Canadian TV, Computing and Home Theatre Forums banner
1 - 20 of 252 Posts

·
Registered
Joined
·
7,360 Posts
4NEC2 Install

I installed the program, and it looks like its into 3 parts: the program, the build, and the view. Using F1 (help) brings up the message "Cannot find 4nec2.hlp". Is there such a file somewhere ??

Edit: Never mind, found it in the C:\4nec2 directory.
 

·
Registered
Joined
·
7,360 Posts
Each element of the antenna that you model, is sub-divided into smaller sections called segments. The "safe" rules for how you determine the segments are:

1. Use at least 10 segments per half wavelength of wire at the highest operating frequency.
(Ten segments per full wavelength is sometimes used to specify the longest allowable segment length -- 0.1 -- but doubling the segment count yields more accurate results for a larger variety of geometric wire assemblies.)
Fine, but I still dont understand the "why" of massive segmentation of a simple wire in the program. Its one of things that gives me "brain freezes" about the program.:eek:

I'm afraid I don't understand what you mean by a cheat sheet ?
You know, like something you buy in a bookstore about shortcuts to Microsoft Word, Excel etc, all the main functions laid out in a page or two.
 

·
Registered
Joined
·
7,360 Posts
I have noticed that 4nec2x DOES take advantage of both processor cores in a dual core cpu in windows xp. :) Has anyone checked processor usage with a quad core system ?
 

·
Registered
Joined
·
7,360 Posts
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).
 

·
Registered
Joined
·
7,360 Posts
Are you talking about the NASA fractal antenna that a computer AI program came up with ?

If so, it only has 6db gain. But at least its super compact, heh.
 

·
Registered
Joined
·
7,360 Posts
Well, in the 4nec2 install package, there is a sub-folder of modeled fractal antennas under Models > Fractals. You can scale and change those if you want.
 

·
Registered
Joined
·
7,360 Posts
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.
 

·
Registered
Joined
·
7,360 Posts
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.
 

·
Registered
Joined
·
7,360 Posts
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.
 

·
Registered
Joined
·
7,360 Posts
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.
 

·
Registered
Joined
·
7,360 Posts
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.
 

·
Registered
Joined
·
7,360 Posts
The Plot button at the right of the graph will give you a choice of the values versus the frequency. You can use these values to calculate the net gain according to Autofils's formula.
Just in case you didnt know, the plot button also produces a plot.txt file in the 4nec2\plot directory which can be used to import the data into a spreadsheet. The plot.txt file must be renamed before the next plot is made, as it is overwritten.
 

·
Registered
Joined
·
7,360 Posts
4nec2 runs under Wine

As a Linux user I'd want the same thing, but I've not heard about whether Wine runs these antenna modeling apps. Someone needs to try it...
Stampeder, 4nec2 runs under Wine. Arie Voors has directions on how to do it in his FAQs.
 

·
Registered
Joined
·
7,360 Posts
After a short time, the "AGT results" were displayed on the Main (F2) screen as "1.02 (0.08 dB)". I think this is trying to tell me that the "Raw Gain" estimated at 585 MHz might be in error by as much a 0.08 dB. In my book that is very small error. If the error had been a lot larger, I would have increased the number of segments in each wire and tried again.
You really should strive to get it to 1.0 (0 db). (otherwise, you have to remember in your head to do the subtraction or addition to a lot of channels) Its not that hard to do to get it perfect after a few times. And after a while, you quickly learn which way to adjust up or down. I dont change the number of segments until changing the wire radius doesnt do the trick. I just change the wire radius from the drop down box in Geometry Editor. That usually does it. If it doesnt, then I manually input a wire radius that is between gauge sizes.
 

·
Registered
Joined
·
7,360 Posts
4nec2 models transmission, not reception. And although you can use "reciprocity" theories to get from one to the other, there are differences.
I thought about that. So that would mean modeling with 4nec2s advanced features over various ground types and conditions would be pretty much an exercise in futility for receiving, as the transmitter that is actually transmitting is far away. So modeling in a "free space" environment is the only way to go.
 

·
Registered
Joined
·
7,360 Posts
Today, the usual practice is to connect a 300 ohm to 75 ohm "Balum" transformer immediately to the antenna and run 75 ohm coax cable down to the TV tuner.
Correct, the antenna is 300 ohms and the coax cable is 75 ohms and the balun is in a 4:1 ratio. Thats the set standard for residential tv antennas. So use Zo = 300. At least maybe until 75 ohm to 75 ohm 1:1 baluns become commonplace. At 75 ohm characteristic impedance, you still need a balun to go from balanced to unbalanced line. Hence the name "balun". (baluns can also be made from coax cable, but theyre tricky for the average person to make)

Also somewhere in my readings, I believe I remember a basic dipole antenna has a Characteristic Impedance of about 72 ohms, a basic bowtie antenna has a Characteristic Impedance of about 75 ohms and a basic folded dipole has Characteristic Impedance of about 300 ohms. Where did thing this Characteristic Impedance thing come from and why does a Gray Hoverman design have a Characteristic Impedance of 300 ohms? Why isn't it say 100 or 200 ohms?
A dipole has a Characteristic Impedance of about 72 ohms, until you put on directors or reflectors, then it changes. Same thing for the bowtie, and most bowties are 4 ganged which changes the impedance drastically. The reason all these antennas are 300 ohm, is because thats the set standard (probably because in the orig old days, the folded dipole ruled and making twin lead lower than 300 ohms was too tricky for the manufacturing of the day. Were lucky they didnt settle on the wide 600 ohm standard which was easier to make consistantly, heh) So the goal is to design your antenna for 300 ohms so everything fits. At least until the next, if ever, standard.
 
1 - 20 of 252 Posts
Top