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@contesting.com] On Behalf Of Bob
Shohet, KQ2M
Sent: Freitag, 10. März 2017 21:18
To: Jim Thomson; towertalk@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@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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