[TowerTalk] Optimum yagi stacking distance

[Billy Cox]

Gary, while HFTA is a wonderful tool to use to design the
station ... there are some "traps" to watch for ... <see text
and source below>. Sounds like you need to check and
confirm that is not what is going on with your model there.
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A Caveat About Closely Spaced Yagis in a Stack

The internal Yagi model in HFTA is a very simple mathematical model. It does
not compute interactions between individual Yagis in a stack-HFTA assumes
that each antenna is a "point source." For antennas stacked more than about
a half wavelength apart this is not a problem. For example, you should be
cautious specifying spacings less than about 20 feet on 20 meters (and
proportionately scaled on other bands) because of mutual-coupling effects
between real antennas. Spacings less about 20 feet on 20 meters will show a
false increased gain in HFTA, even though the real effects of interaction
between the beams will actually be to decrease the gain.

http://www.arrl.org/notes/9876/HFTA.pdf
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Hope this helps!

73 de Billy, AA4NU

>On Behalf Of Gary Slagel
>Sent: Saturday, January 19, 2008 16:15
>To: TowerTalk
>Subject: [TowerTalk] Optimum yagi stacking distance
>
>More questions from a yagi stacking rookie!
>
>I've been using the HFTA software to try and get an
>idea of the optimum distance to stack a couple yagis
>for best performance.  I'm stacking 10/15/20
>tribanders for increased performance.  I was really
>surprised that on most bands the optimum performance
>seems to be with them separated by only about 10'...
>for instance 65' and 56'!  I expected it to be more
>like 20' to 30'!
>
>I wonder how this jibes with folks with real experience?
>
>Thanks,
>Gary