I would recommend a trimtab which clamped around the boom, and
permits it to be slid in or out, to fine-tune the solution.
I did this with an XA 15 years ago, have long since lost
the calculations, and don't have the manual, so won't share
The XA was mounted on a house-mounted wilson tubular mast. In gusts,
with the tower nested down, it tended to rotate toward the back of the beam,
causing the tower sections to rotate within themselves, until they
hit the stop. Screech...bang.
As this tower was firmly tied to the frame of my house outside
our bedroom, sleep was difficult on windy nights.
Here's a method of estimating the size of the trimtab:
The manufacturer specifies 10 sq. ft. as the windload area
for the antenna. The Boom... 32' x 3", accounts for 8 sq. ft.
The mast to boom bracket adds about another half foot to that.
(partly obscured by the boom, and 10.5" on a side)....
So, the elements present around 1.5 sq. ft. to the total windload.
I assumed that all trapped elements were identical, for purposes of
and I assigned them an exposed area of 1 ft ^2. (wrong..but maybe not TOO
Then, I estimated the 10m director area had half as much drag as the trapped
(wrong, but maybe not too wrong.)
>From there, it's a sum-of-moments problem.
For each of the elements, multiply the offset from the mast x the estimated
exposed area. (One end of the antenna has to be negative, obviously.
which.) Then simply sum it up, and figure out how big a trimtab you'll
need, at what
distance. The closer in it is, the bigger it has to be, obviously.
(The boom-offset is probably not material... the center of mass vs.
of boom is offset by 20". 20" x 3" = 60sq. in., so a 1 sq in. tab, 5' from
would balance this out. If that tab were located 15' out from the mast, I'd
only need .3" sq.
It's lost in the estimation errors.)
I won't tell you the dimensions of my trimtab, except to say that it was
less than 1 sq. ft.
(consider.. 1 sq. ft. @ 15' from the mast would produce 15 ft. lbs
torque...the problem isn't
that big!) I leave it to you to calculate the ft. lbs and therefore area
In my case, I cut it once, put it at 15', and the tower stopped screeching.
It still moved
a little in big gusts...but I could sleep at night! Never bothered sliding
it in-out to
perfect the solution, even though I could lower the tower and walk on the
roof to do the job.
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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