A few thoughts on this:
1. Yes, the wind force on a cylinder is essentially perpendicular to its
axis. See for example https://www.kkn.net/dayton2007/w6nl_ant.pdf So the
maximum twisting force (torque) from the offset from the mast on a Yagi
in the wind occurs when the elements are across the wind. When the boom
is across the wind, there may be torque from its unbalanced area, but
not from the offset from the mast; this is a different issue that can be
resolved by keeping the boom symmetrical or adding extra area or length
at the short end.
2. It's hard to imagine a usable mast weak enough to suffer from this
offset force, but there is the possibility of torque on the rotator. I
did some spreadsheet calculations for typical antennas and high
windspeeds, using the HyGain 204BA and 105BA as examples in a 100 mph
wind. I assumed a 4" offset from the mast. The element area of the 204BA
is 10.6 sq. ft, and the 105BA is 2.1 sq. ft. At 100 mph, the wind force
is 25.6 lb/sq. ft., and the drag factor for a cylinder is 0.67. The
result is about 700 in-lb for the 204BA and 150 in-lb for the 105BA. By
comparison, the rotating torque rating of various rotators runs from
1000 in-lb for a Tailtwister to 4500 for the new DXE RT4500 and 16,000
for a prop pitch. See
http://www.arrl.org/files/file/Antenna%20Book%20Supplemental%20Files/22nd%20Edition/Rotator%20Specifications.pdf
The braking numbers are typically much higher. So this offset effect is
well below the rating of most rotators, although a big antenna could
effect a smaller rotator. Radial ice could change this calculation.
3. But this all assumes the wind is constant across the elements, which
seems to be an oversimplification. A while ago, I did an rough analysis
of the rotating forces from wind shear and vortices, and am convinced
that even a completely symmetrical antenna will suffer from the
resulting buffeting. For an example, see
https://www.youtube.com/watch?v=WTZ9q8Fochw I found from the
calculations that an antenna on a longer mast would have a lower
torsional resonance, so this would reduce the coupling of the wind gust
torque to the rotator. In our extreme wind situation here I use the BMW
rubber couplings ("flexdisc" or "guibo") to protect my rotators from the
wind torque. We measured 147 mph max 3-sec gust about 12 years ago, more
typically over 100 mph each winter, a tradeoff for the elevation-pattern
benefit of a ridgetop location.
4. So it seems to be the case that, for two antennas, there may be some
benefit from mounting them on opposite sides of the mast, but it would
be small compared to other effects. One minor issue would be the
increased complexity of mounting the antennas.
73 de Dave, W6NL/HC8L
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