Topband: Vertical Arrays (slightly long)

[W8AV at aol.com]

This didn't make it out to the list the last time around so I will try  again.
 
I have been reading this thread but have held off on posting anything until  
now.  I am assuming that all the modeling that has taken place has assumed  
equal magnitudes for each element of then and the varying of the phase angle of  
the lagging element to achieve the desired cardiod pattern.  Obviously,  when 
you suppress the radiation on the ground in the null area of the pattern on  
the ground the radiated signal has to go somewhere.  Some if it goes to  
generating the gain of the antenna in the main lobe of the pattern and some if  it 
is radiated at some of the higher vertical angles in the null area of the  
pattern that result in less attenuation of the signal at vertical angles at 30  
degrees and higher and therefore effect the F/B of the antenna at these  
angles.  You can see this if you plot the vertical radiation pattern of the  
directional pattern.
 
When I designed my W8AV 80 meter triangle array, I took this into account  
and used a shunt coil in reference element (the one that does not have the  
phasing line attached to it) to suppress the signal at the upper vertical angles  
in the null area of the pattern and make it as close as possible to be uniform 
 at all vertical angles.  From experimentation and from experience I  gained 
by working for 26 years in the broadcast field, I found  that an magnitude of 
about 0.5 on the reference element gave me the best  results.  This same 
principal will work with a two element  array.  You can model it in NEC to 
determine the best number for your  particular application by playing with the 
magnitude on the reference element  and plotting the results.  You can also vary the 
phase delay on  the lagging element to in conjunction with the magnitude to 
achieve the  radiation pattern you desire.  Once you get the desired pattern you 
can  install a coil in parallel with the phasing line at the common feed 
point  and tap down the coil to get the desired magnitude. This is called a  
parallel or shunt power divider and is used in one way or another in all AM  
directional antenna systems to develop the required directional patterns.
 
The caveat, as mentioned before,  is that YMMV if there are any  re-radiators 
in the immediate vicinity of the array that will disrupt the  pattern, 
especially if they are in the null area.  In addition, as  previously mentioned, the 
"real world" values can differ greatly from  the theoretical values obtained 
from NEC and you may have to tweak the  phasing line and/or magnitude to 
achieve your pattern.  My W8AV Triangle  uses three wire elements suspended on 
ropes from the base insulated tower that  serves as my quarter-wave 160 meter 
vertical antenna.  Because of  this, I had to detune the 160 antenna on 80 meters 
to achieve the desired  pattern.  As a note, the big tower wouldn't have had 
as much  impact on the pattern if the tower was a grounded tower as opposed to 
being an  insulated one.  You can also model nearby radiators in NEC to see 
what  impact they will have on your pattern.
 
73............de Goose, W8AV




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