On Sat, 07 Oct 2006 14:20:38 -0400, John Ferrell
wrote:

I have been disapointed in the mechanical strength of the Aluminum
electric fence wire.

Aluminium is not very good material for fence wire and not usually a
substitute for steel in general fencing as it lacks the strength of
steel.

There are fence wires made from a steel core (typically high tensile)
and an aluminium (or aluminium / ~5% zinc) coating, sometimes with a
polymer coating over the top. These products are appearing as the new
"longlife galvanised" fence wires. Commonly the aluminium thickness is
around 30 microns, way less than skin depth at low HF, so they can be
expected to perform about as well as the high tensile steel core.

There are other products with a 200 micron cladding of 60%
conductivity aluminium over a high tensile core, and they look a good
prospect for antenna wire, 80% RF conductivity and 10000% strength
compared to the same diameter HDC. For example Gallagher XL 2.7mm
diameter wire (200 micron aluminium cladding) should have the same
loss as 2.3mm dia HDC, but over 10 times the Gross Breaking Strength.

To determine their likely loss as antenna wires, you need to know the
coating thickness.

Owen
--

Either you didn't read the remainder of what I wrote, or I failed to
explain it clearly.

I was speaking of antennas of a constant length in terms of wavelength
as frequency is changed, for example half wavelength dipoles.

If you cut the frequency in half, the skin depth increases by a factor
of the square root of two, so (assuming a conductor at least several
skin depths in radius) the resistivity decreases by the square root of
two. But to maintain a constant antenna length in terms of wavelength,
the wire length doubles. So the total wire resistance at the lower
frequency is greater by a factor of the square root of two. In other
words, if you make two half wavelength dipoles out of the same diameter
and kind of wire, and cut one for 1 MHz and the other for 2 MHz, the 1
MHz dipole will have about 1.4 times the resistance of the 2 MHz one.
That's why you're more likely to see the loss of steel wire in lower
frequency antennas.

Roy Lewallen, W7EL

Richard Harrison wrote:
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

Roy, W7EL wrote:
"Either you didn`t read the remainder of what I wrote, or I failed to
explain it clearly."

The fault was mine, not Roy`s. It is true that if you scale an antenna
for half the frequency by doubling its length without increasing
cross-section of the wire, its resistance increases. Resistance is
rho(l/a) where rho is the resistivity, l is the length of the wire, and
a is the area of the wire`s cross-section.

Roy noted that lowering frequency by half means a wire twice as long
which tends to double the wire`s resistance but skin effect increases
penetration of the wire at the lower frequency. This reduces resistance
by 1/sq.Rt.2. The same antenna using twice the length of the same wire
but at half the frequency thus will have 1.414 times the effective
resistance of the double frequency antenna.

Best regards, Richard Harrison, KB5WZI

Electric fence wire is aluminum, comes on 1/4 mile, 1/2 mile and mile rolls,
works great for ground radials but is to soft for antennas, IMHO.

Best wire I have found for antennas is what the local phone co-op calls
"field wire". They lay it down during the winter when they can not trench in
a new copper line. In the spring they roll it up and toss it away after they
trench in the new line. This stuff is 7 strands of steel, covered with a UV
protective outer plastic shell. Stuff will last for years, stretch and
return to shape, and if you bundle enough together you can pull a car out of
the ditch in a pinch. Have had it up for years as a 40 and 80 meter dipole.
Hard as the dickens to work with, almost impossible to solder, but, it makes
a dipole even northeast Montana winters can not break, something to be said
for that.

Just my two cents worth.

Sam