Regarding failure points: I've seen photos of two fallen US Tower retractable
towers and both of them failed at the very bottom. The problem with designing
a retractable tower such that it will fail at the top is that of cost. The
bottom section would have to be super massive and I wonder if the tower will be
price competitive.
Regarding tower wind loading: US Tower specifies wind loading in terms of round
members although they don't always make that clear. On my documents they
didn't make it clear. OTOH antenna manufacturers like to expresstheir wind
loading numbers in terms of flat members. Therefore you need to divide the
antenna wind loading number by 0.6 and compare that number to the published
tower wind loading figure. Also, the tower wind loading includes all
accessories such as mast, rotor, coax cable, etc. A mast of 2" OD and 15'
length has a wind loading of 2.5 sq. ft. This is often overlooked by hams.
Regarding wind survivability of retractable towers: my local county building
dept. requires the tower to withstand a 3-second gust of 122mph. The only way
a retractable tower could survive that high of a wind speed is when it's fully
retracted. My tower (US Tower TX-472) will survive a wind speed of 122mph with
a 15.6 sq. ft antenna wind loading when fully retracted. Since my tower is
motorized I leave it fully extended only when I'm on the radio. When I'm done
operating it I leave it half-way or fully retracted. In a nut shell a
retractable tower is very safe as long as the operator uses common sense. If
one doesn't have a motorized tower then he needs to go by the wind speed rating
when the tower is fully extended. By where I live a non-motorized crank up
wouldn't be practical nor safe.
Best Regards,
Jonathan Woo, W6GX
(970) 646-1711
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