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.
Also see the Antenna Modeling Programs page for more information.
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