[TowerTalk] antenna moment of inertia

[jimlux]

It's not all that hard to calculate the approximate moment of inertia of 
a Yagi.  You know what it's made of (sizes of tubing and dimensions) - 
it's a bit tedious to do it by hand but I'd be surprised if it takes 
more than an hour with an Excel spreadsheet.  The arithmetic is pretty 
cookbook, but Yagis are basically symmetrical, so you might not even 
need any sin and cos calculations.

and if you already have the spreadsheet with the lengths, diameters, and 
positions, that would save time.


Then, you'd have something "real" to evaluate the torque requirements.

I suspect it's just tradition: back in the 60s or early 70s, where you'd 
be doing the calculations by hand with a slide rule or sin/cos table, it 
would be a huge pain in the rear to calculate.

Maybe it's the desire to simplify things to a single number that you can 
compare (since knowing the moment only gives you half the answer, you 
also need to know the angular acceleration to get the torque, t = alpha*I)

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Moment of a lump mass is
m*r^2

Moment of inertia of a rod around its center is
Icm = m * L^2/12


Moment of inertia of any object around an axis that is displaced (e.g. 
Yagi elements, the entire structure may have the antenna mast displaced 
from the CM for some reason)
I = Icm + m*r^2