There are so many programs available to model antennas with. Let’s start with a couple of others as well.

FIRST let’s look at 4NEC2. We can read a NEC2 card deck directly such as this one (making sure the units are set correctly). Here’s the deck I used for a starting point:

NEC Card Deck

CM
CE
GW 1 11 0 -32.75 70 0 32.75 70 0.0026706
GS 0 0 .3048
GE 0
EX 0 1 6 0 1 0
FR 0 1 0 0 7.15 1
RP 0 1 360 1000 89 1 1 1
RP 0 181 1 1000 -90 0 1 1
E

When imported, it generates this model:

Here is the plot of the antenna:

Which generates this horizontal pattern:

If we take and enter the same model in NECWin, we get:

Which generates this output file. As you can see, 2.108 dB gain as expected:

Finally, the horizontal antenna pattern is the normal dipole we’ve come to expect.

If you have another program, I’d love to see the model outputs as JPG files which I’ll post here.

UPDATE: Corrected some of the figure sizing

Categories: Antenna Modeling
2 Comments »

In reviewing my recent posts, I noticed that I haven’t provided a link to where you can take the online course in Antenna Modeling. You can find it in the ARRL’s online store at

http://www.arrl.org/catalog/?category=Online+Courses&words=NO-EC-004

I want to emphasize that this is a complete, thorough course for anyone who wants to learn to model antennas. It was written by L.B.Cebik W4RNL and is worth the effort to complete. The complete syllabus is available at

http://www.arrl.org/cce/syllabus.html#EC-004

There are some points to remember about the course:

1. This is probably the toughest course for most people who take it.
2. You have a mentor assigned who is there to answer your questions and confirm your completion. Too many students don’t make effective use of the instructor.
3. The only requirement for successful completion is getting 80% or better on the final examination. While there are practical exercises and quizzes, there for your personal use and the mentor can’t even see how you’ve done unless you raise a question to their attention.

If you’re REALLY interested in learning Antenna Modeling, I’d encourage you to take the course, but remember this is the MOST self-directed course in the ARRL’s continuing education program. More than any other course, you’re on your own.

If you do plan to take it, spend some time up front thinking about some important things:

• Are you really motivated to take the course? - Why do you want to take the course? If you really want to take it, then commit the time to do it, otherwise you’re just wasting money.
• What do you already know? - Do you have any experience with mathematical modeling? Computer modeling? Antenna modeling? Anything else that has some bearing on your learning? Whenever you learn something, your mind will try to link it to what you already know, so take some time recalling what you do know that you can link your knowledge to. I like to build a mind map about what I already know when I start something new. In the process, I build a list of questions that I want to answer which help guide my studies.
• How do you learn best? - Are you a visual learner? Do you learn best from diagrams or pictures? Are you a tactile learner? Do you learn better from hands on? What is your learning style? You’ll do best if you adapt the material to match your personal learning style. If you’re a hands-on learning, then plan to spend lots of time working through the examples with a modeling program. If you do better reading, then study the text. Work in your best mode for the best learning, but also work in your next best mode to reinforce what you’re learning. No matter what mode you choose as best for you, actively participate in the learning. Outline the lessons. Mind map them. Do the examples yourself. Reproduce the answers in the text.
• Can you make the time available? - Take some time and plan out the period of the course. Organize your time and figure out when you’ll get everything done. You don’t need to build a formal project plan or learn time management, in fact, the plan itself isn’t really the important thing here. What’s important is the planning!

If you don’t want to waste your money, then put some effort into the course and actively participate in your own learning. You’ll come out ready to apply Antenna Modeling techniques to a wide range of problems.

UPDATE: Updated links at the request of the ARRL

Categories: Antenna Modeling
1 Comment »

I’ll be adding parallel models in as many different modeling packages as I can. Right now, I’m planning to build models in

EZNEC          http://www.eznec.com/index.shtml
NECWin         http://www.nittany-scientific.com/
4NEC2          http://home.ict.nl/~arivoors/
Antenna Model  http://www.antennamodel.com/

I may also add MMANA-GAL and/or MININEC as well. I’d love to see parallel models from other programs as well.

Categories: Antenna Modeling
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OK, so we’ve got a basic model. Let’s take a quick look to see it’s radiation pattern. I’ll put a 1 volt source at the center of the center element. When I do this, here’s the pattern I get:

Notice that at the elevation angle of 0.0 degrees, we get 2.11 dB gain.

Now let’s have some more fun with this. I’m going to do the same setup in the program ‘Antenna Model’ which is a MiniNEC based program. This is the setup showing the geometry of the antenna as setup. You’ll notice that it’s the same as the previous setup in EZNEC:

When I run the model now, I get this pattern:

It’s hard to see on the diagram, but it also shows 2.11 dB gain at 0.0 degrees elevation. Here’s the log output from the Antenna Model program for the calculation:

----------------- Output from Antenna Model -----------------

                                 ANTENNA MODEL
                   Copyright (C) 1992-2007 Teri Software Co.

                   12-29-2008                        7:44 PM 

                            Antenna File: Untitled

                                  Free Space

               Lowest Frequency of Operation: 7.000000 megahertz
               Center Frequency of Operation: 7.150000 megahertz
              Highest Frequency of Operation: 7.300000 megahertz

              Dimensions below are in feet unless otherwise noted

----------------- Wire Statements

                  End Coordinates, Wire #1             Wire
               X             Y             Z         Diameter      Segments     Material

  End 1:   0.0000000     -32.73000     70.00000     0.005340013       11        Perfect
  End 2:   0.0000000      32.73000     70.00000                                 Conductor

  Approximate near-field/far-field boundary is 99.7610 meters or 2.37928 wavelengths
 
----------------- Warnings  

  No warnings
 
----------------- Antenna  Geometry  

  Pulse         Pulse Coordinates, Wire #1             Wire        Connections
   No.         X             Y             Z         Diameter     End 1   End 2

     1     0.0000000    -26.77909      70.00000     5.340013E-03   End       1
     2     0.0000000    -20.82818      70.00000     5.340013E-03     1       1
     3     0.0000000    -14.87727      70.00000     5.340013E-03     1       1
     4     0.0000000    -8.926364      70.00000     5.340013E-03     1       1
     5     0.0000000    -2.975455      70.00000     5.340013E-03     1       1
     6     0.0000000     2.975455      70.00000     5.340013E-03     1       1
     7     0.0000000     8.926364      70.00000     5.340013E-03     1       1
     8     0.0000000     14.87727      70.00000     5.340013E-03     1       1
     9     0.0000000     20.82818      70.00000     5.340013E-03     1       1
    10     0.0000000     26.77909      70.00000     5.340013E-03     1     End
 
----------------- Source Statements 

     Source #1
                 Pulse               Voltage             Phase
                  No.                (Volts)             (Deg)

                    5                1.00000            0.00000
 
----------------- Current  Data 

     Wire #1

      Pulse          Real         Imaginary      Magnitude       Phase
       No.          (Amps)         (Amps)         (Amps)         (Deg)

       End        0.00000E+00    0.00000E+00    0.00000E+00       ———
         1        3.43175E-03    1.72027E-03    3.83878E-03      26.624
         2        6.36910E-03    3.26170E-03    7.15570E-03      27.118
         3        8.74413E-03    4.58789E-03    9.87464E-03      27.685
         4        1.04184E-02    5.62727E-03    1.18410E-02      28.375
         5        1.12842E-02    6.33766E-03    1.29421E-02      29.320
         6        1.12841E-02    6.03698E-03    1.27975E-02      28.147
         7        1.04180E-02    5.38121E-03    1.17257E-02      27.318
         8        8.74360E-03    4.38976E-03    9.78369E-03      26.659
         9        6.36855E-03    3.12065E-03    7.09202E-03      26.105
        10        3.43136E-03    1.64538E-03    3.80546E-03      25.618
       End        0.00000E+00    0.00000E+00    0.00000E+00       ———
 
----------------- Source Data 

     Pulse #5
                           Voltage = 1.00000 + j0.00000
                           Current = 0.0112842 + j0.00633766
                         Impedance = 67.3688 - j37.8370
                             Power = 0.0112842 watts
                               SWR = 2.01937 for unmatched 50.0000 ohm feed line
 
----------------- Gain and Lobe Data 

   Azimuth profile at 0° elevation:
                               Maximum gain = 2.11 dBi at 0° azimuth
                          Azimuth beamwidth = 79.23°
                   No discernable sidelobes
                        Front-to-rear ratio = 0.00 decibels
                        Front-to-back ratio = 0.00 decibels

   Elevation profile at 0° azimuth:
                               Maximum gain = 2.11 dBi
                  No discernable major lobe
                   No discernable sidelobes

   Field at 0° azimuth, 0° elevation is linearly polarized, horizontal
 
----------------- Gain and Lobe Data, Horizontal Linear Component

   Azimuth profile at 0° elevation:
                               Maximum gain = 2.11 dBi at 0° azimuth
                          Azimuth beamwidth = 79.23°
                   No discernable sidelobes
                        Front-to-rear ratio = 0.00 decibels
                        Front-to-back ratio = 0.00 decibels

   Elevation profile at 0° azimuth:
                               Maximum gain = 2.11 dBi
                  No discernable major lobe
                   No discernable sidelobes
 
----------------- Gain and Lobe Data, Vertical Linear Component

   Azimuth profile at 0° elevation:
                               Maximum gain = -331.24 dBi at 90° azimuth
                          Azimuth beamwidth = 103.36°
                   No discernable sidelobes
                        Front-to-rear ratio = 0.09 decibels
                        Front-to-back ratio = 0.09 decibels

   Elevation profile at 0° azimuth:
                               Maximum gain = -999.00 dBi
                  No discernable major lobe
                   No discernable sidelobes
 
----------------- Gain and Lobe Data, Right Circular Component

   Azimuth profile at 0° elevation:
                               Maximum gain = -0.91 dBi at 0° azimuth
                          Azimuth beamwidth = 79.23°
                   No discernable sidelobes
                        Front-to-rear ratio = 0.00 decibels
                        Front-to-back ratio = 0.00 decibels

   Elevation profile at 0° azimuth:
                               Maximum gain = -0.91 dBi
                  No discernable major lobe
                   No discernable sidelobes
 
----------------- Gain and Lobe Data, Left Circular Component

   Azimuth profile at 0° elevation:
                               Maximum gain = -0.91 dBi at 0° azimuth
                          Azimuth beamwidth = 79.23°
                   No discernable sidelobes
                        Front-to-rear ratio = 0.00 decibels
                        Front-to-back ratio = 0.00 decibels

   Elevation profile at 0° azimuth:
                               Maximum gain = -0.91 dBi
                  No discernable major lobe
                   No discernable sidelobes

Categories: Antenna Modeling
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This will probably be a pretty slow series, but it’s fun. As long as I think I’m not boring people, I’ll keep going.

We left off wondering about the 468 vs 492 in the dipole antenna formula. If we take the speed of light, plug it into the formula for the wavelength, and then take 1/2 wavelength, we get approximately 492 instead of 468 in the standard dipole length formula. Let’s play with this a bit using an Antenna Model to see what we can get.

The Cebik article (QST ‘A Beginner’s Guide to Modeling with NEC’ at http://www.arrl.org/members-only/tis/info/pdf/0011034.pdf) builds a simple model of a 40 meter dipole cut for 1/2 wavelength at 7,15 Mhz. Let’s use the same model (well almost, I’m going to start a bit differently).

We’ll start with the standard formula

dipole length (feet) = 468 / frequency (Mhz)

= 468 / 7.15

= 65.454545 feet =~ 65.45 feet

So each leg of the dipole will be approximately 32.73 feet in length (Cebik came out 1 foot longer, but this is just a starting point, so it won’t make a difference).

Starting in EZNEC, I’m going to build a model of the antenna like this. I’ll define 1 wire like this:

Looking at the view of the antenna, it goes from -32.73 to +32.73 feet along the Y axis. It is exactly ON the X-Axis at X = 0 and is 70 feet off the ground as specified in the article.

This becomes our starting point for calculations. It’s a simple enough model, but as we go along, we’ll start to see the impact of different additions to the model.

UPDATE: replaced the original .tiff files with .jpg files

Categories: Antenna Modeling
3 Comments »

The reading material I’ll be referring to is L.B. Cebik’s QST article series ‘A Beginner’s Guide to Modeling with NEC’. I plan to discuss a number of antenna modeling programs here, so this will go on considerably longer than the original articles.

One of the best courses was originally written by L.B.Cebik W4RNL for the ARRL Online Continuing Education program. It’s pretty much the only game in town at this level. If you go to the right university, you can take courses that will cover it in far more detail, getting into the mathematics and physics much more deeply. The book seems to be out-of-print at the moment. Amazon.com lists it for sale and you may be able to find a used copy. The ARRL Antenna Book also has a very good chapter on Antenna Modeling which is worth referring to.

As we start, you can refer to the 1st Cebik article which you can find at http://www.arrl.org/members-only/tis/info/pdf/0011034.pdf. I’ll also be discussing this in the AntModel antenna modeling discussion list. Hopefully, some people will participate.

My plans are to work through the articles, doing the models and explaining why I’m doing certain things. I look forward to some level of discussion about modeling as we go along.

What is Antenna Modeling

I don’t need to explain to anyone here about the differences between antenna models and model boats and planes, but it IS worth spending some time talking about mathematical modeling and some of the issues relevant to antenna modeling.

If you really want to understand the mathematics and physics behind antenna modeling, one of the classic places to start learning is John D. Kraus’ book Antennas which is the basis for many graduate courses. I learned from an earlier edition of this book during a course in Radio Astronomy. The mathematics us far too complicated for our purposes here, but it’s fascinating to work with if you like building mathematical models.

Mathematical modeling is about creating a model of a system using mathematics as a language. I first got fascinated in mathematical models when we were still working them on slide rules. With my first exposure to computers, I fell in love and went right to work building models. It’s important to understand that models are never exact. Even the best models are limited in some respect.

Consider the simple antenna model for a dipole antenna.

Believe it or not, this is a REAL mathematical model of a 1/2-wave dipole antenna, but how exact is this. The actual speed of light is

186,282.397 0512 miles/second

or approximately

983,568,960 feet/second

AND we know that the wavelength is related to the frequency by the formula

wavelength = speed of light/frequency (Hz)

if we change frequency to MHz (1,000,000 Hz), then

wavelength (feet) = 983.282 960/frequency (Mhz)

and so a half-wavelength would be approximately (I’m using =~ to say ‘approximately equals’)

983/(2 * freq) =~ 491.5/freq =~ 492/freq

Huh? Why 492? Why didn’t we get 468 which is what the thumb-rule model gives us? The number 468 isn’t arbitrary. It has good reasons for being the choice, let’s see if we can come up with something like this working with an antenna model and in the process, learn something about building antenna models.

NOTE: some corrections above identified by italics

Categories: Antenna Modeling
6 Comments »

I first got interested in models and modeling when I was at University. I learned to build them mathematically, to do them by hand, and I learned to build them using a computer. I did a lot of modeling for departments as a way to support myself while at school. Since leaving school, I’ve built systems models, propagation models, antenna models, and many more. What always disturbs me is how little people seem to understand about models and modeling.

Consider a propagation model built into a tool like VOACAP or any other tool that can be used to predict or analyze propagation conditions. Too many people seem to handle them as if they could be expected to make good predictions without thinking critically about the nature of models in general or the specific model in particular. At best, this is unwise, at worst, it can lead to serious consequences.

Models, ANY model, always has a number of assumptions in its construction. In most programs I’ve worked with, the assumptions aren’t stated anywhere. Even when you have access to the source code, you may not be able to piece together all of the assumptions made in building the model. For one thing, models are often constructed without any real regard for internal documentation about algorithms. For another, some people seem to be addicted to tricky programming or complicated ways of doing things.

Some years back as a grad student, I was handed such a model which had been built by other grad students and asked to get it running. I found that it had NEVER run successfully. It filled a whole card box and was written in FORTRAN. What a mess. It took me a while, but I finally got it rewritten and running.

My major point here is that I’ve recently had a conversation which I’ve had over and over again with someone who acted as if a computer model had to give them good answers and that they could simply accept what the computer was providing them without question. That’s never the case.

Whenever you’re working with a computer model, whether it’s to predict radio propagation, antenna patterns, telephone call center performance, or whatever, you have to use the model critically and understand as much as you can about it:

• Do you know what assumptions were made in building the model? ()
• What algorithms are used to do the critical computations? (Is this a solid algorithm for what you want to do? There are often more than one way to accomplish any given calculation)
• What units were used to build the model? (Remember the error in landing the spacecraft on Mars?)

This is just a starting point. You need to ask questions and critically think about the model you’re working with to determine whether the answers you’re getting are sensible. Even if you’re getting sensible answers, that might not prove that the program will continue to provide sensible answers under different conditions.

Categories: General Thoughts
7 Comments »

I’ve wanted for some time to work through the basic of Antenna Modeling, to in effect update much of the work that was done by Cebik W4RNL in his many writings. He’s got some very good stuff, but I think it can be better. So on an irregular basis, I’m going to start here by using Cebik’s original ‘A Beginner’s Guide to Modeling with NEC’, a 4 part series that originally appeared in QST. I plan to go through the articles slowly, piece by piece, and work through them here in case anyone wants to follow along.

I’ve also asked if anyone wants to work through them at my Antenna Model discussion list.

Click to join AntModel

The four articles are:

Part 1: http://www.arrl.org/members-only/tis/info/pdf/0011034.pdf

Part 2: http://www.arrl.org/members-only/tis/info/pdf/0012040.pdf

Part 3: http://www.arrl.org/members-only/tis/info/pdf/0101044.pdf

Part 4: http://www.arrl.org/members-only/tis/info/pdf/0102040.pdf

I look forward to having some fun!

Categories: Antenna Modeling
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