[TowerTalk] Modelling a basic stack

[Pete Smith]

At 07:43 AM 5/10/2004, Bill Tippett wrote:

>         Yes, perfectionists would take the optimized model from YO into
>AO and continue to tweak.  When the measured F/R of my stack was very
>close to what the free-space model predicted, I declared it "good
>enough" for my needs and put it up.  As with many things, there is a
>point of diminishing returns and it's up to each of us to decide where
>that is for ourelves.  As you well know, HFTA also makes assumptions
>about point source antennas, but it's "good enough" as is for my needs.

You can get some really funny results from either HFTA or K6STI's TA if you 
model a really close-spaced stack, because the point-source model breaks 
down.  Not a real-world situation, though.  It's also worth mentioning, for 
those who don't know, that HFTA doesn't use real antennas, so you have to 
choose from its list of hypothetical antennas the one that most nearly 
matches what you have.  TA does use real models, but it has other problems, 
including some hairy modeling artifacts at low angles when using 
highly-detailed terrain profiles.

Modeling over real terrain is invaluable, as is the ability to use HFTA 
figure of merit to compare different stacking heights and distances, 
particularly in a multi-band stack.  The results can be counter-intuitive 
-- I've seen a number of cases where a lower antenna significantly 
outperforms a higher one in terms of signal delivered to the target area.

Perhaps one day we will have a true "Swiss Army Knife" -- a modeling 
program that can optimize actual antennas or arrays, over real 
terrain.  Our desktop computers are certainly powerful enough now.

73, Pete N4ZR
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