Frank wrote:
Thanks Danny, Interesting paper. It certainly points out problems with
silver plating. I must say I have never attempted to measure the Qs of
inductors with and without plating, but it is something to think about. My
point was more about the effects of copper oxide on inductor Q. It makes me
think that the best way to prevent the build up of oxide is to use some kind
of low-loss insulating material on the surface of the copper.

The problem is that copper oxide isn't necessarily what forms on copper
-- you can have copper chloride, sulfide, oxide, and perhaps other salts
in various combinations and thicknesses depending on the environment.
Likewise, silver will form all those salts.

If a chemical coating is a very poor conductor or a very good conductor,
it won't appreciably degrade the Q. What will degrade the Q is a coating
of sufficient thickness of an intermediate conductivity. I've tried for
years to find data on the various salts of both metals, and found only
very few.

But here's a quote from Chipman (_Theory and Problems of Transmission
Lines_, 1968, p.81 - thanks to Wes, N7WS!):

"For many years it was thought that silver was necessarily the best
plating material, since silver has the highest conductivity of all
metals. However, careful measurements have shown that the corrosion
products on a silver surface in ordinary atmospheres have intermediate
conductivity, while those on a copper surface have very low
conductivity. The result is that high frequency currents in a copper
conductor flow almost entirely in the copper, below the surface
corrosion layers, and the conductor's effective conductivity is that of
the copper. For a silver conductor, on the other hand, an appreciable
fraction of the current flows in the corrosion material of intermediate
conductivity (the corrosion products are generally oxides and sulfides)
and the effective conductivity of the conductor as a whole may be
substantially less than that of silver. If a silver surface is protected
against corrosion, including oxidation, by an extremely thin layer of
plated or evaporated gold or by a low-loss dielectric coating, a silver
plated conductor will have the lowest possible distributed resistance."

Chipman is obviously assuming a pure silver plating. As Danny's posted
paper (http://k6mhe.com/n7ws/Plating.pdf) points out, "silver" platings
are seldom pure silver, and are usually considerably less conductive
than copper to begin with.

So it's been known for at least several decades that silver plating
degrades Q over the long run, for a couple of reasons. But myths die hard.

Incidentally, I've found a couple of errors in the equations in the
Fowler paper Danny posted (and which Wes N7WS kindly sent me years ago),
apparently made during the copying of the original equations from their
source. There seem to be some additional errors which I can't identify.
If anyone knows where I might get a copy of the paper with the original
equations, which is reference 10 (Astbury) in the Fowler paper, I'd be
very appreciative. I've requested a copy of the paper through the local
library, but it's likely to take a very long time to get. Perhaps some
of the U.K. readers can make a suggestion, since the I.E.E. is the
British equivalent of our I.E.E.E. I see from the I.E.E. web site that
they're in the process of getting their older journals on line, but
haven't finished the task. I don't mind at all paying a fee for it.

Roy Lewallen, W7EL