CM NEC Input File CM Monopole radius 0.001m, length 17m above perfect ground CM Excitation at base by a 1V source

As you can see, any line at the start of the NEC input file
beginning

Now that the comments are finished, a

CE

GW 1 8 0.00000 0.00000 0.00000 0.00000 0.00000 17.00000 0.00100

Let's look at this in more detail... After the initial card type is a number,
which is the

The next six numbers specify the X,Y, and Z cartesian co-ordinates of the
first and second ends of the wire. So in this case end one is located
at the origin (0.0,0.0,0.0) and the second end is located 17m above this
at (0.0,0.0,17.0). As is usual in mathematics the co-ordinates are such that
the X and Y axes lie in a horizontal plane, and the Z axis points vertically
upwards. The final number in the GW card specifies the radius of the wire
(again in metres so be careful with the units). It's possible to miss
this value out and use a

As that concludes our structure specification, a

GE 1

The one is to indicate that a Ground Plane is going to be used; in fact you probably want one for most amateur radio applications, but it wouldn't be appropriate if you were modelling an aircraft antenna, or something a long way from (conducting) ground.

The ground plane in the NEC code is always taken to be the X-Y plane, that is all the points where the Z co-ordinate is zero. If you have been thinking hard you will have seen a possible problem. Earlier on we said that the antenna would be energised at the ground; but doesn't the antenna touching ground at its base simply `short-out' the energising source? The answer is, that with the `1' flag, no it doesn't. If a `-1' flag had been used, then the current at the ground would be zero.

Before we go on, it is now possible to use a structure viewer like

GN 1 0 0 0 0.00E+00 0.00E+00 0.00E+00 0.00E+00 0.00E+00 0.00E+00

The first number, here `1' means use a perfectly conducting ground. The remaining numbers are all zero for a perfect ground. They can be used however to specify the conductivity and dielectric constant of a non-perfect ground. Hopefully a description of some of the other models will be added to this tutorial in due course.

FR 0 1 0 0 1.81E+01 0.00E+00 0.00E+00 0.00E+00 0.00E+00 0.00E+00This is a simple single frequency analysis, so the card tells the computer to energise the source at 18.1MHz (16.6 metres wavelength).

EX 0 1 1 0 1.00E+00 0.00E+00 0.00E+00 0.00E+00 0.00E+00 0.00E+00This means energise by a voltage source (i.e. and applied Electric Field), at the segment with tag number one, at the first segment (i.e. the bottom end). The eighth segment would of course be the top end. The other numbers are only used for incident plane wave excitation, so they are left at zero.

RP 0 37 72 1000 0.00E+00 0.00E+00 5.00E+00 5.0E+02 0.00E+00 0.00E+00This example asks for a Normal Mode plot (that is compute just the space-wave fields and not the surface wave along the ground). The `37' specifies that the plot is to be at 37 values of Theta, and the `72' the number of values of Phi, at which the field is to be computed. The `1000' isn't a number but a series of binary flags, telling the computer what information to print out. In our example vertical and horizontal together with total gain are to be printed, and the gain printed is to be power gain, not averaged over any region.

Now the initial values of theta and phi are given (both zero degrees), and the increment angles (both 5 degrees).

EN

In the case of a simple model like this the program will automatically
execute the analysis and there is no need for a special

CM NEC Input File CM Monopole radius 0.001m, length 17m above perfect ground CM Excitation at base by a 1V source E GW 1 8 0.00000 0.00000 0.00000 0.00000 0.00000 17.00000 0.00100 GE 1 GN 1 0 0 0 0.00E+00 0.00E+00 0.00E+00 0.00E+00 0.00E+00 0.00E+00 FR 0 1 0 0 1.81E+01 0.00E+00 0.00E+00 0.00E+00 0.00E+00 0.00E+00 EX 0 1 1 0 1.00E+00 0.00E+00 0.00E+00 0.00E+00 0.00E+00 0.00E+00 RP 0 37 72 1000 0.00E+00 0.00E+00 5.00E+00 5.0E+02 0.00E+00 0.00E+00 EN

So copy the model above into a text file; and execute the model. If all went well you should have a fairly large file containing the output which should look something like this.

NEC model output to be inserted.

The output can be broken up into various sections; but for a quick idea
of what it represents a gain pattern can be plotted using

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This text of this tutorial is Copyright © 1999 Alan Bain. It may
be reproduced freely for non-commerical purposes.