[TowerTalk] modeling interaction

[Dick Green WC1M]

Hi. I'm trying to use EZNEC 4.0 to determine whether there would be any
interaction between a 2-el 40m beam at the top of a tower and various beams
placed lower on the tower. For example, I've tried putting a 40-2CD at 105'
over a 3-stack of 4-el 15m monobanders at 90'/60'/30'. The model showed no
significant difference in the 15m pattern with or without the 40m beam, and
no significant difference in the 40m pattern with or without the stack. Same
story for 3-stacks on 20m and 10m at the same heights. I even tried putting
a 5-el 15m beam at 100', only 5 feet from the 40m beam, and got no changes
in pattern for either antenna. Seems to me that there would be some
interaction between 40m and 15m antennas spaced that close. Is that
incorrect, or is there a problem with my model?
 
FYI, I'm deleting the source for the antenna that's not on the band in
question. In other words, if I'm checking the pattern for the 15m stack, I
leave its sources intact and either delete the source for the 40m beam or
set the V to 0 (results are the same either way.) If I'm checking the
pattern for 40m, I leave its source intact and delete the sources for the
3-stack. Is this the right way to do it? The only difference from reality is
that I'm not including booms and transmission lines for any of the antennas.
 
If it appears to the experts that I'm doing things correctly, can anyone
give me a good example of two antennas that should show some interaction
when placed in close proximity? I'd like to model a case of known
interaction so I can make sure I'm doing this right.
 
73, Dick WC1M