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@sdellington.us
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