On Mar 12, 2:27 pm, Roy Lewallen wrote:
wrote:
On Mar 12, 11:01 am, Art Unwin wrote:
Scott, you are introducing sharp corners to your new antenna so NEC
will stray from accuracy.

Is NEC4 better in this regard?

Neither NEC-2 nor NEC-4 has any problem in this regard. Art's statement
is incorrect as quoted.

Roy Lewallen, W7EL

That really is good news. Capacitive coupling between close spaced
wires,
progressive change in capacitive coupling to ground from a
verticle ,or wires
at different spacings and closeness must be a devil to calculate tho I
suppose you can also
insert fudge factors for alignment from impirical results which I
presume is the reason
for program modifications despite the inflexibility to change of
Maxwells laws.
Well you highlight one of the answers the gentleman was asking for so
a forthcoming
complete answer from you should give him closure on this subject
Art.

Art Unwin wrote:
On Mar 12, 2:27 pm, Roy Lewallen wrote:

wrote:

On Mar 12, 11:01 am, Art Unwin wrote:

Scott, you are introducing sharp corners to your new antenna so NEC
will stray from accuracy.

Is NEC4 better in this regard?

Neither NEC-2 nor NEC-4 has any problem in this regard. Art's statement
is incorrect as quoted.

Roy Lewallen, W7EL


That really is good news. Capacitive coupling between close spaced
wires,
progressive change in capacitive coupling to ground from a
verticle ,or wires
at different spacings and closeness must be a devil to calculate

tedious, but not complex. Like all Finite Element approaches, you
basically break the problem up into little tiny chunks and rigorously
apply basic laws of physics. Charge is charge.

The challenge is not in the theory of operation, but in the practical
implementation.


There's also the practical implementation detail of taking a high level
description of something and turning it into many, many smaller pieces.
(For instance, turning an arbitrary 3-d shape into lots of little plane
triangles or quadrilaterals). You want a small number of pieces so the
computational work is less (many of these techniques have "work" that
goes as the cube of the number of pieces, so going from 10 segments to
100 segments takes 1000 times as much computation), but also you want
the pieces small enough that they approximate the original continuous
curve to an adequate level of accuracy (the calculus problem)


As you make the chunks smaller, round off errors and numerical precision
become a bigger issue (e.g. on a computer with finite precision, summing
a million millionths might not equal one). So even if you have a 1000
processor Beowulf cluster, it might not help.



tho I
suppose you can also
insert fudge factors for alignment from impirical results which I
presume is the reason
for program modifications despite the inflexibility to change of
Maxwells laws.


No fudge factors in NEC. What changes there are between versions do
things like:

handle insulated wires, or wires embedded in a medium other than free
space. If you accept the constraint of uninsulated wires in a vacuum,
you can simplify the equations, which takes less computation (i.e. you
don't have to take epsilonr or sigma into account). Since air is pretty
close to a vacuum, and most people build antennas out of uninsulated
wires, the first version of NEC did the easy case.

better approximations of the charge distribution on the segment from a
numerical analysis standpoint. i.e. rather than using sin(x) for values
of x near pi/2, where small changes in x result in very small changes in
sin(x), you use 1-cos(x). You could have also just used a zillion
digit sin calculation, but that gets back to the computational
efficiency thing.

More accurate calculations of the interaction between chunks. NEC
essentially calculates the coupling between every possible pair of
segments in your model. Calculating coupling between two segments some
distance apart assuming the segment is very much smaller in diameter
than the spacing and where they are parallel is fairly straightforward.
Calculating coupling between two conductors of diameter d, separated
by a distance close to d, with them at an angle, is a bit tougher. (a
lot of it is back to the issue of precision of trig functions)

Easier ways to define a model. NEC4 includes a function to enter a wire
with a catenary curve. For NEC2, you'd have to do that outside, and
then enter the wire as a series of smaller wires.


There's a readily available paper out there that explains all the
improvements from NEC2 to the later versions, and how they were
experimentally validated.

{https://e-reports-ext.llnl.gov/pdf/210389.pdf}

Well you highlight one of the answers the gentleman was asking for so
a forthcoming
complete answer from you should give him closure on this subject
Art.


Here's the reference to the paper by Burke:
Title: Recent advances to NEC (Numerical Electromagnetics Code):
Applications and validation
Authors: Burke, G. J.
Affiliation: Lawrence Livermore National Lab., CA.
Journal: Presented at the Conference on Modern Antenna Design Using
Computers and Measurement Application To Antenna Problems of Military
Interest, Ankara, Turkey, 19-20 Oct. 1989
Publication Date: 03/1989
Category: Communications and Radar
Origin: STI
NASA/STI Keywords:
ANTENNA DESIGN, COMPUTER AIDED DESIGN, COMPUTER PROGRAMS, ELECTROMAGNETISM,
MATHEMATICAL MODELS, ELECTRICAL INSULATION, INTEGRAL EQUATIONS, WIRING
Bibliographic Code:
1989STIN...9011917B

Abstract

Capabilities of the antenna modeling code NEC are reviewed and results are
presented to illustrate typical applications. Recent developments are
discussed that will improve accuracy in modeling electrically small
antennas, stepped-radius wires and junctions of tightly coupled wires, and
also a new capability for modeling insulated wires in air or earth is
described. These advances will be included in a future release of NEC,
while for now the results serve to illustrate limitations of the present
code. NEC results are compared with independent analytical and numerical
solutions and measurements to validate the model for wires near ground and
for insulated wires.

Dale Parfitt wrote:
wrote in message
...
Hi,

So I've spent months, rather years, carefully designing a new 5 band
cubical quad for myself. I've always known that I would use #12 solid
copper wire (not stranded), so that is what I used when running NEC2
to optimize this touchy antenna design (over millions of iterations).

Well, real world things are starting to happen. For wire, I've decided
what I would like to use is an enameled coated copperweld wire. I'll
buy the plain copperweld wire and coat it myself.

I think I would spend the extra money and buy copper wire. One nick in your
coating and the copper surface and the wire will disappear. If you must use
copperweld, try The Wireman or The RF Connection for copperweld that has a
high density black polyethylene jacket.

I would hate to spend all the time to design and build a quad and then have
the wire be the weakest link.

Dale W4OP



Dale
He already said he didn't want to use insulated wire because of the
weight of the insulation.
--
Tom Horne

"This alternating current stuff is just a fad. It is much too dangerous
for general use." Thomas Alva Edison

On Mar 12, 6:13 pm, Jim Lux wrote:
Art Unwin wrote:
On Mar 12, 2:27 pm, Roy Lewallen wrote:

wrote:

On Mar 12, 11:01 am, Art Unwin wrote:

Scott, you are introducing sharp corners to your new antenna so NEC
will stray from accuracy.

Is NEC4 better in this regard?

Neither NEC-2 nor NEC-4 has any problem in this regard. Art's statement
is incorrect as quoted.

Roy Lewallen, W7EL

That really is good news. Capacitive coupling between close spaced
wires,
progressive change in capacitive coupling to ground from a
verticle ,or wires
at different spacings and closeness must be a devil to calculate

tedious, but not complex. Like all Finite Element approaches, you
basically break the problem up into little tiny chunks and rigorously
apply basic laws of physics. Charge is charge.

The challenge is not in the theory of operation, but in the practical
implementation.

There's also the practical implementation detail of taking a high level
description of something and turning it into many, many smaller pieces.
(For instance, turning an arbitrary 3-d shape into lots of little plane
triangles or quadrilaterals). You want a small number of pieces so the
computational work is less (many of these techniques have "work" that
goes as the cube of the number of pieces, so going from 10 segments to
100 segments takes 1000 times as much computation), but also you want
the pieces small enough that they approximate the original continuous
curve to an adequate level of accuracy (the calculus problem)

As you make the chunks smaller, round off errors and numerical precision
become a bigger issue (e.g. on a computer with finite precision, summing
a million millionths might not equal one). So even if you have a 1000
processor Beowulf cluster, it might not help.

tho I

suppose you can also
insert fudge factors for alignment from impirical results which I
presume is the reason
for program modifications despite the inflexibility to change of
Maxwells laws.

No fudge factors in NEC. What changes there are between versions do
things like:

handle insulated wires, or wires embedded in a medium other than free
space. If you accept the constraint of uninsulated wires in a vacuum,
you can simplify the equations, which takes less computation (i.e. you
don't have to take epsilonr or sigma into account). Since air is pretty
close to a vacuum, and most people build antennas out of uninsulated
wires, the first version of NEC did the easy case.

better approximations of the charge distribution on the segment from a
numerical analysis standpoint. i.e. rather than using sin(x) for values
of x near pi/2, where small changes in x result in very small changes in
sin(x), you use 1-cos(x). You could have also just used a zillion
digit sin calculation, but that gets back to the computational
efficiency thing.

More accurate calculations of the interaction between chunks. NEC
essentially calculates the coupling between every possible pair of
segments in your model. Calculating coupling between two segments some
distance apart assuming the segment is very much smaller in diameter
than the spacing and where they are parallel is fairly straightforward.
Calculating coupling between two conductors of diameter d, separated
by a distance close to d, with them at an angle, is a bit tougher. (a
lot of it is back to the issue of precision of trig functions)

Easier ways to define a model. NEC4 includes a function to enter a wire
with a catenary curve. For NEC2, you'd have to do that outside, and
then enter the wire as a series of smaller wires.

There's a readily available paper out there that explains all the
improvements from NEC2 to the later versions, and how they were
experimentally validated.

{https://e-reports-ext.llnl.gov/pdf/210389.pdf}

Well you highlight one of the answers the gentleman was asking for so
a forthcoming
complete answer from you should give him closure on this subject
Art.

Here's the reference to the paper by Burke:
Title: Recent advances to NEC (Numerical Electromagnetics Code):
Applications and validation
Authors: Burke, G. J.
Affiliation: Lawrence Livermore National Lab., CA.
Journal: Presented at the Conference on Modern Antenna Design Using
Computers and Measurement Application To Antenna Problems of Military
Interest, Ankara, Turkey, 19-20 Oct. 1989
Publication Date: 03/1989
Category: Communications and Radar
Origin: STI
NASA/STI Keywords:
ANTENNA DESIGN, COMPUTER AIDED DESIGN, COMPUTER PROGRAMS, ELECTROMAGNETISM,
MATHEMATICAL MODELS, ELECTRICAL INSULATION, INTEGRAL EQUATIONS, WIRING
Bibliographic Code:
1989STIN...9011917B

Abstract

Capabilities of the antenna modeling code NEC are reviewed and results are
presented to illustrate typical applications. Recent developments are
discussed that will improve accuracy in modeling electrically small
antennas, stepped-radius wires and junctions of tightly coupled wires, and
also a new capability for modeling insulated wires in air or earth is
described. These advances will be included in a future release of NEC,
while for now the results serve to illustrate limitations of the present
code. NEC results are compared with independent analytical and numerical
solutions and measurements to validate the model for wires near ground and
for insulated wires.

Jim, that was a comprehensive answer for the gen tleman. Probably more
information
that he can deal with.Seems like it is quite easy to have a yagi
behave when using NEC2
but I still hear stories of the diffuculty of tuning them after
following the nec2 instructions.
Never had a quad before so I am really out in left field on that one
Regards
Art

Jim Lux wrote:

tedious, but not complex. Like all Finite Element approaches, you
basically break the problem up into little tiny chunks and rigorously
apply basic laws of physics. Charge is charge.

The challenge is not in the theory of operation, but in the practical
implementation.

. . .

The last time I read any of Art's postings, he was using AO, which is
MININEC-based. MININEC has a number of peculiarities due to choices made
in its implementation, including problems with close spaced parallel
wires, wires connected at an angle, and very simplified ground model.
(See "MININEC: The Other Edge of the Sword", _QST_, Feb. 1991 for more
detail.) NEC-2 and NEC-4 also have peculiarities due to their
implementations, but ones which are different from MININEC and in some
cases from each other.

Roy Lewallen, W7EL

Scott, WU2X wrote:
"I`ve always known that I would use #12 solid copper wire (not stranded)
so that is what I used when running NEC2 to optimize this touchy antenna
design (over millions of iterations).

Bill Orr, W6SAI devotes Chapter 9 in "All About Cubical Quad Antennas"
to tuning and adjustment.

John Devoldere, ON4UN wrote on page 13-52 of the 2nd edition of
"Low-Band DXing:
"I designed the quad (for 80-m) with two quad loops of identical length
(for a 2 mm OD conductor or no.12 wire).

William I.(Bill)Orr, W6SAI wrote on page 77 of the 2nd edition of "All
About Cubical Quad Antennas";
'the individual gamma devices are made of #12 solid copper wire and a
small variable capacitor."

Ed Laport pictures a Wind Turbine Company insulator used for two wire
balanced lines on page 485 of "Radio Antenna engineering". These were
used in WW-2 Signal Corps fhombic antenna kits to support the 600-ohm
feedline. The wire was a cable made from (3) #12 twisted Copperweld
wires. This same cable also was used to make the rhombic curtain which
contained (3) of these Copperweld cables. I used many miles of this
cable and never saw a breakage in normal use despite years of aging in
all weather and the fact that we were using 100 KW in an antenna kit
designed for 5 KW. We did redesign the stainless steel dissipation lines
to withstand the high power. With new dissipation lines installed we got
many letters of complaint from South America from listeners who had
benefited from our bidirectional antennas which were intended to cover
central Europe only. Too bad, but other broadcasters claimed the target
area we were temporarily occupying in South America.

Best regards, Richard Harrison, KB5WZI

Jim Lux writes:

Here's the reference to the paper by Burke:
Title: Recent advances to NEC (Numerical Electromagnetics Code):
Applications and validation
Authors: Burke, G. J.
Affiliation: Lawrence Livermore National Lab., CA.
Journal: Presented at the Conference on Modern Antenna Design Using
Computers and Measurement Application To Antenna Problems of Military
Interest, Ankara, Turkey, 19-20 Oct. 1989
Publication Date: 03/1989

Thanks.

Recent advances, dated 1989.

What's been going on during the 18 years since then? Polishing NEC-4,
and developing other codes for specialized purposes, presumably. I
know it's a big question, but maybe somebody here can tell us
something.

73
LA4RT Jon, Trondheim, Norway

Ok, so I guess my subject line might have open the door to this
discussion. So NEC4 can model insulated wire, which seems like it
might be helpful for me. I see the price is down to 300$ now for US
non-commercial. Now if I can just get them to answer my email. So what
is included in the package - its not listed on the LLNL website. Does
NEC4 use the same input deck as NEC2? I assume there are maybe some
more input cards? What about the output?

I wrote my own cubical quad optimizing software that reads and writes
to the NEC2 input deck and reads the output directly - hopefully it
wouldn't require to much change to adapt it to use a NEC4 engine
instead.

-Scott, WU2X

Jon Kåre Hellan wrote:
Jim Lux writes:


Here's the reference to the paper by Burke:
Title: Recent advances to NEC (Numerical Electromagnetics Code):
Applications and validation
Authors: Burke, G. J.
Affiliation: Lawrence Livermore National Lab., CA.
Journal: Presented at the Conference on Modern Antenna Design Using
Computers and Measurement Application To Antenna Problems of Military
Interest, Ankara, Turkey, 19-20 Oct. 1989
Publication Date: 03/1989


Thanks.

Recent advances, dated 1989.

What's been going on during the 18 years since then? Polishing NEC-4,
and developing other codes for specialized purposes, presumably. I
know it's a big question, but maybe somebody here can tell us
something.

That's about it. NEC-4 is sort of about as good as you can get for a
"wires" based MoM code. The math isn't changing, the implementation does
the math, etc.


These days, it's other modeling techniques that are being developed:
various FDTD schemes
various voxel based schemes
all manner of harmonic balance

lots of work on automatic meshing (i.e., take the mechanical solid model
from Unigraphics or Pro/E or Solidworks or whatever and turn it into
something that a EM code can process)







73
LA4RT Jon, Trondheim, Norway

wrote:
Ok, so I guess my subject line might have open the door to this
discussion. So NEC4 can model insulated wire, which seems like it
might be helpful for me. I see the price is down to 300$ now for US
non-commercial. Now if I can just get them to answer my email. So what
is included in the package - its not listed on the LLNL website. Does
NEC4 use the same input deck as NEC2? I assume there are maybe some
more input cards? What about the output?

yes.. input is basically the same as NEC2, as is the output. A few new
cards (catenaries, for instance)




I wrote my own cubical quad optimizing software that reads and writes
to the NEC2 input deck and reads the output directly - hopefully it
wouldn't require to much change to adapt it to use a NEC4 engine
instead.

Almost none.



-Scott, WU2X