Larry Benko wrote:
Since this topic has been recently discussed, I was wondering how well
EZNEC (ver. 4+ here) would model a wire antenna made with stainless
steel wire. The "Wire Loss" table in EZNEC allows a User Defined
Resistivity and Relative Permeability. Would the fact that SS wire is
usually comprised of a very large number of strands has any appreciable
effect on the loss? Assuming that EZNEC can do the modeling correctly,
what would be approximate values to use for the resistivity and
relative permeability? I did a quick measurement in a short piece of
SS stranded wire measured a DC resistance (with a 4 wire ohmmeter) of
~50 times that of copper.

Stranded wire will increase the RF resistance compared to the equivalent
cross-section of solid - but a DC resistance measurement will not show
this.

In both cases the skin effect forces the RF current to flow on the
surface. The solid wire has a continuous surface, but on stranded wire
the current path has to hop from strand to strand. Because the sideways
contact pressure between strands on the surface is not very great, this
increases the resistance per unit length.

Bare stranded copper is very vulnerable to this effect, because copper
corrodes readily in the atmosphere and even a thin film of corrosion
will force the strands apart. You'd be very unlikely to notice the
gradual worsening of performance in something like an HF antenna... but
it's still there.

Stainless steel is rather an opposite case: the RF resistance of most
grades of SS is so high that stranding may not make much extra
difference, and of course it's also much less easily corroded than
copper.

What kind of wire you choose depends on your priorities. For example,
for marine use the absolute priority is that the antenna is available at
all times, so they use thick stranded phosphor-bronze for their HF
"wires". It isn't the best RF material, but it's so strong, your 80m and
40m inverted-Vs make very good tower guys.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek