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:
I square = 0.55 in4 (assume 0.125 wall for both)
I circ = 0.47 in4
The square boom then will have about 16% less sag
due to gravity.
When the wind blows, the square boom sees higher drag.
Drag coefficient Cd for a circle is 1.2 versus 2.0
for a square. Drag is proportional to Cd and face
area:
D square = 2 in x 2.0 Cd = 4 drag units
D circ = 2.25 in x 1.2 Cd = 2.7 drag units
The drag force is about 48% higher on the square
boom, but the moment of inertia is only 16% higher,
so the square boom will bend more in the wind.
More importantly the square boom will impart a
higher force on the mast and tower than a round
boom.
The only advantage to a square boom (other than
slightly less gravity sag) is that it facilitates
building the elements perfectly horizontal and
fully prevents elements from rotating around the
boom.
73 Jim K6OK
Jim Thomson wrote:
## I dont see any advantages to the use of
square booms. Optibeam uses round booms on a
lot of their other yagis, go figure.
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