"Roy Lewallen" wrote in message
...
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

Very interesting Roy. It makes me feel a lot better about eliminating my
fixation with silver plating inductors. I was aware that there are
different classes of copper oxides, but have only ever seen green and black
oxides. I did find some data on oxide conductivity, but cannot locate it
now. In fact you may have even been the source for this information.

Concerning the Fowler paper; Rudy Severn also published an article in QEX,
for Nov/Dec 2000, where he briefly discusses the math. I believe there are
also errors in his math, but have not looked at it for a year or so.

Frank