Thanks Jim for your quick envelope scribble.
Regards,
Mark, K1RX
On Wednesday, May 29, 2024 at 12:27:34 AM EDT, JVarney <jvarn359@gmail.com>
wrote:
Stan K5GO wrote: "It would be interesting for
someone like K6OK to tell us an example of
how many SF you could have at 3 inches,
7.5 feet and 15 feet with that aluminum
mast and survive about 90 mph ... "
Hi Stan, sure, I'd be happy to scribble
on the back of an envelope.
For 2" OD 0.375in wall aluminum
Assume yield of 35000 psi
Z sect modulus = 1.01 in3
Moment strength = 35350 in-lbs
Wind load = 0.00256V^2 = 20.7 psf
Assume antenna Cd = 1.2 (round elements)
At 3 inches: Max SF = 473
At 7.5 ft: Max SF = 15.8
At 15 ft: Max SF = 7.9
You're right, you can use just about
anything for a mast if the antenna
is positioned very close to the top
of the tower. Since the antenna at
3 inches will be a lot less than 473 SF,
very little of the moment strength will
be consumed, leaving most of it
available for the ants at 7.5 ft and
15 ft.
Disclaimer: this is for entertainment
only and not for construction.
Use of 30-year old simple statics
methods are obsolete. Current codes require
a more comprehensive analysis that takes
site specific conditions into effect
plus considers non-linear structure
response due to large deflections of
the tower system.
73 Jim K6OK
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