MultiNEC - Antenna Modeling Software

bout a month ago, Dan Maguire, AC6LA asked me to try out MultiNEC, a program he developed to allow users to "program by variable" with NEC programs that do not allow that feature. In helping to beta test it, I found that it had a host of useful features that would enable beginners to work with NEC more easily and would enable "experts" to extract useful information at a glance. I am going to tell you right at the beginning of this review that, in its present form, it is an outstanding program! Moreover, Dan is an extremely helpful and cooperative guy who will do anything to add to its features-if feasible. It is FREE, FAST AND EASY TO USE!

The program is an Microsoft Excel spreadsheet "front end" which allows programming by variables, calls a NEC engine - whether EZNEC or a generic NEC-2 which is bundled with it - and then analyzes the data generated by it. Clearly, a user must have access to MS Excel to use the program.

Before we go into what MultiNEC does, it would be productive to see what various NEC programs do and don't do.

NEC-2 Programs Presently Available
Generic NECs, written in Fortran for mainframes, depend on the user programming a host of "pseudo cards." These cards correspond to the actual punch cards which were used to program old computers in the "Paleolithic Computer Era." The software has been adapted to run on desktops with microprocessors but, because of its old code, still has to work with a card format. So separate cards have to be programmed for the wires, loads, transmission lines, sources, etc. This, unfortunately, may be daunting and discouraging to beginners. It also makes simple dimensional changes tedious for experienced users.

Modern NEC software, supplied as EZNEC, NECWinPlus (NWP), NEC/Wires, et al, allows users to define antennas with spreadsheets and by using variables. Therefore A could be the length, B the wire size, C the width, D the height above ground and so forth. The advantage of this method is that, if these parameters need to be changed - as they always do - one would only have to change the value of a single variable. The program will do the rest by adjusting the value of all the wires defined with that variable. It makes for easy and rapid programming and encourages experimentation.

One beneficial feature of some of the old DOS commercial versions of NEC had was that users could make changes to the model via Notepad.txt (plain text) files and could call up batch files which would change values, run NEC and save the output files. You could run large frequency sweeps and make parameter changes in large models by leaving the computer on overnight and coming back in the morning to look at reams of data in output files. But at least you didn't have to stand around, wait for the program to run and then make a change. And start the whole process over again. But looking at hundreds of output files was no mean task.

A Look at MultiNEC
Dan Maguire uses MS Excel as his "front end" to NEC. You can therefore enter data in the spreadsheet and the program uses it define your antenna by programming the "NEC deck" of punch cards for you. This *.nec file is the one which is then fed to the NEC engine. The output from NEC is then used by the spreadsheet to give you numerical and graphic data.

Various such NECs are available for free on the Internet and two come bundled with the software. However, the limit with these is 960 wires, which is plenty for the overwhelming majority of the users. If you should desire a greater capacity, Dan points you to a website where you can download a version that will get the job done.

As you read on in this review, you might want to open another window and look at Dan's website for some of the features in greater detail.

Defining the Model
Figure 1 is a picture of MultiNEC's (MN from now on) "Equations" window. The antenna which I have defined is a P2 for 300 MHz. I did my calculations in the spreadsheet on the right of the window and the values are automatically entered in the variables portion on the left. Note that I defined the height, width, wire diameter (which is in mm), the segment length and the Zo of the tx lines. The number of segments for the height and width are automatically calculated by the spreadsheet by using common functions such as "ODD" and "INT".

The resulting application of these variables in the "Wires" page is shown in Figure 2. A click on the "formulas" button on that page converts the variables into their numerical values or vice versa. This is demonstrated in Figure 3.

The variables can also be used in the "Scr-LD-Tl" page. This page is where you enter the positions and definitions of sources, loads and transmission lines and is illustrated in Figure 4. The value for the Zo of the tx lines was entered as the variable "J" which you can see on the Equations page. You see the numeric value in the window on the Tl page.

Running the Model
We now turn to the "calculate" page on Figure 5. You enter a frequency or a block of frequencies in the first column. With nothing entered in the other 3 columns, the program will run your model over that frequency range. The bad news, with respect to free standing NEC programs such as NWP or EZNEC, is that the commercial programs allow you to program a frequency scan by inputting the start, end and step frequencies, e.g., 300-900 MHz in steps of 10 MHz. If you were to do this with MN, you would have to manually program 61 steps.

The good news is you can enter spot frequencies with MN, especially with large, complex models which can take many minutes to run at each frequency. For instance, with a P4 model of mine -which takes 4 minutes to run - I may be interested in the SWR at the lower and upper ends of a frequency range and at a couple of frequencies in the middle where I know that the SWR peaks. With the commercial programs, I can't just pick a scattered group of frequencies; they must be in the "steps" that are programmed from start to stop. Therefore, there will be a lot of useless NEC runs at unnecessary frequencies other than those that I am interested in. You don't have this problem with MN.

The resulting data is shown on the right hand side of Figure 5. The only minor hassle is that a single run only gives you either the Az or Elev data to look at and to graph. You have to rerun it to get the other pattern.

The feature that really got me excited was MN's ability to do multiple runs with different values for up to three variables. You can see this in columns 2 and 3 of this page where I called variables ("J" - tx line Zo) and ("I" - wire diameter) multiple times with different values over the same frequency range. Until you've tried and compared, you have no idea of what a timesaver this feature is!

I have run NWP over and over again over large frequency ranges with minute changes in Height, Width, wire diameter and tx line Zo. Each time I had to record the data by hand and look for trends and patterns. MN, after some well-invested time in setting up the "calculate" window, was able to run this model overnight and I had the data available to play with in the morning.

This is the best way to run long scans in the "calculate" window. Or you can go and watch TV for a while. The problem is that you can't switch to another window and say write an article or look at your e-mail because MN won't allow it-a drawback if you need the computer for multitasking.

Using the Data
In addition to the numerical presentation of the results in the right hand side of the "calculate" page, MN presents the data graphically. You can parse through radiation patterns in the "FF" window, shown in Figure 6. Similarly, as in Figure 7, you can look at SWR, gain, R and X on the "Plots" page.

But the real power of MN is revealed in the "Custom" page. Thanks to some changes Dan made at my behest, you can take a block of frequencies (seen in Figure 5) and analyze that block, and all other blocks with a similar set of frequencies as seen in Figure 8.

In my illustration shown in Figure 8, I analyzed wire diameter in the X-axis versus antenna Rin on the Y-axis. MN then allowed me to do a linear regression analysis, among many such analyses available, which clearly shows that the Rin decreases with wire diameter. To have done that earlier, I would have had to record my data, input it into my spreadsheet and create graphs to view it.

More importantly, you can play with any data parameters on the X- and Y-axes and analyze them as a single group or in groups defined in the "subsets" menu. This is really powerful and timesaving stuff!

Conclusion
I have attempted to give you a brief overview of the features of MultiNEC. This is by no means an exhaustive analysis, which would take too much time and too many pages. Your time would be best spent in downloading it and playing with it. The features will reveal themselves.

I can't say enough about Dan's efforts. The program is a labor of love and took a long time to create. It is free. Most importantly, you don't have to use all of its features to start with.

If you are a beginner and have never done any antenna modeling, it is the easiest way that I know of getting started. Just do what I did years ago and play. But the playing will be a lot less tedious than what I had to go through when fewer programs were available. You learn from your mistakes and these are easily correctible with the spreadsheet variable entries.

If you are experienced, the program has extensive features which will save you time and enable you to look at the numbers in a more meaningful and exhaustive way.

This is the best "bang for the buck" that I have ever downloaded from the Internet. A useful tool with tremendous features at no cost - what more can I say? -30-

ED: For your convenience, antenneX has placed a copy of this program for direct download on this website. Click here for download.

BRIEF BIOGRAPHY OF AUTHOR

Dan Handelsman, N2DT
Dan Handelsman, N2DT was first licensed as WA2BCG in 1957at age 13. He became interested in antennas at that time when he had to figure out a way to operate from the 6th floor of his apartment house. This resulted in a mobile whip being stuck out from a window without a counterpoise. At that point he became an "expert" in TVI. He was licensed as N2DT in 1977 and is a DX'er and contester. He is now playing with experimental antennas and low power.

Professionally, he is a Pediatric Endocrinologist and holds M.D. and J.D. degrees and is Clinical Professor of Pediatrics at the New York Medical College. As far as his antenna work he is an "amateur" in the truest sense of the word (Dan's words!).