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[TowerTalk] Antenna Modeling Question

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Subject: [TowerTalk] Antenna Modeling Question
From: (
Date: Tue, 24 Jul 2001 20:32:27 EDT
In a message dated 7/22/01 7:19:33 PM Pacific Daylight Time, 

 Request info on obtaining Antenna Modeling software regarding stacked yagis 
 (2 possibly 3) for 10 meters.  Looking for software that is very user 
 friendly.  Your assistance would be appreciated.
 Thank you,
Eznec works just fine.  However I did discover someting about stacked 
vertically polarized beams on a horizontal boom also applicable to horizontal 
stacking.  A general roll of thumb is this.  The optimum spacing is 1/2 wave 
for 2 element beams, 5/8 wave for 3 element bams and wider as the gain of 
each beam increases.  If less than optimum spacing is used there is less gain 
and and more upset of F/B and SWR between the beams.  If wider than optimum 
the 3 dB gain increase stays the same with the lobe sharpening a bit and 2 
ears form.  10M and higher beams can be rotated on a horizontal boom to a 
vertical pick up antenna for a pattern check, the spacing modified as needed 
for "No Ears" and/or then changed to horizontal stacking or use Eznec.

The problem I detected is with 2 element beams using 1/2 spacing and Plumbers 
Delight construction.  The boom acts like a resonant element for both DE's.  
It's hot with RFand then it couples to the mast and tower.  I did a series of 
tests in Eznec and it showed what the different spacings do for RF Spill Over 
on to the booms, mast and tower.  You can see the current loops and relative 
amplitude on the screen with the antenna.  A current chart will give you the 
relative current level all over the array also.  The higher gain beams tend 
to have spacings where the connecting booms are less resonant and "reduce the 
problem".  I willl run tests on 2M on this concept with different gain beams 
with the elements grounded to the boom and then insulated masts etc.  Perhaps 
the hot or at least warm booms, mast and tower explains one reason why the 
F/B is frequently ruined with stacked beams.  

I have a similar installation on 6M with 2 element vertically polarized beams 
spaced 1/2 wave and I didn't quite get what I thought I should compared to 
Eznec in where I didn't use connecting metal booms between the elements and 
boom between the beams and connecting mast and tower to ground.  I added 
metal booms (betwen the gamma matched elements) and mast and they were hot.  
Remember any resonant element connected to the center of the DE will absorb 
RF also.  It travels all directions.  I just added an insulated boom between 
the beams and will retest.  I will add toroids to the coax if the shield is 
connected to the center of the DE like on a gamma match.  Balanced feed is a 
better way to go.  If balanced coax is used, do not connect the shield to the 
boom.  You might try it and see if the SWR changes.

In Eznec booms are seldom added and all that connects to it.  There is no 
feedline as the "RF Source" is connected to the DE and there is no RF Spill 
Over on to a coax feedline as--there isn't any.  So always add the boom and 
all it's actual connections after the ideal free space tests.   The mast or 
tower section supporting the stacked beams have always been assumed to be 
"Immune to RF coupling" just because it's the opposite polarization--NOT SO.  
 Insulating elements from the boom stacked or not has shown to be in Eznec a 
good idea.  More on this later.  K7GCO.

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