I've followed this aluminum mast theme with interest since I also have an
aluminum mast, chosen because my QTH is direct oceanfront on Casey Key Island
here in SW Florida. I selected T6061-T6511, 2" OD, with .375" wall which is
16' long. I have about 3 feet of it into my 9' Glen Martin Roof tower and 13
feet for antennas. I have a Cushcraft 402CD at the top (about 11' mark) and
did have until recently an A4S a few inches above the tower collar. My initial
calculations indicated about 65,000 LB/Sq. In mast stress at 80 mph.. Anything
above that, and I'm not going to be here to worry about the antenna, but rather
worry about my house from somewhere on the mainland.
I've watched this antenna take some pretty violent weather in the past year,
with storm bursts of over 60 mph. It flexed in the wind, but didn't show any
signs of stress. In one particularly violent storm, the 402CD was spun around
and the elements looked like big "U"'s in the wind, but all was fine after the
storm passed.
I'm redoing my tower just now by adding additional stringers to spread the roof
loading (I have a flat roof on my 3 story concrete and steel house which is
itself elevated substantially above sea level). I'm also replacing the A4S
with a TH-7. Now I'm wondering if I should also replace the mast, but I think
the direct sea salt will do the mast in within the first two years. My DSS
antenna is nearly eaten through after only 10 months. Galvanized masts for my
R-7 are severely rusted in severer places after only one year. The aluminum
seems to hold its own after the initial surface oxidation. Antenna patterns
over all this salt water are great but they have very short life expectancies.
What do you think?
Jon Hamlet, W4ZW
Casey Key Island, FL
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