[TowerTalk] High VSWR
Grant Saviers
grants2 at pacbell.net
Tue Sep 3 00:39:11 EDT 2019
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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