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Anyone know where I can find a model for the above antenna, tried KQ6QV, its there but have error on my attempt to use the file for the cm-4221, segment errors.
Yeah, there seems to be a trick to that, as his files are .ez not .nec. What I do is download his files, right click and choose open with. Then browse to the 4nec2x.exe program. It opens the file with warnings, but they work anyway and are converted to nec files. Ive also downloaded and installed the files for additional segments up to 10,000 (I think, I have run a 5600 segment (xg91) far field pattern so far).
 

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Fractals - 4nec2 - Re-Think

Hi Folks,

300ohm... I took your advice and installed 4nec2. I browsed through "A Beginners Guide to Modeling with NEC" by L. B. Cebik. His explanation was pretty understandable. So, I jumped right into the fractal examples that were installed with the 4nec2 software. Very interesting. I have no idea how to re-scale the examples for use in the 470 MHz to 700 MHz range.

Actually, I think I am getting way ahead of myself. I better learn how to use 4nec2 to model something like a simple dipole first. LOL... Somethings in life are humbling.

Have a Great Day,
DTV Student
 

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I browsed through "A Beginners Guide to Modeling with NEC" by L. B. Cebik. His explanation was pretty understandable.
That sounds very good. Do you have a link to download without being an ARRL member ?

When I first started, I went thru the tutorial in the _GetStarted.txt file in the 4nec2 download.

Very interesting. I have no idea how to re-scale the examples for use in the 470 MHz to 700 MHz range.
Basically, to start, if the frequency you want is higher than the model you would scale all the sizes smaller in proportion to the wavelength ratio, ie wavelength you want divided by the wavelength of the model. Vice-versa if the frequency you want is lower than the model.
 

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Hi Folks,

I am not sure where I found "A Beginners Guide to Modeling with NEC" by L. B. Cebik. Perhaps it was in some of the extras I found at Arnie Voors' website (http://home.ict.nl/~arivoors/). I am not a member of ARRL. I bet they have lots of great background information too.

I found the _GetStarted.txt file and have just started reading it. It looks like it will take some serious time to work through this tutorial. I am sure I will have lots of questions.

Thanks for the information and encouragement.
DTV Student
 

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I did find parts two and three of "A Beginners Guide to Modeling with NEC" by L. B. Cebik by googling and downloaded the pdf files, but Im missing part one and is there more after part three ?
EDIT: Never mind, all 4 parts are on Ari's site like you said. I cant believe I missed them. He really needs to rename those files from nec_part1 to something like BeginnersGuideNEC_part1, heh.

One of things Cebik mentions is very important to beginners, that is to think in 3D coordinates X, Y, and Z. And to use standard conventions in doing so, ie the Z axis is always up and down, the X axis is towards the station and away from it, and the Y axis is right and left.

As I also carve decoys and other things for a hobby, I had no problem with thinking in those 3D coordinates. And Im also familiar, for the most part with all the antenna terminology. My brain freezes with it lie elsewhere, heh. But if you keep at it, eventually it gets to be pretty easy.

Another thing thats helpful for people starting modeling is, after going thru the examples in the _GetStarted.txt file, measure and model a UHF/VHF antenna that you already own, the cheaper, the simpler, the better.
 

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Getting Started with 4NEC2

Hi Folks,

I finally made it through the _GetStarted.txt file. WOW! The first half was not so bad. I was able to work through the examples and I even had a vague idea of what the plots were trying to tell me. I really liked exploring the 3D far field plots. LOL... then I got to the Smith Charts. Yikes!!! Time to do some back tracking on Wikipedia.

I have found many references to L. B. Cebik's website and a huge number of articles he wrote on antenna designs and modeling. I remember looking at some of his work last year when I first started reading about basic antenna design. Mr. Cebik, W4RNL, was still writing articles for antennaX at that time. It is my understanding that Mr. Cebik passed away in April 2008. He will be greatly missed.

Many of his articles are no longer freely available. I guess antennaX has to make a buck too.

Are there other antenna modeling tutorials out there written at the same level and style of Mr. Cebik?

Thank you,
DTV Student
 

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Are there other antenna modeling tutorials out there written at the same level and style of Mr. Cebik?
I believe Cebik list some in his sources at the end of each of the sections of "A Beginners Guide to Modeling with NEC".

NEC modeling on a PC is relatively new. Originally Lawrence Livermore Labs bid on a Cray 1 supercomputer in 1977 to do the work for $8 million. It did 160 mips and had 2 megs of memory, heh.

My lowly E2180 dual core does 23,212 mips and I have 2 gigs of memory. At least 145 times as fast as the above Cray 1. So until recently, computations with like 7000 segments, that may take me 13 minutes, used to take a lot of time.
 

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4NEC2 - A Very Cool Tool

Hi Folks,

I have been busy exploring 4NEC2. I wish I understood one hundredth of what this software package is all about. I am very impressed!

It takes some serious time playing around with the graphical editor to get used to all of its features. I keep stumbling into new features. Very nice indeed.

Being able to switch back and forth between the FOUR editors is also very helpful especially when one want to apply a "voltage source" to one's latest antenna idea. LOL... I think I finally figured out how to work around that problem by looking at the EX record or "card" in a few of the models provided with the software.

I have worked through the phasing harness problem for the vertically stacked 4 bay bowtie designed I mentioned in another thread. I first drew the four bowtie stack. Then I added the parallel and equal length harness. I learned a lot about the editor just by building that harness. I saved that file, copied it, renamed it and edited the new file to add a crossover harness like the CM4221. I also did a third design one similar to the second but with no crossover. LOL... I had heard that bowties in this sort of configuration cancel each other out. Sure enough 4NEC2 predicted this design to be a failure. It is nice to know theory matches up with others' experiences.

A little later on, I added a large relector grid. The first 4 or 5 grids had to many segments for the software to process. I kept reducing the number of vertical and horizontal elements in the grid until the software stopped complaining. I would have been better off to have used a series of horizontal rods instead of the built-in reflector grid. The reflector, of course, made a big difference in performance. The reflector grid increased the processing time beyond belief. One can get a good idea of how something like the harness problem might work by leaving off the reflector grid and comparing the two designs by looking at the horizontal gain patterns and/or the forward gain chart from 470 to 700 MHz. Frequency scans are so cool !!!

Maybe I will clean up these three examples and post them for comments. I am still a little confused about the "Source/Load" concept(s).

Have a Great Day,
DTV Student
 

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It takes some serious time playing around with the graphical editor to get used to all of its features. I keep stumbling into new features. Very nice indeed.
Yep. Have you tried selecting all the elements (either by marking or Ctrl-A) in the graphical editor and moving the whole antenna around or copying it to create a stack ? Now, thats really cool.
 

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I am still a little confused about the "Source/Load" concept(s).
Source is a power / frequency source. If you are using one, you are modelling the antenna as a transmitter. However, due to "reciprocity" most of the behaviour of an antenna is equivalent for reception too. (4nec2 doesn't handle modelling reception, so you'd need to use NEC2 directly and do the pre & post-processing yourself.) It's best to stick to one source in your model.

Loads come in multiple types. Spot loads are like soldering a physical resistor, inductor or capacitor to your antenna (LD 0/1). You can also specify the impedance (LD 2/3) or conductance (LD 5) of the materials used to build your antenna.
 

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4NEC2 - Struggling with Concepts

I have worked through the phasing harness problem for the vertically stacked 4 bay bowtie designed I mentioned in another thread. I first drew the four bowtie stack. Then I added the parallel and equal length harness. I learned a lot about the editor just by building that harness. I saved that file, copied it, renamed it and edited the new file to add a crossover harness like the CM4221. I also did a third design one similar to the second but with no crossover. LOL... I had heard that bowties in this sort of configuration cancel each other out. Sure enough 4NEC2 predicted this design to be a failure. It is nice to know theory matches up with others' experiences.
Hi Folks,

Clearly, I am getting way ahead of myself. I got so involved with exploring the graphical editor, generating the far field patterns and drooling over the 3D viewer results, especially the "Total Gain and Multi-Color" mode, that I totally missed what the Azimuth and Elevation angles were all about. LOL... Back to basics.

Scratch my earlier comments about the 4 bay bowtie with no crossover in the phasing harness. Appearently, I was looking at some part of the Total Gain Pattern that had a circular shape and a value less than 0 dBi. Who knows? When I came back to it later, it looked very different than I had remembered.

I better take a short course in what Azimuth and Elevation angles are all about. Your comments and/or explanations are greatly appreciated.

Thanks,
DTV Student
 

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ericball,
What specifically would you recommend for a source for a reception only uhf antenna ?
For reception, you need to use EX 1, 2 or 3 cards for the incident wave and a PT card to print out the resulting currents. Unfortunately, I haven't been able to find a lot of info on this kind of modelling, so don't take my examples as gospel.
Code:
EK			' extended thin wire kernel
GN -1			' no ground - free space
FR 0 55 0 0 470 6	' 470-794MHz (UHF ch 14-68)
EX 1 1 1 0 90 0 90 0 0	' horizontal (Y) polarized wave from +X
PQ -1			' don't print currents
PT 1 999 2 2		' print current for segment 2 in wire 999
XQ			' execute
EN			' end
This is preceded by the usual geometry and loading cards. Segment 2 in wire 999 is where the source would normally go. Instead I use a LD 4 card to put in a 300ohm resistance. (I'm still not happy with this, but I haven't found a definitive example of the right way to do it.)
This results in the following in the output file (once per frequency step):
Code:
                                 - - - RECEIVING PATTERN PARAMETERS - - -
                                           ETA=  90.00 DEGREES
                                           TYPE -LINEAR
                                           AXIAL RATIO= 0.000

           THETA      PHI          -  CURRENT  -         SEG
           (DEG)     (DEG)       MAGNITUDE    PHASE      NO.

            90.00      0.00     1.6998E-03    117.45      246
comparative gain (in dB) = 10*log10(magnitude)
The reason I call it a comparative gain is you use it to compare the results of different runs. If you want the dBd value, you need to model a resonant dipole and calculate the difference. Note: this is actual net gain, no need to compensate for SWR/impedance because you are measuring the current through the 300ohm "source" resistance.
 

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The reason I call it a comparative gain is you use it to compare the results of different runs. If you want the dBd value, you need to model a resonant dipole and calculate the difference. Note: this is actual net gain, no need to compensate for SWR/impedance because you are measuring the current through the 300ohm "source" resistance.
Thanks ericball, thats a help and makes sense.

I believe most uhf/vhf antenna comparisons use dBd = dBi minus 2.1 .


Testing Fonts:
Phi = Φ
Theta = Θ
How about that!
Heh, please dont use the symbols when posting, Phi and Theta are confusing enough in English.
 
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