Folks

So I'm reading the 2006 ARRL Handbook page 22.6, There is a single
line stating "Steel wire is a poor conductor at RF; Avoid it." Any
idea why? Or is this just one of those physical properties?

So how much poorer than copper? Steel clothesline is easily obtained
and not that expensive. Admittedly though I haven't done much
research on copper or the other type of wires the Handbook mentions.

Tony

On Fri, 06 Oct 2006 05:21:00 GMT, Tony VE6MVP
wrote:

Folks

So I'm reading the 2006 ARRL Handbook page 22.6, There is a single
line stating "Steel wire is a poor conductor at RF; Avoid it." Any
idea why? Or is this just one of those physical properties?

So how much poorer than copper? Steel clothesline is easily obtained
and not that expensive. Admittedly though I haven't done much
research on copper or the other type of wires the Handbook mentions.

Tony

Hi Tony,

Go ahead and use steel clothesline, it will work fine.

73's
Richard Clark, KB7QHC

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.

Roy Lewallen, W7EL

Tony VE6MVP wrote:
Folks

So I'm reading the 2006 ARRL Handbook page 22.6, There is a single
line stating "Steel wire is a poor conductor at RF; Avoid it." Any
idea why? Or is this just one of those physical properties?

So how much poorer than copper? Steel clothesline is easily obtained
and not that expensive. Admittedly though I haven't done much
research on copper or the other type of wires the Handbook mentions.

Tony

Use copper clad steal wire. Copper for the RF performance, and the
steal 'core' for strenth. Larger diam. usally yelds wider bandwidth.

Roy,
As my cobwebby brain remembers, for conductive materials such as
aluminum through gold, the rough rule of thumb is that at 10 megacycles
the skin depth is .01MM (01 is 10 backwards, only reason I remember)
So 1mm is 0.0394" therefore a skin depth of 0.1mm is 0.00394", call it
4/1000 of an inch for round numbers... So, the other rough rule of
thumb I have always used in my wasted career in industrial electronics
is to have the conductive plating 5 times the skin depth... So, 0.020"
would suffice for 10 megacycles...
Now, that begs the question for steel, or zinc plated steel... Anyone
interested can google up answers with a bit of personal effort...

What has always intrigued me though, is the concept that a moving
charge at RF frequences, spreads over the surface and penetrates only
0.020" the majority of the charge ( @10 mc ) , while still having lines
of flux penetrating radially to the electrical center of the metal
object... Yet, by the same token, if the metal shape is a hollow tube,
no signal will be detected upon the inner skin of the tube...

denny

I think you dropped a decimal point, there, Denny. How did you get
from 0.01mm skin depth at 10MHz to talking about 0.1mm? If you stay at
0.01mm, it would be about 0.4 mils. The rule of thumb I remember is
2.6 mils at 1MHz, so it would be 0.8 mils at 10MHz. That's for
_copper_ and as Roy wrote, steel will be much less.

Another rule of thumb: the RF resistance of copper wire is about 1
milliohm/foot * sqrt(freq. in MHz) / diameter in inches. For 14AWG
wire at 4MHz, that's about 31 milliohms per foot, which is pretty much
inconsequential for a 75M half-wave dipole with about 75 ohms feedpoint
radiation resistance.

The resistivity of nonmagnetic stainless steel is roughly 50 times that
of copper, so the loss would be about seven times as great at RF,
assuming that the permeability really is low at RF; that wouldn't be
bad. But high permeability would not be good, especially in a small
diameter wire..

Moderately wide bandwidth, high strength, low loss dipole: a center
support steel cable, surrounded by 4 or so small copper conductors in a
"cage" spaced out from the center support to make a conductor perhaps
1/200 of a wavelength effective diameter.

Cheers,
Tom


Denny wrote:
Roy,
As my cobwebby brain remembers, for conductive materials such as
aluminum through gold, the rough rule of thumb is that at 10 megacycles
the skin depth is .01MM (01 is 10 backwards, only reason I remember)
So 1mm is 0.0394" therefore a skin depth of 0.1mm is 0.00394", call it
4/1000 of an inch for round numbers... So, the other rough rule of
thumb I have always used in my wasted career in industrial electronics
is to have the conductive plating 5 times the skin depth... So, 0.020"
would suffice for 10 megacycles...
Now, that begs the question for steel, or zinc plated steel... Anyone
interested can google up answers with a bit of personal effort...

What has always intrigued me though, is the concept that a moving
charge at RF frequences, spreads over the surface and penetrates only
0.020" the majority of the charge ( @10 mc ) , while still having lines
of flux penetrating radially to the electrical center of the metal
object... Yet, by the same token, if the metal shape is a hollow tube,
no signal will be detected upon the inner skin of the tube...

denny

On 6 Oct 2006 01:15:58 -0700, "Will" wrote:

Use copper clad steal wire. Copper for the RF performance, and the
steal 'core' for strenth.

Yes, the ARRL handbook mentioned that.

Tony

Roy, W7EL wrote:
"---the loss with a given wire size gets greater as you go lower in
frequency,---.

Effective resistance to r.f, is approximately proportional to the square
root of the frequency due to "skin effect" as Roy mentioned in
describing how current penetrates the conductor less completelty due to
inductance deeper in the wire. So, loss is greater at higher frequency
due to reduced effective cross-section in the wire. Conversely, the loss
with a given wire size gets lower as you go down in frequency.

Best regards, Richard Harrison, KB5WZI

On Fri, 06 Oct 2006 05:21:00 GMT, Tony VE6MVP
wrote:

Folks

So I'm reading the 2006 ARRL Handbook page 22.6, There is a single
line stating "Steel wire is a poor conductor at RF; Avoid it." Any
idea why? Or is this just one of those physical properties?

So how much poorer than copper? Steel clothesline is easily obtained
and not that expensive. Admittedly though I haven't done much
research on copper or the other type of wires the Handbook mentions.


I am guessing that the "steel clothesline" to which you refer is
probably actually stranded (7x1?) heavy galvanised soft steel wire.

The galvanising is zinc or zinc/aluminium alloy and its thickness has
bearing on the answer for a specific frequency.

The stranding also has adverse effect on the effective RF resistance,
though not as predictable as the zinc coating.

Though it works, there are a number of mechanisms that increase the
loss, and the extent of some of them are quite difficult to predict or
to measure (for the average amateur).

The additional loss of steel wire is less important in an antenna
design that is loaded with bulk resistance, eg T2FD. A reason why
small guage stainless steel wire commonly used commercially on these
antennas isn't necessarily unsound. But that application should not
imply that small guage stainless steel is just as suited to a half
wave folded dipole.

Antenna wire would be one of the lowest cost elements of a complete
system, which questions the cost effectiveness of savings.

Owen
--

On Fri, 06 Oct 2006 22:17:10 GMT, Owen Duffy wrote:

Antenna wire would be one of the lowest cost elements of a complete
system, which questions the cost effectiveness of savings.

Sure, but clothesline wire is easily available in this small town.
Copper wire means I'd have to search it out in the nearest big city.

Tony