As I have found the presence of the undriven antenna is effecting the gain
and F/B a lot.
Actually in a stack of two the lower undriven antenna decreases the gain of
the upper one by about 1db if turned into the same direction.
That is almost half of the stacking gain!! That's why I think that the
upper-lower-stack tests might sometimes lead you to wrong conclusions - to
think that your stack is performing better than it actually is. The effect
of the lower antenna vanishes when you turn the upper about 40 degrees (for
6 el owa yagis).
Some other tips/ideas I have come to when testing and modelling:
Don't place yagis in a stack lower than 1 wl!
If it is a three or four stack you really don't get more gain from the
low antenna and if you want high angles you can switch the other antennas.
Don't stack different beams, you lose gain!
Try to avoid stacking different beams as you usually get the gain of a
stack of the lower gain beams if even that much.
Put the beam with higher gain lower in the stack!
I have found that gives you more gain. If you still want to stack
different beams, say 1wl boom and 0.75 wl boom then put the longer boom
lower on the
tower as models suggest that the lower gain beam desperately wants to be
higher to work better. I think most of the people would be tempted to put
their "killer"
beam to the top:)
73s
Tonno
ES5TV
----- Original Message -----
From: "Bill Tippett" <btippett@alum.mit.edu>
To: <towertalk@contesting.com>; "alsopb" <alsopb@gloryroad.net>
Sent: Saturday, July 28, 2001 6:32 PM
Subject: Re: [TowerTalk] Optimum Stacking Distance
>
> K3KO:
> >I understand you did extensive analyses considering the terrain around
> >you. To what extent did this impact the stack spacing and/or final
> >antenna heights?
>
> Yes and no...I did use N6BV's Terrain Analysis but primarily to help
me
> learn how to use the stacks...i.e. best takeoff angles vs azimuth vs time
of
> day. For each contest, I usually make a chart by UTC hour showing which
> directions and combinations of the 3-stack should be best. Of course
> nothing beats checking actual signal levels (and TOA's) during the contest
> but it also helps to have a general idea of what you should be doing
> (especially for when your brain gets tired!)
>
> YO7 assumes flat terrain but can output a file in AO format which can
> then be used to model local terrain effects. I do not have AO and my
> surrounding terrain is not extreme so I felt that was an unnecessary
> exercise. I did verify that my measured F/B for the stack was close
> (actually a bit better) to what YO7 predicted so I have confidence that
> YO7's assumptions are working OK for my case. W4AN says his mountaintop
> terrain works better with closer stack spacings than I've found to be
> optimum so he obviously believes complete AO modeling is necessary for his
> location.
>
> One related and interesting thing happened when I measured my actual
> F/B. The F/B for a single antenna was MUCH better than YO7 predicted so I
> mentioned this to K6STI. Brian then modeled the stack in AO and found
that
> the presence of the undriven antenna significantly (5-6 dB I recall)
> improved the F/B of a single driven antenna...and this was with them
spaced
> over 1 wavelength apart!
>
> Anyway, it's been a fun learning experience and I continue to be
> surprised by some of the things I see (like long path JA's often being at
> high angles!)
>
> 73, Bill W4ZV
>
>
>
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