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Re: [TowerTalk] the value (or not) of modeling

To: "on4kj" <on4kj@skynet.be>, "Jim Lux" <jimlux@earthlink.net>,"Towertalk" <towertalk@contesting.com>
Subject: Re: [TowerTalk] the value (or not) of modeling
From: "Gene Smar" <ersmar@comcast.net>
Date: Wed, 14 Apr 2004 11:25:01 -0400
List-post: <mailto:towertalk@contesting.com>
TT:

     As I've written on TT before:  The difference between theory and
practice in theory is less than the difference between theory and practice
in practice.

     Put up a wire and get on the bands.

73 de
Gene Smar  AD3F

----- Original Message ----- 
From: "on4kj" <on4kj@skynet.be>
To: "Jim Lux" <jimlux@earthlink.net>; "Towertalk" <towertalk@contesting.com>
Sent: Wednesday, April 14, 2004 11:03 AM
Subject: Re: [TowerTalk] the value (or not) of modeling


>   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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>   > TowerTalk@contesting.com
>   > http://lists.contesting.com/mailman/listinfo/towertalk
>   >
>
>
> _______________________________________________
>
> 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.
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk

_______________________________________________

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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