[TowerTalk] Antenna interactions

[Rick Stealey]

Paul,
I did a quick model of something very similar to your environment using 
4NEC2.  It comes with a model of a 3 element beam at 50 feet. That antenna 
has 12.2 dbi gain at a takeoff angle at 20 degrees with a f/b ratio of 32 
db. It also has a large lobe in the 60-75 degree range.
When I set a reflector and director 6 ft above them (representing two of the 
elements of another 20 meter beam) the gain drops to 10.5 dbi, the f/b to 
18.8 db and the minor lobe becomes huge, centered at 80 degrees.  Probably 
the reflector is directing the energy downward causing a reflection off the 
ground?

This is the sort of thing you can have a lot of fun with by learning the 
basics of modeling.  I use 4NEC2 which is free, and comes with a bunch of 
antennas already modeled.  I used the 3 element 20 meter yagi model that 
came with it, and just copied the reflector and director elements, and 
changed their height (the Z axis) to 56 feet instead of 50 feet. So what I 
had that was close to your situation was a 3 element beam at 50 ft with a 
reflector and director sitting directly above them 6 feet higher.  In the 
program you could model your exact 204BA elements but I think the outcome 
would be similar, because you are looking for the distance at which the 
interaction is minimal.  I could send you my NEC file if you like.  It is 
just text.

Rick  K2XT

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