The program’s name is short for Numerical Electromagnetic Code for WINdows 95 Virtual Memory. NEC was a FORTRAN program written in the 70’’s by the Naval Ocean Systems Center. It uses the “Method of Moments” to work out the parameters of an antenna design. One small bug with NEC was that there was a limit to the total number of wires and segments due to the limits of RAM in the system. With the virtual memory option this program has, the limit is set by amount of hard disk that the program can use. I have seen some demo programs that limit the total of segments and wires to 25, and since the antenna I build used 37 wires, these demos could not be used to design this antenna. Since NEC was a FORTRAN program, its main input method was punch cards. With punch cards if a wire were mis-defined, a new card would replace it. The freeware DOS version of NEC was a nightmare for inputting because there is no way to store or edit an input file — a mistake meant starting all over. With the Windows version, input is by means of a spreadsheet, so editing of the design is easy. 

The input for the program is done by entering the starting and ending XYZ coordinates for each wire. Since many antennas use only one wire, but have many bends, there is a button to allow for adding wire connected to the last wire. Looking at my antenna, there are many places where wires come from a node. The connect function only allows for one wire to come from a node, it would have made input faster for me if the function could have allowed many wires to be connected to one end of a wire node.

During input of the wire coordinates, I could click the show button to see the antenna as it was being entered. For an antenna with a large amount of wires, this could save time in trying to track down a bug in the antenna design. During input, the program will automatically segment the wires. After entering the wires, the antenna feed point is defined as a source and any capacitors and coils are added as loads. The antenna has now been defined for the program and fine designing can begin.

This will be one of the first functions run: the total far field. It shows the antenna radiation pattern, and gives other datum in text. By looking at the impedance of the antenna, it could be decided if the antenna needed to be adjusted. A resonance sweep then would be done, and frequency ratio found. This frequency ratio then could be used as a scale factor. Then by going back to the input spreadsheet, selecting all wires, entering the scale factor into the scale window, the program will automatically rescale the wires. By rerunning the total far field, a new redesign cycle is begun. Other functions that can be run are SWR plots, Smith graphs, and 3D plots of the antenna with the total radiation pattern. 

The program flags most common mistakes, but the output is still a computer model. Sometimes the model has problems. A common mistake is mixing units, metric/ English, feet-inches, centimeters-meters, etc. Sometimes the automatic segmentation will cause the model to become unstable. The cure is to change the number of segments on longer wires. Also if a large impedance change occurs when the number segments is changed, then the model is unstable. For an example of an unstable model, I tried a long wire antenna at 14 feet above the ground. As the height was lowered the gain increased. At one foot the antenna had a gain of about 12dBd (dipole). When I changed the ground from bad to perfect, the gain drop to only 6dBd (which is about what theory says it should be). I do not know if the figure was correct for the antenna, since I did not build it, but I figured any antenna gain that depended on a bad ground could not be true. 

The tutorial that comes with program is very good for using the program. However, a new-comer to computer antenna modeling may have problems. Running a computer program does not make one an antenna expert. The tutorial assumes the user has a good understanding of antennas. There are ways that the computer looks at an antenna that are not the same way I look at an antenna. For example, I see a dipole as two wires, but the computer sees a dipole as one wire. A T wire node can be seen as two nonconnected wires or as three connected wires. Also, since NEC has problems with closely spaced parallel wires, and wires close to the ground- NEC4WIN95VM will have problems. This is why it is necessary to have good working knowledge of antennas. 

NEC4WIN95VM is by Orion Microsystems and can only be bought at their Website. The cost for NEC4WIN95VM is $90. Also, there is NEC4WIN95 (no virtual memory option), which costs $60. I sent in a money order and Orion Microsystems sent a password by e-mail to download the program via the Internet before the CD came by snail mail. 

The program can work with a fast 486, but for larger projects it is recommended to have Pentium III and 128 MB of RAM so as to keep the file swapping to a minimum. If you like antennas, I think you will like this program.