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@hotmail.com
http://www.jamesburgdish.org
Subscribe: http://bambi.net/jamesburg.html
Jamesburg Earth Station Moon Bounce Team
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