|Subject:||[TowerTalk] Antenna interactions|
|From:||"Rick Stealey" <firstname.lastname@example.org>|
|Date:||Fri, 07 Sep 2007 14:45:17 +0000|
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.
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