Date: Mon, 2 Sep 2019 21:39:11 -0700
From: Grant Saviers <grants2@pacbell.net>
To: towertalk@contesting.com
Subject: Re: [TowerTalk] High VSWR
<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
## Leeson got it all wrong with his use of ..dummy eles..... they dont
work..cant.
The correct concept works on the crossflow principle. As long as each
ele half
is symmetrical to the boom..... , and the boom is symmetrical to the
mast, the entire
yagi is tq balanced. A small counterweight is placed at light end of
boom.....to weight
balance the boom. IF counterweight is placed inboard of far end of
boom, it has to be
a lot heavier. Start / stop tq on rotor does not get any worse
with either counterweight config
used. Without the counterweight, boom is weight unbalanced, and start
/stop tq on the rotor is
worse.
## If boom is mounted to mast at booms CG, then a sail..aka..tq
compensator plate has
to be used.
## I tested these concepts, the theory is sound. Vortex shedding of a
short boom is a non issue.
Longer booms are typ trussed.... which will kill any vortex shedding.
If eles designed correctly,
vortex ele shedding is a non issue.
Jim VE7RF
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