I have calculated which surface provides the least wind load for each of the
multiple beams on my tower... But, I do not expect the tower to blow over or
to fail in any manner, this side of a tornado, in which case it will not
matter in the slightest where the beams were pointed .... so, knowing the
surface area is interesting, but not particularily useful in less than
ultimate storm conditions... Those who expect a hurricane will point the beam
based on their calculations of least surface area and best mechanical
position... Keeping the beam pointed during the storm passage is another
story....
I am more interested in what angle to the wind provides the least torquing
and shaking.... I found this by observing my beams on windy days, and then
turning them a bit and watching some more until I found the angle to the
prevailing wind that provided the steadiest ride for each beam.... For my
installation it appears that the mounting point on the tower and how far each
is above the buildings/trees determines the wind response more than the
projected area of the elements/boom...They seem to to do best with combined
element and boom sideloading... This probably maximizes the load on the
tower, but as I noted above, this is not going to result in failure so it is
of no concern....
The yagi on top of the tower at 150' is steadiest at 30 to 40 deg off the
wind... The yagi at 120' on a ring mount, is best at 10 to 20 deg off the
wind... The yagi side mounted at 90' seems best directly into the wind...
This is only part of the story... Wind seldom blows from the same direction
all day in michigan ( the coasts may be a different story) with a 90 degree
change from the time I leave in the morning until I get home 14 hours later,
being the norm.... Also, windy days in michigan are gusty, with the
instantaneous wind varying 30 or 40 degs, as often as they are steady from
one direction.... So, in rough weather, I point the beams based on the
average expected wind direction for the day...
I recommend ON4UN's book on antennas... He discusses this topic and offers
methods to calculate ultimate wind survival for your antennas...
Cheers ... Denny k8do@aol.com
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