Modeling software is a wonderful tool. It saves a lot of time, roof or tower
climbing, waste of material and more. But it must be used with caution. There
are lots of assumptions and simplifications to reduce the size and number of
equations and the huge quantity of unknown variables in the game.
As one Structural Dynamics teacher always said: "The tools for numeric
calculations are useless without the engineer's knowledge and criteria."
It is amazing that this modern tools used to calculate antenna patterns,
building structures, stress on spaceships and many other applications are based
on certain numeric methods designed two or three centuries ago, but at the time
of its creation there was no calculating machine that could handle it.
Anyway, if you like to play with antennas, it is worth the effort to learn it.
David
HK1A
EC5KXA
ex-HK1KXA
> Date: Sat, 6 Mar 2010 23:34:21 -0500
> From: olinger@bellsouth.net
> To: topband@contesting.com; antennaware@contesting.com
> Subject: Topband: Why I use modeling software
>
> When I first started using modeling software, the temptation was to
> accept those results as real and I quickly succumbed. I came up with
> some models that showed excellent gain and put one up.
>
> I quickly discovered that there were some gotcha's in modeling where
> free/inexpensive programs got lost in computer floating point issues.
>
> I discovered that my particular backyard dirt was not the same as the
> assumptions in the programs and measurements could vary wildly from
> the model, especially with low-band vertical antennas, that wire in
> and on dirt was still a considerably unsolved problem.
>
> I discovered that the simple act of winding wire around an end
> insulator changed the apparent electrical length of the wire and could
> not be handled by some programs, requiring freehand SWAG "adjustments"
> to wire lengths for end insulators.
>
> "Modeling kingdom" was a kind of magic space where wire elements came
> to a straight ends with nothing to support them. Where feedlines did
> not make the middle sag. Where one did not need a space shuttle to
> put up an antenna in "free space". Where dirt could be a perfect
> conductor. Where you could buy super-conductor wire that had no
> resistance for free. Where common mode current on feedlines never
> occurred. Where there were no miscellaneous conductors to warp and
> destroy patterns. Where one could put up an inverted L without
> including the tower system that supported it. And on and on.
>
> I started out ham life doing haul up, prune, haul up, prune.
> Accumulate anecdotal results, recycle.
>
> I have returned to that mode, BUT with some powerful new tools.
>
> I no longer trust modeling for 'pute, cut, hang and play. HOWEVER....
>
> Modeling lets me throw away the ridiculous without all the trouble to
> cut, hang and operate only to discover AFTER all the work that it's
> ridiculous.
>
> Armed with a list of gotchas to avoid, modeling allows me to find
> solutions that have no mountain of anecdotal success stories like
> dipoles and yagis, lets me discover the counter-intuitive. When I go
> to the backyard, I know the solution IS going to require haul up,
> prune, measure, and accumulation of anecdotal evidence. But I also
> know it has a far better chance of working than the proverbial
> snowball in h*ll.
>
> Modeling allows me to rediscover old solutions that have become
> neglected, and in some cases discover the essential modification that
> overcomes the issue that caused it to fall into disfavor, such as the
> single wire windom. It allows me to discover a whole other world
> beyond RG8 coax and 1:1 SWR.
>
> I cringe when I see elaborate dissertations based on no more than
> models, but the models allow me to watch the effect of adding some new
> consideration to an antenna's performance and gives me a line on what
> to expect when it gets real and measured.
>
> There are places where modeling does not explain the surprising degree
> of success, as in end-fed half-wave inverted L's. There are places
> where modeling quite precisely identifies certain modes of failure and
> mostly explains wild variations in anecdotal results as in ground
> mounted current feed HF verticals, or BOG listening antennas.
>
> Since ham radio is half antennas and half equipment, one can say we
> have two eyes, understanding antenna theory and understanding
> electronic theory. Many hams start out limited to being appliance
> operators, using both equipment and antennas designed by others, and
> clearly understanding neither.
>
> Building equipment and immersion in the fellowship of ham operators
> are but two of the ways of gaining sight in the equipment eye. But
> looking back over more than a half century of hamming, "sight" in the
> area of antennas did not really occur until we had PC's and modeling
> programs with the monstrous capacity to compute the millions of
> interactions between locations on wires and add them all up for
> display and analysis.
>
> Without learning modeling one is at least partially blind to some of
> the great pleasures of the hobby. To be quite honest, for me, getting
> started in modeling was a royal pain in the *ss. NOTHING explained
> anything. I got totally lost in the really very excellent help files
> in EZNEC.
>
> To fairly critique myself at the starting point, it is really hard to
> explain cars when one doesn't understand how a wheel rolls and has no
> knowledge of certain exploding liquids. Only immersion and a growing
> p*ssed-off determination that I was not going to allow "it" to beat me
> got me past the original confusions. It was just like 8th grade
> English class when all of a sudden one day diagramming sentences came
> clear. I neither understand the earlier confusion nor what flipped
> the switch.
>
> I completely identify with some of the complaints about getting
> started. But don't look to me for any answers about getting started
> quick. No such experience for me. One either wants to be able to
> "see" there, or wants to spend their precious spare time elsewhere. I
> get it either way. But if antenna behavior p*sses you off, I would
> offer that without understanding modeling you will stay p*ssed off,
> give up on it.
>
> On the other hand, if you tough it out getting into modeling, you will
> only be p*ssed off for six months or a year, with all that
> enlightenment on the other side.
>
> 73, Guy.
> _______________________________________________
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