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[TowerTalk] Optimum Stacking Distance

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Subject: [TowerTalk] Optimum Stacking Distance
From: (Bill Tippett)
Date: Sat, 28 Jul 2001 16:32:46 +0100
>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

     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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