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 firstname.lastname@example.org
Jamesburg Earth Station Moon Bounce Team
TowerTalk mailing list