Re: [TowerTalk] High VSWR

[Grant Saviers <grants2@pacbell.net>]

I understand the objectives and mechanics of wind torque balancing of a 
beam. Leeson explains how to do this with round elements so it works at 
all wind angles.  (Physical Design of Yagi Antennas, chapter 6)
However, I don't understand why a counter-weight balanced beam is  a 
benefit other than when tramming the beam and for on the tower work.
Could some one explain the dynamic mechanical principles of 
counter-weighting?  It seems to me that increasing the rotational 
inertia of the beam is a bad idea as it increases the dynamic start-stop 
load on the rotator.
Then a radial top bearing takes care of most of the center of mass 
offset moment of an unbalanced boom and the rotator can handle whatever 
is left.
When the wind blows the bending moments on radial bearing and rotator 
will be many times whatever the imbalance mechanical mass moment exerts.
And anything that brings the resonances of the boom on both sides of the 
mast closer together increases the chances of resonant coupling from 
vortex shedding.  Resonances are a very complex parameter to calculate 
but the reasoning applies.
Grant KZ1W

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