[TowerTalk] the value (or not) of modeling

Wed Apr 14 18:03:03 EDT 2004

  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
  >
  >
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