Frequently Asked Questions
This page is provided for NEC-Win Basic, NEC-Win Pro, and GNEC users. Most of the topics concerning NEC-Win Pro also apply to GNEC. Please browse the list below to see if it contains your problem or question.
If you do not see the answer to your problem then please go to the Technical Support area for help.
Another source for finding a solution to your problem is to check the examples page. This page consists of example input files that you can download.
| NEC/MoM | International User's | NWB/NWP/GNEC | Necvu |
| Polar Plots | Rectangular Plots | Surface Patterns | Printing |
General NEC Questions
How
do I run large models? (many segments)
Why
is input impedance and VSWR invalid with Current Sources?
Far Fields - What do the results mean when I don't specify a
distance?
Why
is the resistive component of the input impedance negative?
NEC-Output file shows wire radius to be zero?
How
do I feed an Array? (source impedance)
What
is the frequency limitation of NEC?
How does the Green's function work?
How do I use the Green's function?
What are the units for voltage and current?
What are the units for the power gain calculated by NEC?
When the NEC core runs why is there a DOS window that opens?
When can a radial wire ground screen be used?
What is the reference point for the linear and radial cliff ground
planes?
Necvu
Why
does the Necvu window stay on top of all my other windows?
Why does the antenna flip or look reversed in Necvu?
Necvu keeps causing General Protection Faults.
When I choose different ground planes, why does Necvu only show a
square ground plane?
Why can't I view Surface Patches?
Why
doesn't Necvu display structures contained in a Green's function?
Why do the surface patterns in Necvu move erratically when rotating
the structure using the mouse?
When loading a file into Necvu, why doesn't the quickload feature
work?
NEC-Win Pro/GNEC/NEC-Win Basic Specific
Auto-DMA/DMA..what does it mean?
NEC-Win Pro/GNEC starts up but I am immediatly warned about Low
Resources. What is the matter?
What
is the best way to bring graphics into word processors?
How do I use a commercial spreadsheet in conjunction with Wire
Assist (NEC-Win Pro/GNEC)?
Do
the NEC-Win Programs allow me to import measured data into the
plots?
Does
NEC-Win Pro support the GC command?
Why am I always prompted to save my file?
Why
do I have to rerun NEC everytime I want a surface plot or a current
plot?
What
is the purpose of the Resource Monitor?
Polar Plots
Why
does the analysis box in the polar plots stay on top?
When plotting a polar plot of power gain, what is the difference
between linear and log?
Rectangular Plots
How can I change the axes labels in the rectangular plots.
Surface Patterns
Why can't I do a full surface request with the Surface Patterns?
Printing
Is
there any way to print out sections of the tabular data?
International User's
Plotting is incorrect. Resembles a spider web with lines all over.
Errors
requesting input impedance or VSWR.
Questions
Why does the antenna flip or look reversed in Necvu?
The optical illusion is caused by two important facts:
-
1) There is no perspective (no depth adjustments).
2) The segments are displayed in the order that they are created not their location in 3D space.
When using the GM to move a group of tags the last parameter in the command is a floating point number. This number is broken into two fields the beginning tag number and the ending tag number of the move. The tag numbers are separated by a decimal point.
e.g. GM 0 0 0 0 0 0 0 5 12.15
The above command will move tags 12 through 15 5 meters in the z direction.
You can not do a specific move to any tag numbers greater than 999 bsecause the limit for the last parameter is: 999.999
To avoid this problem you can use duplicate tag numbers for wires.
ex:
GW 1 5 0 0 0 0 0 2 .001
GW 1 9 .5 0 .5 0 0 2 .001
GW 1 9 1 0 0 1 0 2 .001
This answer was provided by Dr. James Breakall, Penn State University.
The inputs on the LD command are ignored if you put zero in for the values. The parser in NEC knows to look for zero inputs as a sign that there is no element of that type. For instance if you put an LD series with 50 ohms and zero for L and the C and did the same for parallel, they would both give you the same result.
You can see what the element is by looking in the NEC output listing. In the listing you will see nothing listed for the R or C or L type of element if a zero was input for the value, respectively.
If you wanted a short for one of the values (really doesn't make much sense), you would input something like 1E-10 or 1E-3 for the resistor. If you wanted an open or high value for the R, then you would enter something like 1E10 or 1E6.
Does NEC-Win Pro support the GC command?
The NEC2 core included with NEC-Win Pro does support the GC command but Necvu, the antenna viewer, will not display the segment data correctly.
Plotting is incorrect. Resembles a spider web
with lines all over.
-or-
you get errors requesting input impedance or VSWR.
This problem is frequent with international users. International versions of windows use the "comma" as a decimal seperator. NEC expects the seperator to be a "period".
To fix the problem go into the Windows control panel and under the number option for regional settings make sure the decimal symbol is a period (.) and the digit grouping symbol is a comma (,).
How do I feed an Array? (source impedance)
FEEDING AN ARRAY:
Assume the array is being fed by a transmitter, NOT a signal generator.
Any decent transmitter DOES NOT have a 50 ohm internal impedance! If it did you would need to create 2XPout RF watts and would heat up the xmtr with the same amount of power you wanted to deliver to the load/antenna. Transmitters are not constant voltage sources. They are nearly constant power sources, with internal impedances as low as possible so as to minimize internal losses. (Signal generators do low power devices so a 50 ohm internal Z is ok and assures that you will get the output you dial up, into a 50 ohms load.
But how do I specify a source impedance...?
Use a network card. The antenna feed point impedances are available
in the network table.
The way to control an array excitation is to feed it with constant current sources via an NT card. If the array is an AM Broadcst array, the matching network elements and the phasing network elements must be known and must be incorporated into the array via NT cards.
NEC-Win Basic, which is included with NEC-Win Pro, uses a Network card to implement Current Sources. NEC-Win Pro allows you to modify the network tables generated by NEC-Win Basic.
Note: Within NEC-Win Basic the user does not have access to the network tables.
To modify the Current Sources:
- Add the current sources with the NEC-Win Basic insert or manually enter the commands in the NEC-Win Pro editor. (commands required for a current source are explained in the NEC-Win Pro User's Manual).
- After returning from NEC-Win Basic, double click the left mouse button over any commands beginning with NT to edit the network table. If you enter the commands for a current source manually you will have the opportunity to modify the network table as you place the sources.
What is the frequency limitation of NEC?
There is no limit on the frequency range although NEC is usually not used for frequencies above 2 gHz.
NEC is dependent on the structure geometry not the frequency. If the structure can be modeled following the NEC guidelines then the appropriate frequency can be used.
Necvu keeps causing General Protection Faults.
This problem is caused by the video display being configured for less than 256 colors.
Necvu remaps the 256 color palette. If the video display is set at anything less Necvu will attempt to run but fail.
Please see your Windows documentation for information on how to configure your adapter.
Auto-DMA/DMA..what does it mean?
The new core has been optimized for the 32 bit environment. In this respect, it has the capability to use all of your system resources. This method is used in the auto-dma choice from the options menu.
The other option is the 32 bit core with DMA. (The only difference when talking about these methods is the word auto.)
Auto-dma parses your input file and automatically sets the variables for how many sources you have, number of segments...etc. It will then go off and run your file using as much RAM (or virtual memory) as it can find.
With just dma you are back to the old NEC method in which you modify an include file. The difference is that with the new core you modify the include file but don't need to recompile. With this method you can simply state how many segments you want to do in core, how many total segments do you want to run...etc. The name of the include file is nec2d.par. The file resides in the bin dir. The NEC-Win Pro users manual explains how to set these parameters. Look in the installation section.
For almost all cases you can use auto-dma. There are a couple of cases when you shouldn't use this method. You can not use auto-dma when you wish to use the Green's function (NGF). The other case is when you use surface patches. Other than that, auto-dma should work fine.
Both of these methods allow users to do a problem of any size as long as they have the resources.
How does the Green's function work?
When you use the Green's function, NWP does some automatic things with file names. I should also mention that the Sommerfeld uses the same method of filenaming as the Green's.
If you use the WG (write Green's file) command without a filename the default filename used for the Green's file (*.ngf) will be the nec input file name with an extension of (.ngf). Therefore if you have a nec file called ship.nec and you use the WG command without a filename you will get a Green's file called ship.ngf which will reside in the same directory as the nec input file.
If you use the WG command with a filename you will get a Green's file with the name you selected.
Now for the reading of the .ngf file. If you use the GF (read Green's file) command without a filename the nec core will attempt to read a Green's file named the current nec input filename but with the extension of ".ngf".
Note: When specifying the filename for Reading and Writing of the Green's file never enter a full path, just the filename. The NEC core expects to find the Green's file in the same directory as the NEC input file which is reading or writing to the Green's file.
How do I use the Green's function?
The Green's function is used for large structures. Generally users will have a large structure in which they wish to modify various parameters. The problem is that you don't want to have to wait the large amount of time each run may consume to view the results of the change.
For example you might want to model a ship antenna varying the location of the antenna for each run. The way to efficiently simulate the run would be to create the ship and save the structure in a Green's file. Then you would build a nec input file which reads in the Green's file. After reading in the Green's file you can add wires to the structure. You can also modify parameters such as excitation and pattern requests.
The one limitation is that when you create the Green's file you have to specify a frequency. The Green's structure is only good for that frequency. You DO NOT specify the frequency in the NEC input file which will call the Green's file.
Let's look at an example!
In the following example you first specify the wire geometry and then run NEC and save the results to a Green's file. The file which creates the Green's file should contain a Comments section and a Geometry section.
Ex: File which builds the Green's file.
CM KHAY 42" FACE MAGNUM TOWER - N LEG AT ORIGIN CE GW 1,1, 0,0,0, 0,0,4.125, .75 ! BOT LEG STUB GM 0,1, 0,0,0, 0,0,235.875, 001.001 ! TOP LEG STUB GW 2,2, 0,0,4.125, 0,0,21.375, .75 ! BOT 1/2 BOT CELL LEG . . GS 0 0 .02540 GE FR 0,0,0,0, 100.7 ! Set frequency for the structure WG fmtower.ngf ! Write structure to a Green's file XQ ! Execute the file EN
Then, to use the Green's file which you just created, your NEC input file needs to read the structure file. After the comments section you should immediately read in the Green's file. (This is shown in the example below.) After reading in the file you are allowed to add wires, loads, and sources to the Green's structure. You can also request patterns.
Ex:
CM Test for FM tower ! First place the comments section CE GF 0 fmtower.ngf ! Read in the Green's file GE 0 ! End Geometry section EX0,1, 1, 0, 1 ! Place sources..request patterns etc. RP 0 181 1 1000 -90 0 1 1 ! Note: there is no freq request. EN
Run the NEC simulation. Your results are now based on a structure which contains the Green's file. Your results are also based on the frequency you entered when creating the Green's file.
Note: Necvu cannot read in Green's files. Build the NEC file for the structure using Necvu and then, when you are sure the model is correct, save it to the Green's file.
If you later want to view the model using surface patterns or currents you will have to combine the file which created the structure to the file which sets the correct parameters. See the NEC-Win Pro users manual under Quick Examples for an explanation on how to do this.
Why can't I do a full surface request with the Surface Patterns?
There is a limitation with the surface patterns which is based on the viewer.
Necvu can only read in 32,000 segments. This amount is large enough to easily handle any antenna structure. The problem is that the surface patterns can generate a much larger number of segments.
A full hemisphere surface pattern request at 1 degree increments results in a pattern which contains over 64K segments. Therefore you are prompted whenever you request a pattern that the viewer cannot handle properly.
If you need a high degree of resolution for a particular antenna you should zero in on the area that you wish to view and only request a pattern for that area.
Why can't I view Surface Patches?
Unfortunately NEC-Win Pro offers limited support for surface patches. Hopefully you will see this support increased in the future.
Most antenna modelers use wire grids for modeling patch like geometries. Surface patches were common before computing speed could easily handle wire grids.
Some general comments about surface patches:
They should only be used for an enclosed surface such as a box. To connect a wire to the surface you need to approximate the center of the patch and place the wire at that location.
An example of a box with a monopole is available for download from our web site:
http://www.nittany-scientific.com/examples
Note: Necvu does not display surface patches.
Why is input impedance and VSWR invalid with Current Sources?
NEC-Win Pro uses a modeling trick to implement current sources. The problem with this method is that it uses an NT (network) command to transorm a voltage source located at a point far off in space to a current source located at the antenna.
The VSWR, input impedance, and Smith Charts look at the voltage sources. Since this is the case when you request a plot or tabular data of VSWR or input impedance, you will not be looking at the source connected to the antenna but the source connected to the network card.
The correct value for the input impedance at the antenna can be found by parsing the NEC output file for the Network Excitation Table. This table contains the impedance after the network transform.
Note: Far-field Patterns are not affected by this problem.
Far Fields - What do the results mean when I don't specify a distance
In NEC, IF you DO NOT specify a range, the far field reported is listed as V/m in NEC2, but is correctly listed as (V.) in NEC4. The value is calculated by multiplying the E field (which has an R in the denominator) by R (making the units now V., not V/m), and the phase is referenced to the origin (or at r=0 in the e^-jkr term). So it is the value of E field with R=1 and r=0. Of course, R=1 is a phoney location because it could well be too close for far-field.
This method allows one to find the RELATIVE V/m for pattern shapes, but not for gain. It will track the excitation values you apply and will match up with near fields if NE is run at a far field distance.
Why is the resistive component of the input impedance negative?
Why is the resistive component of the input impedance negative?
The first part of the answer was taken from NEC-List (See http://www.nittany-scientific.com/nec-list.htm for further information on NEC-List.)
NEC-List: Author is unknown.
"We have observed this many times when working with short antennas whose input impedances are high. The angle of the impedance vector is close to zero and NEC apparently rounds the resistive component to a small negative number. We never did come up with a good solution."
This part was submitted by L.B. Cebik.
The above section on negative input impedances is only correct for single source models that give this result. There are instances of using multiple sources where one or more may correctly show a negative resistance. These values occur in cases where parasitically induced currents are greater than source currents. In those instances where multiple sources are used in advance of tying sources together via transmission lines or networks, inclusion of those features and specification of a single source will normally return a correct value for the new feed point resistance.
NEC-Output file shows wire radius to be zero
Your input file has a problem. Usually this is caused by trying to run an input file which contains 80 characters or more on one of the lines.
The interface will only read 80 characters per line. If greater precision is needed please use scientific notation.
ex: .0000001 = 1E-7
Another way to save line space is to truncate geometry data depending upon the degree of accuracy needed.
E.g., 1.234567E-6 = 1.234E-6
Why do I have to rerun NEC everytime I want a surface plot or a current plot?
Necvu relies on a total match between the antenna structure file (nec input file) and the calculated data. In order to ensure that the file and calculated data are in perfect agreement the program forces a NEC run.
What is the best way to bring graphics from NEC-Win Pro and NEC-Win Basic into word processors?
Although NEC-Win Pro allows you to save all plots as bitmaps, the recommended procedure is to save the image to the clipboard. This can be done by choosing save screen or save area from the edit menu of the current plot window. After saving the image to the clipboard open your word processor and choose paste from the edit menu. You will then be allowed to copy the clipboard image into your document. This technique will also work with programs other than word processors.
Is there any way to print out sections of the tabular data?
NEC-Win Pro will always print the complete tabular data for your chosen selection. The only way to print sub-sets of the data is to copy the data into a word processor or other windows program.
Why doesn't Necvu display structures contained in a Green's function?
The Green's structure is stored in a binary format. At this time the viewer only supports ascii format.
The two types of files that can be read by the viewer are nec input files and flatfiles. The NEC input files should consist of the structure geometry without the Green's structure.
The flatfile format is similar to a plotter file. It consists of point coordinates and pen movements.
By default NEC-Win Pro will attempt to run the 16 bit NEC core. The NEC core can be changed from within the options menu of the main interface.
For more information on the NEC2 cores please refer to the Installation section in the NEC-Win Pro User's Manual.
Why does the Necvu window stay on top of all my other windows?
The viewer defaults to "Keep Window on Top". This default can be overridden in the Configuration Menu. To make your choice the new default click on the Save Options feature in the Configuration menu.
Why does the analysis box in the polar plots stay on top?
The analysis box for the polar plots has been set to always remain on top. Unfortunately this causes problems when you minimize the polar plot window. To avoid any problems you should always close the analysis window after doing a screen capture or performing any additional operations.
Does NEC-Win Pro allow me to import measured data into the plots?
As of NEC-Win Pro v1.1 importing of data is not supported. Hopefully the next release will include this feature.
What is the purpose of the Resource Monitor?
The Resource Monitor was included in the event that you are having memory problems with NEC-Win Pro.
It is not intended for REGULAR USE BY the user but more for providing information if technical support is required.
In a file which contains a frequency sweep, the sweep will only apply to the first radiation pattern request. All other pattern requests will be calculated at the last frequency in the sweep.
In order to get around this limitation your input file should contain multiple requests for a frequency sweep.
e.g.
FR 0 10 0 0 550 5
RP 0 361 1 1000 0 0 1 1
FR 0 10 0 0 550 5
RP 0 1 360 1000 90 0 1 1
EN
When loading a file into Necvu, why doesn't the quickload feature work?
This was a reported bug that we have not been able to recreate. If the problem occurs please let us know at tech-support@nittany-scientific.com.
A work around for the problem is to open the viewer with a smaller structure. Then under the configuration menu, click on Save Options. Under Defaults click on the checkbox next to Quickload on Startup. Then click on Save Options. Now the next time you run Necvu it will quick load the file.
Why do the surface patterns in Necvu move erratically when rotating the structure using the mouse?
This problem is due to the mouse sensitivity settings in Necvu. The mouse sensitivity has been set to provide smooth operation when viewing the antenna. Unfortunately, when the antenna and the surface pattern are displayed simulaneously, the mouse sensitivity causes the rotation to be somewhat erratic.
To get a smoother response, please use the arrow keys on the keyboard or the scroll bars.
Note: The arrow keys on the keyboard work for all operations. (rotate, pan, and zoom)
How do I use a commercial spreadsheet in conjunction with NEC-Win Pro?
The Wire Assist feature in NEC-Win Pro allows you to copy and paste columns and rows of data between NEC-Win Pro and commercial spreadsheets. To use Wire Assist, you must first highlight one or more of the GW (generate wire) command lines in the NEC-Win Pro editor. After highlighting the commands click on the Wire Assist button.
Within Wire Assist you can do a block copy of any cells. You can then go into a commercial spreadsheet and paste the data into the spreadsheet. After modifying the data you can then copy it from the spreadsheet and paste it back into Wire Assist.
NEC-Win Pro starts up but I am immediatly warned about Low Resources. What is the matter?
This problem has only been known to occur on Windows 3.1x systems. Generally it is the result of having networking software such as Novell on the system.
The NEC-Win Pro installation diskettes include a program called Resource Monitor. Start up the resource monitor without running NEC-Win Pro. All of the resource values should be somewhere in the range of 80 - 95%. If these values are in the 60% range on a clean boot-up, then you do not have enough resources on your machine to run NEC-Win Pro reliably. If you still wish to run NEC-Win Pro please save your file periodically.
When I choose different ground planes, why does Necvu only show a square ground plane?
NEC-Win Pro v1.1 will only show a square ground plane in the geometry viewer regardless of the ground plane you have selected for your model.
To verify your ground plane check the RP(radiation pattern) and GN (ground parameters) commands in the editor or with NEC-Win Basic.
Note: To use different ground planes, the GN as well as the RP command must have the correct options selected.
How can I change the axes labels in the rectangular plots.
Anything in the rectangular plots can be modified by double clicking the left mouse button over the item you wish to edit.
Also, clicking on the right mouse button brings up a menu of typical operations you might want to perform with the plots.
What are the units for voltage and current?
NEC caculates the peak value of voltage and current. Therefore to calculate power the following formula should be used:
Power = I^2R/2
Note: The power calculations in NEC take into account that the current and voltage are in peak values.
What are the units for the power gain calculated by NEC?
The units for the power gain calculated by the NEC core are dBi.
When plotting a polar plot of power gain, what is the difference between linear and log?
Since the NEC core calculates power gain values as dBi what is called a linear plot is actually a linear-log plot. When you request a log plot you are actually requesting a log-log plot.
Why does NEC-Win Pro always prompt me to save my file?
Although some users consider this feature a pain it was implemented to ensure that you do not accidentally overwrite a model. If you don't care if the original file is overwritten the easiest thing to do is click on the save button before you run NEC. This will prevent the save-as box from popping up.
When the NEC core runs why is there a DOS window that opens?
The NEC core runs external to the main interface. This was done so that cores can be swapped in and out of the code easily. This also allows us to maintain compatibility with older input files.
Note: Even though the core runs in a DOS window it has been optimized for Windows. The NEC core can not be run from standard DOS. It can only be run from a DOS window.
When can a radial wire ground screen be used?
The radial wire ground screen was introduced into the NEC code for Broadcasting Antennas. The ground screen should only be used for a single vertical monopole antenna. It should not be used as a method for modeling vehicles and other type of structures. (for vehicles a wire grid should be used)
The radial wire ground screen is typically used for LF, MF, and LHF.
What is the reference point for the linear and radial cliff ground planes?
The reference point is the origin. For the radial cliff the origin is in the center of the middle circle. For the linear cliff the origin is in the center of the first ground type.
The NEC-Win Basic insert shows detailed pictures of the linear and radial cliff.
Note: NEC does not take into account diffraction.
tech_support@nittany-scientific.com