At 02:14 PM 10/10/2005, Barry Merrill wrote:
>Using the nomograph in the ARRL Antenna Book,
>which only goes up to a SPAN of 1000 feet
>(and only after an hour did I see that "SPAN" is
> one HALF the distance between end points),
>I can't find any wire that can survive that SPAN,
>when you've only got 50 feet for sag.
>
>I've got a distance of 2400 feet for the long wire
>between two 70 foot high bluffs on opposite sides
>of a saltwater inlet that is shoaled and un-navigable.
>
>I plan on 100 watts, and I'd like to use small wire to
>make the wire less visable, but using the maximum
>SPAN of 1000 feet with #12 wire, the chart says that
>to keep the sag less than 50 feet, I need 400 pounds
>of tension (as if I could every supply that force!),
>but the recommended #12 tension is only 75 pounds.
>
>using something as large as #12 wire,
>for the maximum table SPAN of 1000 feet,
> (my actual SPAN is 400 meters)
>the chart says that to keep the sag to less
>than 50 feet, I'd have 400 pounds of tension;
>unfortunately, the recommmended tension is
>only 75 pounds (at 10% of breaking strength).
>
>Or, my tension would be 400/750= 53% of breaking
>strength of #12 wire.
>
>So, what can I do? Is there a strong, light wire that
>will work for 800 meters with 50 foot sag?
I assume you're using copper clad steel wire. It's much stronger than
regular old hard drawn copper. My chart shows breaking load of AWG12 hard
drawn copper is 337 lbs, and for 30% conductor copperclad it's 770 lbs.
Don't forget the weight of the wire, 1000 ft of AWG12 copperweld is 18 pounds.
But, you've hit on the real problem... very long spans take either really
strong wire or you have to allow for a lot of sag. The problem is that if
you make the wire bigger in diameter (to increase the strength), the weight
of the wire goes up in exact proportion, so you don't gain anything. The
tension, for a given sag, is exactly proportional to the weight per unit
length, and so is the strength.
So your challenge is to find something stronger than steel, for the weight.
Perhaps an aramid fiber (kevlar) with a metal coating. Tungsten wire is
very, very strong for the size, but I'm not sure of the density (it's quite
dense). In general, piano wire is the strongest steel wire, but I don't
know if it's available copper clad.
If you need the equations, they're here:
http://home.earthlink.net/~w6rmk/math/catenary.htm
There's a link to an excel spreadsheet to try things out, too.
Jim
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