[TowerTalk] Fwd: Tower Permitting Spec's
jimlux at earthlink.net
Tue May 1 08:45:44 EDT 2018
On 5/1/18 12:31 AM, J Chaloupka via TowerTalk wrote:
> Wondering, couldn't you wrap heavy gauge wire in a coarse spiral around the mast, similar to the wire wrapped around the proverbial automobile receiving antenna mast, in an attempt to dampen the vibration? (Aluminum wire on an Aluminum mast)(look at the 2013 Chrysler Town and Country van as example)
You can - and it can be plastic, or rope, or almost anything. What I
don't know off hand is how big that spiral has to be. Obviously,
wrapping a AWG 20 wire isn't going to do it.
One thing to think about, though, is that the vibration may be excited
somewhere else (guys?) and the tower just happens to be the resonator.
The other thing is to make it stiffer (raise the resonant frequency) so
it's not excited, or to make the diameter different in different places.
This is a pretty complicated phenomenon - you don't see aluminum
flagpoles having the problem.
Interestingly, the phenomenon is probably more severe in moderate winds,
rather than high winds (turbulence in the wind inhibits the effect)
Here's the formula:
f = 0.185 * V/D
f in Hz
V in m/s
D in meters
1 mi/hr = 0.48 m/s
1 " = 0.025 m
so a 2" mast in a 10 mi/hr wind (0.05 m, 4.8m/s) would tend to vibrate
at 1.78 Hz.
On the other hand a 1/4" guy wire will be oscillating at 8 times that,
around 14 Hz.
I'd guess that the guy wires are higher Q than the mast.
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