[TowerTalk] Wooden Poles;Vibrations, Vortex Shedding

[Pat Barthelow]

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:
http://www.tms.org/pubs/journals/JOM/0205/Byko-0205.html
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.
http://www3.imperial.ac.uk/portal/pls/portallive/docs/1/18395703.PNG
 I think there were also components in the very low frequency range  (1-3 hz), swaying, that I felt in response to stong gusts.
See
http://www.n6ij.org/n6ij_pro67.htm
http://www.n6ij.org/images/n6ijsunlp.jpg
http://www.n6ij.org/images/new_0803/view_from_twr_site_2.jpg
http://www.n6ij.org/images/n6ij5_clr2.JPG

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

All the Best, 73,
Pat Barthelow aa6eg at hotmail.com
http://www.jamesburgdish.org
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Jamesburg Earth Station Moon Bounce Team