[TowerTalk] Fwd: Fwd: (80 Meter yagi question) Cu <=> Ag

[jimlux]

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.