63. GH and GM: The NEC-4 Versions

L. B. Cebik, W4RNL

We have examined in the past two episodes the basic uses of the GH (Helix-Spiral Specification) and the GM (Coordinate Transformation) inputs in their NEC-2 incarnations. Along the way, we had occasion to briefly note that in certain particulars, the NEC-4 versions of these geometry in[put lines differed from the NEC-2 counterparts. Some modelers may have occasion to use NEC-4, and so it may be useful to trace the way in which this core employs these input lines.

Although the inputs are a function of the NEC-2 and NEC-4 cores, implementing software provides the user with certain helps. Therefore, we shall examine the GH and GM cards through NEC-Win Pro (NEC-2) and GNEC (NEC-4), both by Nittany Scientific. The screens of these programs will have similar appearances, since they are roughly counterpart programs. However, it will be the differences that most interest us.

The Helical Dipole for 28.5 MHz

Let's begin be re-creating the helical dipole from the preceding column. In Fig. 1, we have the ASCII inputs that define this model.

There are only two lines in the model version that differ. One is the LD5 material conductivity line. In the NEC-2 version, places 2, 3, and 4 specify the tag number, the start segment, and the stop segment of the wire to be loaded. The NEC-4 version uses a shortcut: these same places all contain zeroes, indicating that all segments in the model will be loaded by the conductivity value (in S/m) listed in the last entry position. We have noted this shortcut in past columns, but likely have not illustrated its use until now.

The more germane difference lies in the GH line that defines the helical dipole. The basic design consists of AWG #12 wire (0.0808" diameter) would in a helix in which the turns occupy 12" each. The radius is 4", and the overall length is 106" or 8.8333 turns.

NEC-2 enters the data in this format:

```GH  1    168  12  106  4   4   4   4   .0404
GH  ITG  NS   S   HL   A1  B1  A2  B2  RAD
I1   I2   F1  F2   F3  F4  F5  F6  F7```

Note that we use the space between turns and the total length to define the helix, where both values are in the unit of measure chosen for the model and transformed to meters by the GS line.

In contrast, the basic defining data required by the NEC-4 version is the number of turns, where the number of turns may be a decimal value rather than a simple integer, and the total length of the helix. Hence, the line input format undergoes a reshaping.

```GH  1    168  8.3333  106    4    4    .0404  .0404  0
GH  ITG  NS   TURNS   ZLEN   HR1  HR2  WR1    WR2    ISPX
I1   I2   F1      F2     F3   F4   F5     F6     F7```

The integer entries retain the same meanings to indicate the tag number of the spiral and the total number of segments with the helix. F1 and F2 contain the number of turns and the total length. The length is designated ZLEN, because--common to both cores--the initial helix is grown along the Z-axis from zero to a positive limit. If ZLEN is negative, the output is a left-handed spiral; if positive, the helix is right-handed. Since the helical dipole does not care about its hands, we have assigned a positive number.

Whereas in NEC-2, we might assign different values to the radius along the X-axis and the Y-axis (allowing an oval), HR1 and HR2 assign a single radius value to the Z=0 end and to the Z=ZLEN ends of the spiral, respectively. WR1 and WR2 refer to the wire radius at each end of the helix. If we enter different values for two entries, then the program automatically scales the radii of the segments logarithmically.

The final entry, ISPX is effective only when HR2 and HR1 are not equal-- which creates a spiral structure. When HR1=HR2, values of 0 or 1 make no difference. However, when we have a spiral, 0 defines a log spiral and 1 defines an Archimedes spiral.

Fig. 2 shows the NEC-2 and NEC-4 GH-help screens to further identify the differences between the GH geometry input lines. The help screens simply provide places to enter the line data in order (or to revise individual entries), so correlation to the respective model lines should be straightforward.

In the original model, we were not content to leave the helix extending from Z=0 to Z=106 (inches). Therefore, we rotated the helix -90 degrees on the X-axis, displaced it by -53" on the Y-axis, and elevated it 360" on the Z-axis. We accomplished all of this with a single GM line input. For operations that act upon the entirety of the tags and segments within a model, there is no difference between the NEC-2 and the NEC-4 GM inputs. Therefore, in the final helical dipole model, the NEC-4 version will appear as in Fig. 3.

To verify that the resulting model is identical to the one we produced in NEC-2 in the preceding column, we may take a truncated look at the NEC output file.

``` . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
- - - STRUCTURE SPECIFICATION - - -

WIRE                                                                               NO. OF    FIRST  LAST     TAG
NO.        X1         Y1         Z1          X2         Y2         Z2      RADIUS   SEG.     SEG.   SEG.     NO.
1      THIS WIRE IS A LOG-SPIRAL OR HELIX                                          168        1   168       1
SPIRAL DATA: TURNS=    8.8333  LENGTH=  1.0600E+02  H.RAD=  4.0000E+00  4.0000E+00  W.RAD=  4.0400E-02  4.0400E-02
TOTAL LENGTH OF WIRE IN THE SPIRAL =  2.45104E+02
THE STRUCTURE HAS BEEN MOVED, GM COMMAND DATA IS -
0    0 -90.00000   0.00000   0.00000   0.00000 -53.00000 360.00000    0    1    0  168
STRUCTURE SCALED BY FACTOR   0.02540

- - - - SEGMENTATION DATA - - - -

SEG.   COORDINATES OF SEG. CENTER     SEG.     ORIENTATION ANGLES    WIRE    CONNECTION DATA   TAG
NO.       X         Y         Z       LENGTH     ALPHA     BETA      RADIUS    I-   I    I+    NO.
1   0.09885  -1.33819   9.12752   0.03706  -62.79489 108.92291   0.00103     0    1    2      1
2   0.08816  -1.32216   9.09635   0.03706  -52.48438 134.75235   0.00103     1    2    3      1
3   0.06794  -1.30613   9.07032   0.03706  -37.67732 146.87851   0.00103     2    3    4      1
4   0.04036  -1.29011   9.05227   0.03706  -21.29291 152.34449   0.00103     3    4    5      1
5   0.00842  -1.27408   9.04414   0.03706   -4.34628 154.29633   0.00103     4    5    6      1
6  -0.02443  -1.25806   9.04681   0.03706   12.69518 153.68493   0.00103     5    6    7      1
7  -0.05463  -1.24203   9.05998   0.03706   29.44213 150.22438   0.00103     6    7    8      1
8  -0.07893  -1.22600   9.08225   0.03706   45.24814 142.10108   0.00103     7    8    9      1
9  -0.09470  -1.20998   9.11120   0.03706   58.42714 124.31186   0.00103     8    9   10      1
10  -0.10022  -1.19395   9.14369   0.03706   64.37504  90.37231   0.00103     9   10   11      1
11  -0.09490  -1.17792   9.17621   0.03706   58.62722  56.17145   0.00103    10   11   12      1
12  -0.07932  -1.16190   9.20526   0.03706   45.52955  38.12187   0.00103    11   12   13      1
------------
80   0.04264  -0.07212   9.05330   0.03706  -22.55679 152.07638   0.00103    79   80   81      1
81   0.01091  -0.05609   9.04438   0.03706   -5.63326 154.24218   0.00103    80   81   82      1
82  -0.02200  -0.04007   9.04623   0.03706   11.41384 153.81986   0.00103    81   82   83      1
83  -0.05252  -0.02404   9.05865   0.03706   28.19970 150.61245   0.00103    82   83   84      1
84  -0.07737  -0.00801   9.08030   0.03706   44.11422 142.96060   0.00103    83   84   85      1
85  -0.09385   0.00801   9.10884   0.03706   57.60255 126.18027   0.00103    84   85   86      1
86  -0.10018   0.02404   9.14119   0.03706   64.32866  93.34658   0.00103    85   86   87      1
87  -0.09567   0.04007   9.17384   0.03706   59.40187  58.17071   0.00103    86   87   88      1
88  -0.08082   0.05609   9.20326   0.03706   46.64635  39.04741   0.00103    87   88   89      1
89  -0.05723   0.07212   9.22627   0.03706   30.98815  30.29622   0.00103    88   89   90      1
90  -0.02744   0.08814   9.24039   0.03706   14.29461  26.50571   0.00103    89   90   91      1
-------------
157   0.01339   1.16190   9.04468   0.03706   -6.91958 154.17381   0.00103   156  157  158      1
158  -0.01955   1.17792   9.04571   0.03706   10.13111 153.93953   0.00103   157  158  159      1
159  -0.05038   1.19395   9.05737   0.03706   26.95266 150.97653   0.00103   158  159  160      1
160  -0.07576   1.20998   9.07839   0.03706   42.96775 143.77077   0.00103   159  160  161      1
161  -0.09294   1.22600   9.10652   0.03706   56.74138 127.94746   0.00103   160  161  162      1
162  -0.10008   1.24203   9.13869   0.03706   64.20848  96.30087   0.00103   161  162  163      1
163  -0.09639   1.25806   9.17144   0.03706   60.13303  60.27713   0.00103   162  163  164      1
164  -0.08227   1.27408   9.20123   0.03706   47.74816  40.02950   0.00103   163  164  165      1
165  -0.05926   1.29011   9.22482   0.03706   32.21887  30.74264   0.00103   164  165  166      1
166  -0.02984   1.30613   9.23967   0.03706   15.57213  26.67627   0.00103   165  166  167      1
167   0.00281   1.32216   9.24418   0.03706   -1.44894  25.63317   0.00103   166  167  168      1
168   0.03516   1.33819   9.23785   0.03706  -18.43862  27.12109   0.00103   167  168    0      1
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .```

The values are essentially the very same as those we developed in the NEC-2 model for the helical dipole. Indeed, NEC-4 returned a source impedance of 22.6 - j 2.1 Ohms, with a free-space gain of 1.73 dBi. The NEC-2 model returned the same gain with a source impedance of 22.6 - j 1.9 Ohms.

Log vs. Archimedes Spirals

If we set HR1 and HR2 to different values, we obtain a spiral structure. Only if we set the helix length (ZLEN) to zero will we obtain a flat spiral. Just for the exercise, lets create a flat spiral with a starting radius of 4" and a final radius of 20". In fact, either HR1 or HR2 may be the larger or the smaller figure. However, if HR2=0, then its values becomes the value of HR1. Hence, for a nearly closed end to HR2, we must use a very low number, but one greater than zero.

For the sake of simplicity, we shall use a constant wire radius throughout and retain the 168 total segment count. In addition, we shall specify 9 turns for our spiral. The resulting help screen version of the new GH line will look like Fig. 4.

The only remaining option is whether to choose a log spiral (entry = 0) or an Archimedes spiral (entry = 1) in the ISPX position. The differences in the ways of calculating rate of spiraling lie in the development of a new radius based on the preceding radius using a program-calculated constant.

In practical terms, alternately selecting between the two spirals and leaving the other spiral-determining factors constant results in the two spirals shown in Fig. 5.

The differential in spacing between the successive rings of the two spirals is clearly apparent. However, there are other features worth noting. In both spirals, the segment lengths increase at the same rate from the innermost point to the outer limit. The selection of the number of turns and the number of total segments results in segment junctions that do not align particularly well. For the highest accuracy when using closely spaced wire segments, segment junction should be aligned as closely as possible. With 171 segments, the junctions would align at 19 segments per turn.

All other recommendations and limitations applicable to wires set up with the GW input apply to the GH input. The user should be specially aware of these limitations when using closely spaced spiral rings in conjunction with sizable wire radii. The log spiral may prove tricky unless the modeler pays close attention to the innermost rings and their spacing. The following extract from the NEC output report tracks the first 2 and the final 2 rings of the log spiral in our example as a sample of the ring-spacing differentials that may emerge.

``` . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
- - - STRUCTURE SPECIFICATION - - -

WIRE                                                                               NO. OF    FIRST  LAST     TAG
NO.        X1         Y1         Z1          X2         Y2         Z2      RADIUS   SEG.     SEG.   SEG.     NO.
1      THIS WIRE IS A LOG-SPIRAL OR HELIX                                          168        1   168       1
SPIRAL DATA: TURNS=    9.0000  LENGTH=  0.0000E+00  H.RAD=  4.0000E+00  2.0000E+01  W.RAD=  4.0400E-02  4.0400E-02
TOTAL LENGTH OF WIRE IN THE SPIRAL =  5.59747E+02
STRUCTURE SCALED BY FACTOR   0.02540

TOTAL SEGMENTS USED=  168     NO. SEG. IN A SYMMETRIC CELL=  168     SYMMETRY FLAG=  0

- - - - SEGMENTATION DATA - - - -

COORDINATES IN METERS

SEG.   COORDINATES OF SEG. CENTER     SEG.     ORIENTATION ANGLES    WIRE    CONNECTION DATA   TAG
NO.       X         Y         Z       LENGTH     ALPHA     BETA      RADIUS    I-   I    I+    NO.
1   0.09921   0.01694   0.00000   0.03421    0.00000  98.02802   0.00103     0    1    2      1
2   0.08890   0.04923   0.00000   0.03454    0.00000 117.31374   0.00103     1    2    3      1
3   0.06830   0.07655   0.00000   0.03488    0.00000 136.59945   0.00103     2    3    4      1
4   0.03956   0.09573   0.00000   0.03521    0.00000 155.88516   0.00103     3    4    5      1
5   0.00578   0.10442   0.00000   0.03555    0.00000 175.17088   0.00103     4    5    6      1
6  -0.02931   0.10143   0.00000   0.03589    0.00000-165.54341   0.00103     5    6    7      1
7  -0.06176   0.08689   0.00000   0.03624    0.00000-146.25769   0.00103     6    7    8      1
8  -0.08783   0.06221   0.00000   0.03659    0.00000-126.97198   0.00103     7    8    9      1
9  -0.10444   0.03000   0.00000   0.03694    0.00000-107.68626   0.00103     8    9   10      1
10  -0.10953  -0.00624   0.00000   0.03730    0.00000 -88.40055   0.00103     9   10   11      1
11  -0.10230  -0.04247   0.00000   0.03765    0.00000 -69.11484   0.00103    10   11   12      1
12  -0.08333  -0.07459   0.00000   0.03802    0.00000 -49.82912   0.00103    11   12   13      1
13  -0.05454  -0.09886   0.00000   0.03838    0.00000 -30.54341   0.00103    12   13   14      1
14  -0.01900  -0.11240   0.00000   0.03875    0.00000 -11.25769   0.00103    13   14   15      1
15   0.01937  -0.11345   0.00000   0.03913    0.00000   8.02802   0.00103    14   15   16      1
16   0.05629  -0.10166   0.00000   0.03950    0.00000  27.31374   0.00103    15   16   17      1
17   0.08754  -0.07810   0.00000   0.03988    0.00000  46.59945   0.00103    16   17   18      1
18   0.10947  -0.04524   0.00000   0.04027    0.00000  65.88516   0.00103    17   18   19      1
19   0.11941  -0.00661   0.00000   0.04065    0.00000  85.17088   0.00103    18   19   20      1
20   0.11599   0.03352   0.00000   0.04105    0.00000 104.45659   0.00103    19   20   21      1
21   0.09936   0.07062   0.00000   0.04144    0.00000 123.74231   0.00103    20   21   22      1
22   0.07114   0.10043   0.00000   0.04184    0.00000 143.02802   0.00103    21   22   23      1
23   0.03430   0.11943   0.00000   0.04224    0.00000 162.31374   0.00103    22   23   24      1
24  -0.00714   0.12525   0.00000   0.04265    0.00000-178.40055   0.00103    23   24   25      1
25  -0.04857   0.11698   0.00000   0.04306    0.00000-159.11484   0.00103    24   25   26      1
26  -0.08529   0.09529   0.00000   0.04347    0.00000-139.82912   0.00103    25   26   27      1
27  -0.11305   0.06236   0.00000   0.04389    0.00000-120.54341   0.00103    26   27   28      1
28  -0.12853   0.02173   0.00000   0.04431    0.00000-101.25769   0.00103    27   28   29      1
29  -0.12973  -0.02215   0.00000   0.04474    0.00000 -81.97198   0.00103    28   29   30      1
30  -0.11625  -0.06437   0.00000   0.04517    0.00000 -62.68626   0.00103    29   30   31      1
31  -0.08931  -0.10011   0.00000   0.04561    0.00000 -43.40055   0.00103    30   31   32      1
32  -0.05173  -0.12518   0.00000   0.04605    0.00000 -24.11484   0.00103    31   32   33      1
33  -0.00756  -0.13654   0.00000   0.04649    0.00000  -4.82912   0.00103    32   33   34      1
34   0.03833  -0.13264   0.00000   0.04694    0.00000  14.45659   0.00103    33   34   35      1
35   0.08076  -0.11362   0.00000   0.04739    0.00000  33.74231   0.00103    34   35   36      1
36   0.11485  -0.08135   0.00000   0.04784    0.00000  53.02802   0.00103    35   36   37      1
----------------------------
133   0.29053   0.20650   0.00000   0.12117    0.00000 123.74231   0.00103   132  133  134      1
134   0.20801   0.29367   0.00000   0.12234    0.00000 143.02802   0.00103   133  134  135      1
135   0.10030   0.34922   0.00000   0.12352    0.00000 162.31374   0.00103   134  135  136      1
136  -0.02087   0.36624   0.00000   0.12471    0.00000-178.40055   0.00103   135  136  137      1
137  -0.14201   0.34206   0.00000   0.12591    0.00000-159.11484   0.00103   136  137  138      1
138  -0.24940   0.27862   0.00000   0.12712    0.00000-139.82912   0.00103   137  138  139      1
139  -0.33057   0.18235   0.00000   0.12834    0.00000-120.54341   0.00103   138  139  140      1
140  -0.37583   0.06354   0.00000   0.12958    0.00000-101.25769   0.00103   139  140  141      1
141  -0.37934  -0.06477   0.00000   0.13082    0.00000 -81.97198   0.00103   140  141  142      1
142  -0.33991  -0.18822   0.00000   0.13208    0.00000 -62.68626   0.00103   141  142  143      1
143  -0.26116  -0.29271   0.00000   0.13335    0.00000 -43.40055   0.00103   142  143  144      1
144  -0.15127  -0.36603   0.00000   0.13464    0.00000 -24.11484   0.00103   143  144  145      1
145  -0.02210  -0.39926   0.00000   0.13593    0.00000  -4.82912   0.00103   144  145  146      1
146   0.11208  -0.38785   0.00000   0.13724    0.00000  14.45659   0.00103   145  146  147      1
147   0.23614  -0.33223   0.00000   0.13856    0.00000  33.74231   0.00103   146  147  148      1
148   0.33582  -0.23787   0.00000   0.13990    0.00000  53.02802   0.00103   147  148  149      1
149   0.39934  -0.11470   0.00000   0.14124    0.00000  72.31374   0.00103   148  149  150      1
150   0.41881   0.02386   0.00000   0.14260    0.00000  91.59945   0.00103   149  150  151      1
151   0.39115   0.16239   0.00000   0.14398    0.00000 110.88516   0.00103   150  151  152      1
152   0.31861   0.28519   0.00000   0.14536    0.00000 130.17088   0.00103   151  152  153      1
153   0.20852   0.37802   0.00000   0.14676    0.00000 149.45659   0.00103   152  153  154      1
154   0.07266   0.42978   0.00000   0.14817    0.00000 168.74231   0.00103   153  154  155      1
155  -0.07407   0.43379   0.00000   0.14960    0.00000-171.97198   0.00103   154  155  156      1
156  -0.21524   0.38869   0.00000   0.15104    0.00000-152.68626   0.00103   155  156  157      1
157  -0.33473   0.29864   0.00000   0.15250    0.00000-133.40055   0.00103   156  157  158      1
158  -0.41857   0.17298   0.00000   0.15396    0.00000-114.11484   0.00103   157  158  159      1
159  -0.45656   0.02527   0.00000   0.15545    0.00000 -94.82912   0.00103   158  159  160      1
160  -0.44352  -0.12817   0.00000   0.15694    0.00000 -75.54341   0.00103   159  160  161      1
161  -0.37992  -0.27003   0.00000   0.15845    0.00000 -56.25769   0.00103   160  161  162      1
162  -0.27201  -0.38402   0.00000   0.15998    0.00000 -36.97198   0.00103   161  162  163      1
163  -0.13116  -0.45666   0.00000   0.16152    0.00000 -17.68626   0.00103   162  163  164      1
164   0.02729  -0.47892   0.00000   0.16307    0.00000   1.59945   0.00103   163  164  165      1
165   0.18570  -0.44730   0.00000   0.16464    0.00000  20.88516   0.00103   164  165  166      1
166   0.32612  -0.36434   0.00000   0.16623    0.00000  40.17088   0.00103   165  166  167      1
167   0.43228  -0.23845   0.00000   0.16783    0.00000  59.45659   0.00103   166  167  168      1
168   0.49146  -0.08309   0.00000   0.16944    0.00000  78.74231   0.00103   167  168    0      1
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .```

Another advantage of aligning the segment junctions is that one can more easily calculate the spacing between rings by using aligned segment centers from the table.

A Reversible Yagi: The NEC-4 GM Input

We have so far overlooked the final 4 entry positions in the NEC-4 GM card. The structure of the GM input follows this model:

```GM  ITG1  NRPT  ROX  ROY  ROZ  XS  YS  ZS  IT1  IS1  IT2  IS2
I1    I2    F1   F2   F3   F4  F5  F6  F7   F8   F9   F10```

Up through floating decimal input F6, the GM input line is identical to the NEC-2 version. However, NEC-4 uses 4 final places to input the start and stop tag numbers and segments numbers for the structure to be copied and replicated at a new position (and orientation). Omission of these 4 entries results in the movement or duplication of all segments in the model. We used this feature in out first example.

If IT1 is zero, then IS1 refers to the absolute segment number in the model. If IT1 is greater than zero, then IS1 refers to the relative (tag-number-related) segment number specified, except that an IS1 value of zero in this case becomes a value of 1. Similar rules apply to IS2, with IT2 referring to the last tag number in the range. If IT2 and IS2 are both zero, the range extends to the last segment defined in the model up to the entry of the GM line.

Let's use a simple example of a reversible 2-element Yagi. such antennas are sometimes used in the lower HF range and made from wire. A permanent installation would not be rotatable, and so one might install alternative driver elements, one on each side of a common reflector wire. The unused driver would have small effects on the overall pattern of the antenna, relative to its omission.

Fig. 6 shows the model to be used for this antenna. We create two wires by standard GW entries. The longer wire (GW 1) is obviously the reflector. The shorter wire (GW 2) defines one driver, spaced 168" from the reflector for this 10.125-MHz array.

Although we save no modeling space, let's use the GM input to define the third wire, that is, the alternate driver. We shall want this driver to be a new structure and to have its own tag number. There, we specific a tag increment of 1 and also 1 new structure. Since we wish to space the wire equally distant from the reflector, but on the opposite side, we order a translation of 336" along the X-axis.

The final 4 entries show the tag and segment numbers for the start and stop of the existing wire to be duplicated and moved. If we look at the antenna view graphic, we get a picture of the total final model shown in Fig. 7 (minus the identifications of the element functions).

The addition of the 4 places to the GM line offers some interesting possibilities for the modeler. In NEC-2, we could only duplicate and move entire structures defined by a tag number. However, NEC-4 permits us to duplicate partial structures within the limits of a given tag number. Indeed, there is no restriction against beginning in the middle of one tag number and ending in the middle of another.

Fig. 8 shows the GM help screen with the start-stop entries opened. We might have begun with tag 1, segment 11 and ended with tag 2, segment 8 (although in the context of our example, I cannot think of a reason for doing so). (Those interested may wish to open the help screen for the GM line in our first example. There we only rotated and translated an existing structure. The differences between that help screen and the present one may be useful in becoming accustomed to the differing appearances of the GM lines in that example and this one.)

Since we have only specified a single source, we may run the model both with and without the GM input line. Fig. 9 compares the free-space E- plane pattern for the original 2-element wire Yagi and the new reversible model. The effects of the undriven alternate driver are clear in its slight addition to gain and the slight decrease in front-to-back ratio.

In order to obtain the reverse-direction pattern, we need only alter the source location on the EX input line. Instead of specifying tag 2, we would enter tag 3 in the second entry position. (See Fig. 6.)

Conclusion

We have used simple models in this exercise because our aim was to illustrate the differences between the NEC-4 and the NEC-2 formats and functions for the GH and GM entries. The true utility of these geometry entry lines begins to emerge when our structures become far more complex. Consider creating a rectangular grid of wires. First, create 3 sides of one grid square with 3 GW entries. Then duplicate the second 2 wires in a single GM line as many times as it takes to make a single row of grid squares, each with an open bottom edge. Now, with a second GM line, duplicate the entire row as many times as it takes to fill the rectangular plane. Since the bottoms of the last row of squares are all open, let's enter a GW line to close the first square. Now add a final GM line to duplicate this line and close the remaining square bottoms. The model remains at a constant ASCII input size whether we are creating a 5-by-5 grid or a 50-by-50 grid. However, since every new GM structure replicates wires and segments, the core run times will be quite different for the two sizes of wire grids.

One common practice among modelers is to run the same model on both NEC-2 and NEC-4, sometimes to detect any differences between results and thereby catch any sensitivity of the model to limitations of the programs. We may perform such tests only where the entry lines for the model are the same for both NEC-2 and NEC-4. A model using only GW entries for the wire geometry and simple control input entries are amenable to being run in both programs without modification.

However, there are many subtle differences in the advanced input structures for lines having the same identification letters within NEC-2 and NEC-4. We have sampled a few of those differences as they relate to the GH and GM inputs. There are many others, and numerous ones apply to the control inputs. If these small exercises have made you a bit self- conscious about the input line differences among the NEC cores, then they will have done some useful work.