[TowerTalk] Wind Loading redux

[Kurt Andress]

EUGENE SMAR wrote:
> 
> TT:
> 
>      I won't feel offended if you hit "delete" at this point.
> 

>  SNIP

 
>      A final point:  If you blindly point your mast to due north (please
> don't start!) and attach all your long-boom Yagis from the same side of the
> tower, you're unbalancing the torque on your tower system.  Dick wrote a
> couple of papers on aerodynamic balancing of antennas to minimize tower
> torque.  I'm now going to mount my four antennas (Skyhawk, 2M Yagi (23 foot
> boom), D40 rotatable dipole, and triband V/UHF vertical) on the mast on
> alternating sides, depending on how the math shakes out.  No point in
> deliberately putting unnecessary stress in your tower system, if it can be
> avoided simply.
> 

Don't forget to think about what's happening when the antennas are
broadside to the wind.

The common practice of attaching a mast at the center of gravity of the
antenna, is quite convenient for handling the antenna. But, it is really
quite an awful practice, for what happens after we get it installed! 
If the mast is not connected at the physical center of the boom, the boom
will generate torque when exposed to the wind. A small offset could be used
to balance the asymetry of the boom/element mounting hardware, but that is
usually pretty small.

A good example of this is the good old KLM 5 el 20, on a 42' boom. At a
sustained wind speed of about 35 mph (if I remember it correctly), the
antenna torque generated by the placement of the mast equals the stall
torque of a T2X rotator. The "effective moment" type of rating says it
should be ok.

I think this is a significant factor in the number of antenna's eating
rotators in our empirical database. There are some commercially sold
antennas that generate some spectacular torque values.

Now, if you're real clever you can figure out where to point the antenna to
get this torque to cancel out the one caused by the offset mounting of the
antenna to the mast :-)

If you mount the antennas on the mast to mitigate the antenna/mast mount
torque source and don't fix the other probelm, you will still have torque
loads, they'll just happen at other points on the compass.


> 73 de
> Gene Smar  AD3F
> P.S.  I'd advise you actually to calculate the area of your antennas, and
> NOT rely on manufacturers' specs (Unless you know how they came up with the
> numbers.  And I apologize for starting another re-thread.)  If you're
> contemplating the purchase of an antenna, maybe the manufacturer will
> sell/give you a manual for this purpose.  (But then again, maybe not.)
> 

Force 12 ,Hygain ,and Cal-Av tell us enough to use their areas. We should
commend them for doing that! 
We would be doing ourselves a favor by pestering the heck out of the others
to get the info. They might figure out it is easier to just publish figures
we can use. Then everyone will be happy!

-- 
73, Kurt, K7NV 

http://yagistress.freeyellow.com

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