[TowerTalk] hard drawn copper

[K9MA]

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.

73,
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

k9ma at sdellington.us