I am sitting in the middle of our big midwest ice storm wondering if my new
Steppir is going to succumb to the same fate as my Bencher Skyhawk during last
year's ice storm. I just re-read Dave Leeson's article on strengthning the
402-CD and wonder if the current practice of using internal doublers butted
together to strengthen booms is a sound practice. I am obviously not an
engineer, but it seems to me that where two internal doublers butt together
the effective strength of the joint is that of the single overlying boom
material. That would seem to be born out by Dave Leeson's article where all
butted joints are overlapped with two sections of tubing.
Example - the Skyhawk's boom is composed of 4 - six foot, 2 inch sections
pinned together with underlying doublers which are 1 7/8ths diameter. The
middle section is double wall and the next two single wall. During last
year's ice storm the boom failed three feet out from the mast - right in the
middle of the first boom section on the director side, RIGHT AT THE POINT
WHERE THE INTERNAL DOUBLERS BUTT TOGETHER.
Since the middle doubler is double wall the claim is that the boom tapers from
triple wall, to double wall,to single wall thicknesss at the outboard three
feet of the boom. I think the boom is effectively double wall at the center
section ( the double wall thickness of the center doubler), triple wall for
the next three feet, single wall at the point of the first internal butt,
double wall for the length of the outboard doublers and single boom thickness
at the tips.
The new Steppir (3 el) only has a 1 3/4" boom pinned together with internal 1
5/8" splices. Wouldn't the effective strength of the boom only be that of the
1 5/8" splices where the boom sections butt together? I know the 3 el is a
real short boom, but right now it has a real nifty droop!
I think strength is proportionate to tubing diameter (at least that is what
the bicycling community claims) so if for cost reasons single wall butt joints
can't be avoided, wouldn't it be better to have larger diameter splices on the
outside of the boom, rather than prettier internal ones?
I am not sure how boom to mast plates affect this. With both antennas the
boom is butted together in the center and might be effectively doubled by the
u-bolts of boom to mast.
If nothing else, I am convinced we ought to employ boom trusses when we live
in ice and wind country.
By the way I posted earlier on element rotation and took the collective advice
to pin the boom to the boom to mast plate. So far the antennas elements are
all still horizontal despite some pretty hefty (cannot be symetric) loads.
Thanks for the good advice!!
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