Owen Duffy wrote:
. . .
I do remember Roy discussing the resistivity of rust, but I don't recall
figures... does anyone have any figures for the resistivity of the rust
coating on a rusted steel wire? I guess its permeability is also relevant
to RF resistance.

No, I don't believe I've mentioned rust, although I've commented on
copper and silver salts a couple of times. I wouldn't try to guess what
the resistivity of rust might be, since it would surely vary a great
deal with the amount of hydration.

A highly resistive coating won't add appreciable loss, nor will a highly
conductive one. There's an in-between range which will. But keep in mind
that the skin depth is inversely proportional to the square root of the
conductivity, so a thick coating with twice the DC resistivity will
increase the RF resistivity only by a factor of about 1.4.

The real problem with thinly plated steel wire is that if and when the
coating corrodes off -- or if it's too thin to begin with -- the current
ends up flowing in the steel itself. Steel is terribly lossy stuff at RF
chiefly because of its permeability, not that its conductivity is all
that hot to begin with. Skin depth is inversely proportional to the
square root of permeability, so a steel with permeability of 100 has
1/10 the skin depth it otherwise would, resulting in 10 times the RF
resistance. If you consider half wavelength antennas at various
frequencies all made from the same size wire, you find that the loss
gets worse as frequency decreases. So it's often more of a problem on
the lower frequency bands.

Some stainless steels are magnetic and some aren't. Magnetic ones are
much lossier at RF for the same reason as ordinary steel.

Roy Lewallen, W7EL