To: |
towertalk@contesting.com |
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Subject: |
[TowerTalk] hard drawn copper |

From: |
jimlux <jimlux@earthlink.net> |

Date: |
Tue, 10 Dec 2019 17:11:50 -0800 |

List-post: |
<mailto:towertalk@contesting.com> |

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.
On 12/10/19 4:46 PM, jimlux wrote: On 12/10/19 2:41 PM, Bob Shohet, KQ2M wrote:Hi Jim, ”A major advantage of using stretched wire is that un-stretched copper will stretch under tension over years. Before changing to stretched wire,” _______________________________________________ _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk |

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