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Re: [TowerTalk] hard drawn copper

Subject: Re: [TowerTalk] hard drawn copper
From: K9MA <>
Date: Tue, 10 Dec 2019 20:16:10 -0600
List-post: <>
Long ago, I brought some approximately AWG 14 hard drawn copper back from Finland. The 40 meter full wave I made from it stayed up for nearly 30 years. So did the 80 meter dipole, of the same wire. I think the full wave broke once many years ago, and I spliced it. I finally replaced them with AWG 18 copperweld last summer, when I replaced the tower. Maybe there was something special about that Finnish copper.

That fatigue limit is why all my bicycles have steel forks.

Scott K9MA

On 12/10/2019 19:11, jimlux wrote:
One other complexity is fatigue failure. traditionally, Steel is taken as having a fatigue limit, below which it can withstand an infinite number of cycles.  Copper and Aluminum do not have such a limit. The more cycles, the lower level of stress for failure.

A wire antenna "blowin in the wind" can get millions of cycles pretty easily.

f = So * V/d

So = Strouhal number 0.185 for metric units
V = 5 m/sec (11 mi/hr)
d = 2mm (12 AWG)

f = 0.185 * 5/0.002 = 460 Hz

Time for million stress cycles is 1e6/(460*2) (because the axial load is at a max twice per cycle of the sinewave) = pretty close to 1000 seconds - 20 minutes.

There's an even worse situation, when the natural resonance of the wire happens to align with the aeolian vibration - the Q is pretty high (internal damping of a wire is about 0.25% - a Q of 200), so the loads can be dramatically increased.

l (loop length) = 1/(2*f) *sqrt(T*g/w)

loop length is "half a wavelength" of the vibration mode (the distance between "nodes") (just like a resonant dipole)
T is the tension in  Newtons
g is 9.8 m/sec^2 (accel due to gravity)
w is the conductor weight per unit length (kg/m)

Taking our AWG 12 copper wire..19.76 lb/1000 ft = 9 kg/328 meters =0.027 kg/meter

Let's say we've got 50 lbs (225 N) tension.

So, l = 1/(2*460) * sqrt( 225 * 9.8 / 0.027)
 = 0.31 meters (1 foot).

yeah, for a 20 or 40 meter dipole that's going to be a pretty high order mode, so the deflection won't be all that big.   But remember that the frequency is proportional to wind speed.  So if the wind is 1 m/s (2 mi/hr), the frequency is about 100 Hz, and now the loop length is more like 1.5 meters.

So, it's those gentle afternoon zephyrs that will probably afflict your antenna more than the howling gale.

And I assume that as a responsible ham, everyone will follow the NEC to the letter, do a complete aeolian vibration analysis, calculate the loads, test coupons of your antenna wire to destruction, and then, confirm all the calculations with precision laser measurements of the span during all wind conditions. <grin>

Oh yeah, and I was talking to someone a while ago who claimed that there really isn't a fatigue limit for steel, just the slope of the curve is a lot less than for other metals. So even with copper clad steel, you're still ultimately doomed.

Perhaps single crystal fibers of fused silica plated with silver would be best.

Scott  K9MA


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