Roy Lewallen wrote:
Steel has a resistivity many times that of copper. It varies a lot with
the alloy, so it's not possible to put a single number on it. But the
real problem is that steel is ferromagnetic -- in other words, it has a
high permeability.

At radio frequencies, current flows in a thin layer near the surface of
the conductor. (It actually continues clear to the center of the
conductor, but the density decreases very rapidly with depth, so it's
essentially zero anywhere except very near the surface.) This
concentration of the current has the same result as passing the current
through a wire of much smaller cross-sectional area: it greatly
increases the resistance of the path carrying the current.

The problem is that the thickness of this layer (more technically, the
rate at which the current density decays with depth) is determined by,
among other things, the permeability of the material -- the higher the
permeability, the shallower the layer. So the higher the permeability,
the higher the resistance. The permeability of steel is probably even
more variable than resistivity, but I'd be surprised if you ever found
any in common use with permeability under 100. Or if you found some with
permeability of several thousand. Since the relationship between the
depth of current flow and permeability is a square root, this means RF
resistance of 10 to 100 or so times that of copper, as well as the
higher resistance due to the higher DC material resistivity.

If the antenna has a large enough surface area, even steel is fine. A
common example is an FM mobile whip, which has insignificant loss, or a
tower operated as a vertical. But because of the way the current depth
and antenna size change with frequency, the loss with a given wire size
gets greater and greater as you go lower in frequency, assuming the
antenna stays the same size in terms of wavelength. So while moderate
diameter steel wire might have insignificant loss on the higher
frequency HF bands, that same wire might have substantial loss at the
lower end of the HF range.

Most hams can measure SWR, but almost none can quantitatively measure
the strength of the signal their antennas radiate. And most run way more
power than needed to communicate, so can easily lose quite a few dB
without a major effect on communications. Consequently, the wider
bandwidth gained due to loss in steel wire is considered an asset, while
the few dB loss is probably not noticed. (Although hams spend a
staggering amount of money trying to buy a few extra dB of gain. Go
figure.) In fact, I recall an article some years ago -- in QST if I'm
not mistaken -- featuring a wide-band 80 meter antenna whose secret was
just that -- loss from using steel wire.

If you try it, you might just like it!

By the way, copper wire is easily obtained and not that expensive,
either, should you choose to go for a stronger signal rather than wider
bandwidth.

Brings up a question in my mind which is related to the points you've
made about conductivity vs. skin depth. As it then relates to bare vs.
insulated copper wire for HF work.
Bare copper wire out in the WX will oxidize which adds a layer of
copper oxide on the O.D. of the wire over time. What is the effect of
this layer on skin resistance losses at HF frequencies in practical
terms? Somewhere along the line I picked up the notion that copper
oxide is a pretty lousy conductor and the problem can be resolved by
using insulated wire for wire antennas . . comments??


Roy Lewallen, W7EL

Brian w3rv