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[TowerTalk] Wooden Poles;Vibrations, Vortex Shedding

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Subject: [TowerTalk] Wooden Poles;Vibrations, Vortex Shedding
From: Pat Barthelow <>
Date: Fri, 5 Oct 2007 08:31:12 -0700
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Hi John,

Thanks for your contribution about your experience of antenna tubing failures 
on wooden pole supported antennas.  I recall now that you mentioned it, that I 
often felt fairly strong vibrations while aloft on the wooden pole farm at N6IJ.

There is a great case history of a vortex shedding oscillation-structural 
failure at an amusement park, at:
Look for "Thrill Ride Support Snaps..."

On the old, MARS standard design, guyed, tri-legged wood pole supported, 
platform for the large Log Periodic, there definitely was a rocking movement, 
that I could feel when on the platform when the wind was blowing.  Probably 
around 2-3 hz.

The N6IJ antenna/pole farm, has a bunch of  really solid, and free standing 
poles, most about 70 ft, and buried in sand, 9 ft deep.  They are on an open 
scrub brush covered hillside with frequent, steady breezes off the Monterey 
Bay, of 10-30 mph at height.  Whenever there was a fair breeze blowing  (say 
maybe 10-15 kts) you could  definitely feel fairly strong vibrations, perhaps 
in the frequency range of up to 100 hz, probably due to turbulence and vortex 
shedding of the wind around the pole.
 I think there were also components in the very low frequency range  (1-3 hz), 
swaying, that I felt in response to stong gusts.

Maybe poles supporting antennas, could benefit from some kind of aero damper 
that would reduce vibration.

All the Best, 73,
Pat Barthelow
Jamesburg Earth Station Moon Bounce Team

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