With a square boom, you don’t have to worry about elements rotating around the
boom from the horizontal plane in a severe wind and/or icing situation. Just
to keep it in place on a round boom, you need to use a very large and strong
clamp with multiple screws. A less robust clamp in size and weight will
suffice for a square boom.
Having said that, I still prefer round booms and all my yagis have round booms
for several reasons:
1) Less surface area on which to accumulate snow and ice, accumulation of which
increase the effective surface area acted on by the wind and which will also
further increase the weight of the antenna and the stress on the tower.
2) Without snow and ice, increased surface area in wind compared to that of a
square boom. I believe that the physics of the wind acting on a round object
are more favorable compared to a square object, so that even with the same
effective square footage of antenna/boom, the force of the wind and wind
resistance acting on the tower would be greater with a square boom than round
one. If my memory is not correct on this, please set me straight.
3) Most clamps are round shaped and much easier to obtain than square ones
which aids in maintenance and the reduces the cost of maintenance.
73
Bob KQ2M
From: Jim Thomson
Sent: Friday, March 10, 2017 2:58 PM
To: towertalk@contesting.com
Subject: [TowerTalk] square vs round booms
The section modulus of a 4 inch OD x .125 inch wall thickness tube is
greater than a 3 inch square boom, also with a .125 inch wall thickness.
Both have a 12 inch circumference. The 4 inch OD tube is a hair more. They
would both weigh aprx the same. Assume both are 6061-T6 and both are 40 ksi
yield strength.
For the round tube..... bending moment = section modulus X yield strength
.
Does that also apply to rectangular tubing ?? If it does, square tubing
offers no advantage at all..in terms of wind survival.
Wait, it gets worse. The 3 inch sq boom has a smaller projected area vs the 4
inch round boom, but, per the latest wind specs,
you have to multiply the projected area of the square tubing, with its flat
surface... by 2.0 The round tubing is multiplied by 1.2 So the 4 inch
round tubing still ends up with less effective area vs the square tubing.
So now the square tubing has lost...twice. For the same circumference and
wall thickness, and yield strength, the square tubing has a smaller sectional
modulus, plus more effective area.
Am I on track here ? So why are we seeing square booms used, other than
perhaps a slight ease in mounting eles. What am I missing ?
Jim VE7RF
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