At 01:38 PM 10/11/2005, Eric Scace K3NA wrote:
>Barry --
>
> You are looking at a very challenging mechanical engineering situation.
>
> As others have pointed out, a sag limit of 50 ft will require
>substantial tension in the line.
>
> Any post-type anchor must be back-guyed. Two back-guys at different
>angles (away and the to 'front' and 'rear' of the wire span) would be
>wise, since wind will introduce side loads on the wire which much be
>restrained by the anchors. You will want the wire to fail before the
>anchors pull out.
>
> You will also need to consider additional loads:
> -- wind load: not necessarily trivial on long runs. #10 AWG is
>about 0.1 in diameter. For 800 ft of wire this represents about 6.7 sq
>ft of flat projected surface area. As a cylindrical object this can be
>reduced by a third to 4.5 sq ft.
Small wires at the Reynolds numbers characteristic of this kind of
application have a higher Cd.. probably 1.2 is a safe number. (This is a
classic problem in airplane design.. streamlined struts, although bigger,
have lower drag than wires)
(10 m/sec = 22 mi/hr, 0.0025m diameter = 0.1 inch = AWG10)
For example, Re=UD/nu = 10 m/sec * 0.0025m/1.51E-5m^2/sec = 1655 (which is
a pretty low Reynolds number)
Table 10-18 in Blevins' "Applied Fluid Dynamics Handbook" (a truly useful
handbook for this kind of thing, by the way) gives
for a smooth cylinder, Cd = 1.4 for Re=100, 1.0 for Re=1000, and 1.1 for
Re=10,000.
This means that a very small wire (small Re) might have more drag than a
somewhat bigger wire (bigger Re) For what it's worth, the lowest Cd is at
Re around 500,000 to 1E6. Most of the examples in the book for drag on
cables uses Cd=1.2 and Re in the 40-50,000 range.
For a stranded steel cable, 1000<Re<10,000, Cd=1.5 (with or without a jacket)
> For a 60 lb/sq ft wind load, an
>additional 270 lb will be added to the tension. Using #10 Copperweld
>(breaking strength 1200 lb) and a 10% pre-tension (120 lb, which will
>not meet your sag goal), you are now up to 390 lb. You are now above
>the safe working limit of 340 lb (see below).
> -- ice load: this is very serious, even for small loads.
I'd say, especially for small loads, because the ice is a bigger fraction
of the overall size. 1/8" of ice on a 1" diameter member is not nearly the
problem that 1/8" of ice on a 1/10" diameter member.
For guys, the
>"Handbook of Rigging" says the minimum safety factor is 3.5; i.e., that
>1200 lb breaking strength for #10 AWG Copperweld is around 340 lb safe
>working load when you've added up all the loads.
You're still going to be shafted by the strength of materials problem.
Double the size of the wire (making it 4 times stronger) and you've
increased the weight by a factor of 4, so the stress on the wire (in a
pounds per square inch of cross section sense) is exactly the same.
Larger wire helps with wind and ice loads though... Wind loads go up
roughly as the diameter, while strength goes as the square. Ice likewise.
If you want long spans with little sag, you need to find a material that is
very strong for the weight (someone down the hall suggested Beryllium, but
I think that would be impractical.)
Titanium, though, might be something to think about. It's available as
wire and fairly inexpensive. It's corrosion resistant, and about the same
strength as steel, but half the density. I don't know if the high strength
Ti alloys are the same as they make wire out of, nor if they are available
plated. Ti isn't all that great a conductor, so you'd really want
something like copper or silver clad titanium wire.
As long as you're thinking exotic, why not silver plated.. silver oxide is
a good conductor, and you're only looking at a thin layer anyway.. what's
skin depth in silver at 1.8 MHz.. something like 25 microns or 1 mil, and
if you're going to use this beast on 80m or 40, it's even thinner.
Be the first on your block or call region, or heck, the world..
Just think... during that QSO:
THE ANTENNA HERE IS A SILVER PLATED TITANIUM LONG WIRE.
? SAY AGAIN ALL?
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