[TowerTalk] [Tower Talk] Opposing boom to mast plates?

[Leeson]

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