Exactly my philosophy Jim.
Programs, literature, discussions brings people closer to better
understanding of theory. Practise leads to the sollution, needs often hard
work, and takes a lot of time.
Jos
----- Original Message -----
From: "Jim Lux" <jimlux@earthlink.net>
To: "Towertalk" <towertalk@contesting.com>
Sent: Wednesday, April 14, 2004 7:27 AM
Subject: [TowerTalk] the value (or not) of modeling
> Periodically on TT (and in other forums) the issue of modeling, accuracy
of
> modeling, and the value of modeling comes up. I thought I'd throw out
my
> thoughts for comment.
>
> First, I'd like to start with a quote from the late R.W. Hamming (of the
> eponymous error-correcting codes): "The purpose of computation is
insight,
> not numbers".
>
> Modeling is wonderful! It lets you experiment in nice cozy warm comfort
> while it's raining outside, or while the sunspots are few, or while
you're
> just too darn lazy to go out and build it to try it. Especially now,
with
> 2+GHz computers available for $350 from Dell, running half a dozen
> speculative cases is so quick that there's no excuse not to.
>
> On the other hand, there's no substitute for going down to Home Depot
and
> buying 500 ft of AWG 12 THHN wire and stringing up some antenna, and
just
> giving it a shot!
>
> So where does modeling fit in the overall scheme of things? I think the
> first thing to realize is that the "professional" modeling area (which I
do
> at work) is very different from the "amateur radio" area. We have
different
> goals, different budgets, different objectives. If you're working on a
> design for an antenna for a spacecraft that costs $100 Million to
launch,
> then you can afford to spend a bit of cash to make sure it's going to
work
> before you build it. On the other hand, if you're scrounging parts at
> hamfests and want to make a good showing in the SS, maybe your money is
> better spent on a good P.E. to wet stamp the plans for that 100 foot
tower
> than on a copy of NEC4 from LLNL.
>
> There's also the issue of using models to understand what's going on, in
> general, rather than predict to the gnat's eyelash what your
forward/back
> ratio is going to be. Given that construction and environmental
tolerances
> for most antennas will be in the few percent range, expecting gain
numbers
> to be better than a few dB is unrealistic. For instance, I've been
hunting
> for a good NEC model of a tract house for years, and haven't found one
yet
> (for that matter, I haven't even found a bad model). Since the vast
majority
> of us have houses near our antennas, this would be a useful thing.
>
> So, given that modeling isn't going to give us exact answers, what good
is
> it?
>
> Going back to Hamming, it gives us insight.
>
> I don't have a model of my house, or of the rain gutters, in any sort of
> precision. However, I can make a decent assessment of the impact of my
> gutters fairly easily, using models. I model my antennas (dipoles
laying on
> the tile roof). Then, I add in wires where the gutters are
(approximately..
> within 10-20 cm). I run the model and look at the current in the
"gutter
> wires". If the current is low, I figure, hey, the gutters don't matter.
I
> compare the pattern with gutters and without. Hmm, 0.02 dB difference.
> Yep, the gutters don't matter.
>
> Or, maybe I want to know how critical the dimensions are (given that I'm
> lame with a tape measure). Build the model with the ideal dimensions.
Run
> it. Now, change the dimensions by 10cm. Run it again. Sure enough, the
> pattern looks pretty similar, but the feed point impedance has changed a
> fair amount. Hey, I've got an autotuner at the feed point: what do I
care
> about feed point impedance. What I care is that the pattern isn't
horrible.
> Whether the pattern matches, exactly, what I've modeled is immaterial.
>
> The big gotcha in modeling is losses. Ground losses, losses in
surrounding
> conductors, losses in the antenna conductors themselves.
>
> There was a great article by Brian Beezly (K6STI) in one of the ARRL
Antenna
> Compendiums describing his experience simulating a (he thought) new,
nifty
> antenna design. At first it looked great., until he started putting in
> lossy components. All of a sudden, that great performance turned not so
> great. The W8JK type antennas are notorious for this. Great
directivity,
> getting ever better as you move the two elements closer. Ooops, if you
> factor in element resistance, it doesn't look so hot, because the
element
> currents get real high, so the IR losses get huge, not to mention the
> problems in feeding an antenna with a feedpoint impedance of 0.1 ohm.
Small
> resonant loops have the same problem.
>
> Again, though, even though losses are tough to model accurately, you can
get
> a feel for the impact of the loss. Change the resistivity of the
elements
> by a factor of 2. See what happens. Does the efficiency drop like a
stone?
> Does the F/B ratio go away? Or, does it work fairly well still. A bit
more
> loss perhaps, but still a decent pattern? You've got a winner. Build
it,
> and 10 years from now, when all the joints are corroded, it will still
work
> pretty much like how it worked when you built it. On the other hand, if
> changing resitivity by 2 causes the pattern to die, and the efficiency
to go
> to heck, watch out. You'll be posting comments to TowerTalk asking
about
> what sort of climbing gear you should have, and how to keep your
tramlines
> in order, because you'll be bringing that antenna down every year for
> maintenance.
>
>
> So, use that model for insight, not to predict your run rates in the
next
> contest!
>
> 73,
> Jim, W6RMK
>
>
> _______________________________________________
>
> See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
questions and ask for Sherman, W2FLA.
>
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>
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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