From: JVarney <jvarn359@gmail.com>
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Optibeam OBLY14-5
<It's straightforward to show that a square boom is
<inferior to a round boom.
<A fair comparison is 2.25-in round boom vs. 2.0-in
<square boom because they have the same cross-sectional
<area. Bending is proportional to the moment of inertia
<of the cross section:
## Its actually a lot worse. A 2.25 inch OD round boom has a
circumference of 7.069 inches. A 2.0 inch square boom has
a larger circumference of 8.0 inches.
## assuming same wall thickness, the 2 inch sq boom is
8 / 7.069 = 13.2% HEAVIER...and more expensive, if cost
per pound is the same. Circumference of the 2 inch sq boom is
13.2 % bigger.
## A real world fair comparison would be equal circumferences.
In that case the round boom would have to be 8 / pi = 2.546 inches
in diameter. They would both weigh..and cost the same.
## Ok, now compare the 2 inch sq boom vs the 2.546 inch round boom.
Using your calcs, the round boom now solidly trounces the square boom..hands
down.
Closest tubing readily obtainable is of course a 2.500 OD boom.
## The square boom will require the use of one off ... square shaped...U
bolts.
Flat plates could instead be welded directly the the square boom.
## I will stick with my round booms. A buddy tried building full sized 40m
eles,
using rectangular, telescopic tubing, 6061-T6. This was in the early 80s.
With the
long side of the rectangular tubing in the vert plane. 3 els on a 48 ft
boom. Boom made
from 4 inch OD tubing. Worked great..till the wind eventually destroyed
the eles.
Jim VE7RF
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