[TowerTalk] the value (or not) of modeling
Gene Smar
ersmar at comcast.net
Wed Apr 14 12:25:01 EDT 2004
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 at skynet.be>
To: "Jim Lux" <jimlux at earthlink.net>; "Towertalk" <towertalk at 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 at earthlink.net>
> To: "Towertalk" <towertalk at 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.
> >
> > _______________________________________________
> > TowerTalk mailing list
> > TowerTalk at 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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