[TowerTalk] Modeling question - for the experts!

[Jim Brown]

On Thu,6/2/2016 5:38 AM, StellarCAT wrote:
> So here are the questions:
>
> 1) WHY when I model a full size 2 element 40 does it not have a low SWR on 15 when I know in the real world people have said it should/does?

At least to some extent, you're asking the wrong question. Interaction 
is FAR more than VSWR. Interaction is PATTERN DISTORTION and/or 
performance degradation. Pattern distortion can be in the vertical 
plane, the horizontal plane, or both. To see it, we must look at the far 
field pattern with and without the interfering antenna, and carefully 
compare the plots on the same drawing.

One method I've found to do this is to model the "primary" antenna, save 
the model, and save plots for H and V field strength, then add the 
interfering antenna, look at the H & V plots, and with each, add the 
saved plots to the display. NOW, you can see very clearly the effect of 
the interfering antenna, if there is any.

Another method that I've used is to build the model with both antennas 
and compare H and V plots of the primary antenna as I rotate (or raise 
and lower) the "wires" defining the interfering antenna. Again, to see 
the interference, we must look at plots "with and without" the 
interfering antenna on the same graph.

When I evaluate an antenna, I typically look at the vertical pattern on 
axis, and the horizontal pattern at 5, 10, and 15 degrees elevation. 
When a second antenna (or wires in the vicinity) are interacting, the 
"pretty" pattern of the primary antenna will be less pretty. :)

73, Jim K9YC