[TowerTalk] square vs round booms

Kevin kstover at ac0h.net
Sun Mar 12 10:50:29 EDT 2017


Optibeam's have been in the field for for how long now? Their multi-band 
beams have higher surface area (more elements) than most multi-band 
beams and I haven't heard of huge numbers of elements falling off, or 
broken booms.


On 3/10/2017 3:30 PM, Peter Voelpel wrote:
> All 49 elements on my KLM antennas hold nicely for more then 30 years with a
> single band clamp on the round boom.
>
> I see problems with the square booms.
> To get a rigid clamp of elements to the boom large size screws are necessary
> for the clamp.
> On Optibeams for instance the small diameter screws just get elongated until
> they brake but do not hold the elements as good as those on my KLMs.
>
> 73
> Peter, DJ7WW
>
>
> -----Original Message-----
> From: TowerTalk [mailto:towertalk-bounces at contesting.com] On Behalf Of Bob
> Shohet, KQ2M
> Sent: Freitag, 10. März 2017 21:18
> To: Jim Thomson; towertalk at contesting.com
> Subject: Re: [TowerTalk] square vs round booms
>
> 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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-- 
R. Kevin Stover
AC0H
ARRL
FISTS #11993
SKCC #215
NAQCC #3441


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