Going tp put this antenna up today
 

Roy Lewallen wrote:
Jim Lux wrote:

I doubt that any antenna an amateur is likely to build has some
physics not adequately modeled by the math in NEC. Exactly what might
be these discrepancies in equations, formulae and assumptions?
Considering that NEC has had decades of validation against actual
measurements and a lot of really, really smart people looking at how
it works, I'd be kind of surprised.

However, I can also easily believe that an amateur (or professional)
could build an antenna that has measured performance different than
expected from their NEC model of that antenna. The differences would
lie, most likely, in these areas:
1) Inaccuracies in the model itself. Things like earth properties (NEC
assumes uniform dielectric, it isn't) are an important source of error
for antennas close to the ground. Most amateur models do not include a
very good model of the surroundings (supports, trees, feedlines, etc.)

2) Inaccuracies in the measurements or not measuring the right things.
A good example is using NEC to get feedpoint characteristics, then
measuring at the rig, and not properly accounting for the transmission
line, particularly if the feedpoint Z is reactive.

In the professional antenna world, if someone models an antenna, then
builds it and tests it on the range, and the measurements differ from
what the model predicted, the usual assumption is that what was built
differed from what was modeled, or the measurements were off.

A lot of people are aware that some knowledge and skill is necessary in
order to construct a good model which will give accurate results. What's
not so widely appreciated is how difficult it is to make decent
measurements of even such seemingly simple things as impedance, let
alone gain and pattern.

Some years ago I was at what was then NOSC (Naval Ocean Systems Center)
in San Diego, where they had the very best equipment to do full 3D
pattern measurements of antennas mounted on carefully constructed and
accurate scale models of U.S. Navy ships. They were also modeling the
antennas and ship structures with NEC-4. One of the engineers confided
to me that they'd learned that when the measurement disagreed with the
model result, the model result was probably better. Of course, these
people were very highly skilled in using the modeling software and how
to avoid and detect its limitations. But they were also very highly
skilled in making the best possible measurements.

A claim by an antenna manufacturer or creator that an antenna "can't be
modeled by NEC" very often means that NEC's accurate results don't back
up the manufacturer's or creator's inflated claims. If the claim
includes a statement that some alleged physical rule or phenomenon
("critical coupling" comes to mind) isn't "accounted for" by NEC, you
can be certain that the disparity is due to inflated claims rather than
a shortcoming of NEC. There are, of course, some antennas that truly
can't be modeled with NEC, for example a patch antenna with dielectric
between the patch and ground plane. But those of NEC's limitations that
aren't apparent from the basic program structure have been pretty well
discovered and documented in the 30 years or so it's been in use.

Antennas are being designed every day with NEC and EZNEC which provide
critical communications functions for military and government agencies,
aerospace companies, space agencies, domestic and international
broadcasters, scientific researchers, and many others. Many of those
antennas have been carefully measured and verified before being put into
service, and countless of them are in current use. I personally have
designed a number of antennas with EZNEC, tested physical prototypes at
a professional range, and seen them put into daily service performing
critical functions as designed.

Roy Lewallen, W7EL


Oh, I C ... You will excuse me then, I have had a bit too much to drink,
to be coherent ... but then, from your response, you are in the same
shape, right? :-)

Regards,
JS

Going tp put this antenna up today
 

Roy Lewallen wrote:

...
Roy Lewallen, W7EL

Gawd, that is a lot Roy. Like I have said before, I have read your
articles, since I was a boy ...

And, I shouldn't even be here, from the party ... but then, ya' already
know that ... wink

Did I get that right? I mean, I like what I'm drinkin', what are you
drinin', can I get you one? goofy-smile

Regards,
JS

Going tp put this antenna up today
 

Correction:

Roy Lewallen wrote:
. . . An experienced
EZNEC or NEC modeler would immediately run an Average Gain test and seen
that the reported gain is about 16.7 dB too high due to numerical
problems. If desired, this can be subtracted from the reported gain to
give the gain that's much closer to reality.

I should have said "experienced *and not careless*". The average gain is
about 30.4 dB, not 16.7.

. . . In this case, the model
violates at least one NEC rule by spacing two 40-foot #14 wires 1/2 wire
diameter (0.032") apart. (NEC guidelines specify that parallel wires
should be at least several wire diameters apart.) It would be a great
challenge to actually construct this antenna. But, even at that, the NEC
results are probably not bad once the average gain correction is made.

The wires are just about one wire diameter, not 1/2, apart.

This antenna is a very difficult one for NEC. In a few minutes of
fiddling, I couldn't get a decent average gain at much less than about
0.3 foot wire spacing, and then it required specific segmentation. I'll
modify my earlier statement to say that this is an antenna that is
indeed pretty difficult for NEC to handle. But it's easily recognized as
such by looking at the average gain result. If you don't know about the
average gain test, look in the EZNEC manual -- included also with the
demo program -- index under "Average Gain". It's an important and
valuable tool.

Roy Lewallen, W7EL

Going tp put this antenna up today
 

Roy Lewallen wrote:
All it shows is that models can be carelessly or intentionally
constructed in such a way as to cause NEC to malfunction.

SUPRGAIN.EZ was a complete accident from many years ago.
I noticed that the closer together the wires were, the
higher the reported gain. When it got to about 5 dBi
omni, I knew I had done something wrong.

All I was trying to do is show there are certain things
that NEC cannot do. I suspect that modeling Art's antenna
is one of those things.
--
73, Cecil http://www.w5dxp.com
"According to the general theory of relativity,
space without ether is unthinkable." Albert Einstein

Going tp put this antenna up today
 

Cecil Moore wrote:
Roy Lewallen wrote:
All it shows is that models can be carelessly or intentionally
constructed in such a way as to cause NEC to malfunction.

SUPRGAIN.EZ was a complete accident from many years ago.
I noticed that the closer together the wires were, the
higher the reported gain. When it got to about 5 dBi
omni, I knew I had done something wrong.

All I was trying to do is show there are certain things
that NEC cannot do. I suspect that modeling Art's antenna
is one of those things.

Let me see, is there a way to "carelessly or intentionally" construct
data into my formula to compute the volume of a sphere to make the
formula "malfunction?"

Nope! Guess that formula is returning valid results! Nice being able
to have real confidence in it! ;-)

Regards,
JS

Going tp put this antenna up today
 

On Fri, 03 Oct 2008 10:20:12 -0700, John Smith
wrote:

Let me see, is there a way to "carelessly or intentionally" construct
data into my formula to compute the volume of a sphere to make the
formula "malfunction?"

Nope! Guess that formula is returning valid results! Nice being able
to have real confidence in it! ;-)

What is the volume of a sphere with the measured dimension of
a chord = 1 meter?

That was too easy!

73's
Richard Clark, KB7QHC

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Richard Clark wrote:


What is the volume of a sphere with the measured dimension of
a chord = 1 meter?

That was too easy!

73's
Richard Clark, KB7QHC

Ahhh, once again that magnificent intellect of yours is displayed ...

However, somehow, it has lost its' shock value ... :-|

Regards,
JS

Going tp put this antenna up today
 

On Fri, 03 Oct 2008 12:31:34 -0700, John Smith
wrote:

On Fri, 03 Oct 2008 10:20:12 -0700, John Smith
wrote:

Let me see, is there a way to "carelessly or intentionally" construct
data into my formula to compute the volume of a sphere to make the
formula "malfunction?"

Nope! Guess that formula is returning valid results! Nice being able
to have real confidence in it! ;-)

What is the volume of a sphere with the measured dimension of
a chord = 1 meter?

That was too easy!


Ahhh, once again that magnificent intellect of yours is displayed ...

However, somehow, it has lost its' shock value ... :-|

Which is to say even with your confidence and your formula that you
don't know. It demonstrates you can neither construct careless nor
careful data. This was anticipated and all would note that Sarah Lee
could answer her own questions even if she couldn't answer others'.
(Did they draft Cecil to Sedona last weekend for her rehearsals?)

Brett, let's just cut to the chase and point out all of your
"objections" to modeling are offered with rather pale logic and even
less demonstrables.

Simpler yet, you are just trolling - but that isn't groundbreaking
news here....

73's
Richard Clark, KB7QHC

Going tp put this antenna up today
 

Richard Clark wrote:


Brett, let's just cut to the chase and point out all of your
"objections" to modeling are offered with rather pale logic and even
less demonstrables.

Simpler yet, you are just trolling - but that isn't groundbreaking
news here....

73's
Richard Clark, KB7QHC

Hey, I do a lot of what some would consider "illogical", like a belief
in a superior being/race!

However, the failing of NEC need not be taken as a leap of faith, they
can be demonstrated ...

But, ya'all wouldn't know a real troll if it bit ya' on the arse ...

Regards,
JS

Going tp put this antenna up today
 

John Smith wrote:
[chit]

Richard Clark wrote:
...
Simpler yet, you are just trolling - but that isn't groundbreaking
news here....

73's
Richard Clark, KB7QHC

Or, let me summarize, by stating an example:


Take two boxes, mark one "TROLLS", the other "NON-TROLLS."

Now, put members of this group (or from your life experience--that'll
work also) into the two boxes, sorting as you see fit.

Now, if you notice that one box is full of all those who agree with you
and who you worship, and the other is full of all those not agreeing
with you--you don't have a "troll problem" at all!

Regards,
JS