and looks like hell? Based
on the study, lacquered polished copper or over-sized aluminum would
appear to be the winner for coil construction.
One advantage of silver or gold plating over copper is that copper oxide
(and chloride, and sulfide, and ..) are all semiconductive which is
potentially lossy (depending on the thickness). Silver oxide,
chloride, and sulfide are either insulators or conductive. Gold is
pretty inert.
That said, if you plate silver or gold over copper, without a nickel
flash coat, it will eventually form an alloy as the atoms diffuse
between the layers. And for anything RF, nickel is bad, because it's
magnetic.
There are some more exotic techniques used to plate silver/gold over
copper (or aluminum) that doesn't have that problem, but i don't recall
the details.
For spaceflight stuff, we do gold over nickel over aluminum - it has the
right thermal radiation properties (shiny metal), and is reliably
electrically conductive.
Copper and varnish would be good
Aluminum and some sort of conductive chemical conversion coating
(iridite, etc.) would probably be good too - that way when you clamp to
it, it actually makes contact.
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