1. Aluminum vs. steel
My recollection from a materials engineering course long ago was that
aluminum and steel are different in how they handle
cyclical loads. With aluminum, the yield strength gradually declines as the
material cycles, with no limit to the degradation. If
a piece of aluminum is vibrating, that represents hundreds of cycles per
second. It doesn't take very long to accumulate millions
of cycles... and eventually the material will fail with a load that it would
have withstood when new.
Steel, on the other hand, cycles differently. Its yield strength does
degrade, but asymptotically to a limited value.
I don't recall seeing a chart for titanium. And I don't know if 6061-T6 or
other aluminum blends exhibit this performance.
Hopefully a structural engineer on the reflector with access to the right data
can contribute more precise information than my
15-year memory can!
2. Dampening vibration:
I've noticed that long spans of high tension power lines (and other aerial
cabling) are built with vibration dampeners. One
common form I've seen is a small dumbbell shaped objected clamped at its center
and suspended below the cable. And I think someone
told me that other shapes also exist. While a solution that doesn't add any
external surfaces to the antenna element is probably
preferable, in a troublesome case perhaps a similar approach could solve a
W3LPL and I were installed yagis for a PVRC Field Day operation one year.
While we were on the tower we noticed a vigorous
vibration in a particular 10m element. Frank reached out and just lightly
touched the element at the point of maximum deviation --
a few grams of pressure at the right point was enough to kill the vibration (at
least in that slight breeze). I suspect that, if
you go up at look at a vibrating element, you can quickly determine which point
in the element needs to be dampened, and try a few
alternatives to kill that mode of vibration.
-- Eric R3/K3NA
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