Jim Lux wrote in
:

Owen Duffy wrote:
....
thickess so that the RF resistance is similar to copper, but the
strength comes largely from the copper core.

I think you mean the strength comes from the steel core?


Yes, thanks Jim.

FWIW, when they say 30%, that means that the wire has 30% of the
conductivity of the same diameter copper wire, not that it is 30%
copper. You have to look in the mfrs tables to see what the cladding
thickness is.

Yes, IIRC it is rated on its 60Hz resistance as a competitor to HDC
aerial power applications.

Odd that there aren't suppliers in Australia.. Maybe because there
isn't an existing telegraph/telephone cable infrastructure in rural
areas? In the U.S., there were and still are millions of miles of the
stuff installed before carrier telephony, coax, optical fibers, etc
became common. And, it's still used for MV power distribution, much
of which is still above ground.

Yes, pity that. Remember that we had a monopolistic telecomms market
until ~1995. I did scavenge a littel copper clad steel (3.2mm) on one
occasion, but most aerial phone cables were HDC.

My article mentions an aluminium clad HT steel fencewire which looks
interesting for antennas, but it does bring the issue of corrosion
resistance connection to aluminium. Unfortunately, this wire is only
available in 1km rolls.

It turns out that HDC is probably the best available wire in Oz (JERD is
a VK5), but gal fencewire or stainless steel are eminently suited to
lossy antennas like TTFD etc.

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.

Owen

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

The 'best' wire for long antennas is whatever you happen to have
the most of that will support it's own weight. After that, it's more
a matter of what color insulation you think is nice. And after that,
it's whatever you can convince yourself that you can afford. Have
several pieces of different wire size/type? If they will support
their weight, splice the @#$ things! Paint it all pink. Pink works
very well for antennas.
- 'Doc