[TowerTalk] High VSWR

[Jim Thomson]

Date: Mon, 2 Sep 2019 21:39:11 -0700
From: Grant Saviers <grants2 at pacbell.net>
To: towertalk at 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