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[TowerTalk] stacking distances -- YT and TA

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Subject: [TowerTalk] stacking distances -- YT and TA
From: (Pete Smith)
Date: Tue, 08 Jan 2002 08:03:39 -0500
At 01:23 AM 1/8/02 -0500, Tom Rauch wrote:
>> You have to be careful with YT, using stacks with close spacing.
>> If you plot a stack of two antennas even one foot apart,
>> YT will still show a 3 dB gain over a single antenna.
>Very interesting. It is amazing what is in those manuals
>that I, like most people, never bother reading. 
>Anyway, that's why I like the new Eznec with it's average gain! You 
>can look to see if average gain exceeds 0dB as an automatic red 

I think Dave's statement could be misread to mean that YT adds 3 dB gain
for a stack over a single antenna, regardless of spacing.  It doesn't.
Instead, there is a gain error that gets more obvious (larger and less
consistent with NEC-2 models) as stack spacing is brought closer.  

What the YT manual says is "The internal Yagi model in YT is simple and
does not compute interactions between individual Yagis in a stack -- YT
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."

I verified this just now with the classic 2-high 4-element 20m yagi stack.
At 50 and 100 ft, the stack's gain is given as 16.2 dBi, with the stack
2-2.2 dB better at peak than either individual antenna.  However, with the
stack at 50 and 60 feet, the peak gain rises to 17.0 dBi, with the
individual antennas 2.8 to 3.2 dB below that.  At intermediate spacings
from 20 to 40 feet, the computed gain of the stack is consistently above
that at 50 foot spacing.  This looks nothing like the curve of gain versus
spacing produced by NEC-2 modeling.

In the documentation, Dean recommends that YT be used for what it is good
for, which is assessing the pattern effects of irregular terrain, and that
"I would trust the results within plus/minus 3 dB."

Does anyone know whether K6STI's TA, which uses NEC antenna models, does a
more accurate job of predicting stack gain over terrain? 

73, Pete N4ZR

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