[TowerTalk] square vs round booms

[Bob Shohet, KQ2M]

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 at 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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