Thanks for the link to the very detailed Weber papers. They clarify the
issue. The effect is even bigger for side-mounted antennas, say on a
rotating tower.
The calculations are based on the assumptions (a) of constant windspeed
in all directions across the antennas, and (b) wind pressure is the only
source of force on the boom and elements. But this gives a basis to
understand the issue and proposes a clever solution.
However, wind shear and vortices (variation across or along an antenna,
for example) are ever-present features of real weather, as are up- and
down-drafts. Also, the variation of windspeed with height above ground
can make a small difference -- it varies as the 1/7 power of height, so
an antenna at 60 feet above ground has 5.2% more wind force than another
at 50 feet.
Last, experiments I did years ago with small boom and element structures
on a pole through the sunroof of my car at freeway speeds showed forces
and torques that were not simply perpendicular to the cylinders. A
symmetrical structure tended to have stable points at 0°, 90°, 180° and
270°, so there were some shape, shadowing or viscous forces as well.
So what's the lesson here? For big antennas and small rotators, opposing
boom mast plates may be helpful, and Weber's analysis provides a
quantitative understanding. But a mathematical model is only as good as
the assumptions that underlie it. "The model is not the thing", and
"experiment trumps theory." Try it and see how it works for you.
Dave, W6NL/HC8L
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|