On Wed, 23 Aug 2006 00:17:25 -0700, Richard Clark
wrote:

On Wed, 23 Aug 2006 03:35:44 GMT, Owen Duffy wrote:

I think "ordinary house wire" may be aluminium, a by product of 110V
utilisation I guess. House wiring here is still principally copper.

Hi Owen,

Aluminium/um house wiring here was but a brief, failed experiment some
30-40 years ago.

Hi Richard,

Thankfully, it didn't come here. I just Googled for "aluminium house
wiring fires" and got 1,080,000 hits, whereas for "copper house wiring
fires" I got 736,000 hits. I guess there may have been an issue.

Subject to more reliable information on the wire's GBS, my initial
calcs are that a span of 40m (half of a half wave dipole on 160m)
would need 3.3% sag (~1.4m) to survive wind at 60m/s

With a Category Four Hurricane roaring outside, I would think you
would be worried about more than wire.

with a safety
factor of 3.5. Yes, of course, the mountings must also survive the
wind, and this analysis assumes not deflection of the mounting points
and no stretch of the wire.

My wire tables offer that 40M of #12 wire would weigh 2.6 pounds in
bare copper. The breaking load would push beyond 100 times that at
337 pounds (40% copper clad is slightly more than twice that).

For a sag of 1.4 meters in 20 meters would be an angle of depression
of 4°. If I take the inverse of the sin( 4°) it would multiply the
weight by 14.3 for a tension of 37 pounds. The wire by itself would

Using that approximation, shouldn't you have taken the weight of a
half span? Then, isn't that weight evenly distributed over the half
span, so you should halve it again?)

A parabolic approximation is better than the triangular approximate,
and it would suggest tension is W*S^2/sag/8 or 0.28*40^2/1.2/8 N or
46.7N (~10.5lbf) which is a small fraction of the GBS of 1350N for
HDC, so yes, the wire is easily able to support itself. Wind forces
are much greater, and wind at 60m/s (highest design speed for
non-cyclonic localities under building standards here) loads the wire
to 386N (which is GBS/SF). The lowest design speed under our standards
is 41m/s, that results in about half the tension.

hardly constitute any jeopardy, but there is still a choke and
transmission line's weight to be added (and I probably missed this by
a factor of two in simply winging the math).

I was considering a span with no concentrated loads (it is a quarter
wave, half of a half wave dipole).

Yes to missing a factor or two. It is too late to be grinding numbers
on your side of the world!


Let it sag 3 meters and the multiplier drops to less than 7.

The only thing that compares on strength and conductivity is
Copperweld, but it is not easily obtained here... I suspect the cost
of freight might double or triple the price of a 100lb pack of 30% #12
wire.

I did look at heavy galvanised wires, but it seems the move has been
to Zinc/Aluminium alloy with an overall synthetic coating, and since
it erodes much slower, the coatings are only 10 to 20 microns... not
thick enough for good conductivity.

At something like 4 dB additional loss, this may matter. Additional
sag would seem to be a very efficient return on investment in
comparison to the additional 1.6 meters proximity to earth (that
wouldn't nearly add 4 dB loss, would it?).

My usual supplier looks like he can't do 3mm HDC economically any
more, hence the search.

Try using a rope runner.

Do you mean a rope as a carrier for the conductor... runs into some
other issues like differential stretch, and huge wind resistance. The
structure may stay in the sky, but the wire might be fractured anyway.

Owen
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