NEC code examples

1. Dipole
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1.1 CM dipole with feed line and real ground || start of comment
1.2 CE || end of comment
Antenna geometry description. GW --geometry wire.
2.1 wire 1 made up of 21 segments, from point (-16.85,0,70) to
(16.85,0,70). The wire located on XZ plane(Y=0) with radidus 3.367e-3
2.2 wire 2 made up of 3 segments, from point (-1,0,1) to (1,0,1) with radius 3.367e-3
2.3 all dimensions are multiplied by 0.3048(=12*25.4/1000), it indicates previous dimension are in feet.
2.4 Geometry end with 0(0=no ground plane)
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2.1 GW 1 21 -16.85 0 70 17 0 70 3.367e-3
2.2 GW 2 3 -1 0 1 1 0 1 3.367e-3
2.3 GS 0 0 0.3048 ' All in ft.
2.4 GE 0
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Transmission line connection the antenna wires.
3.1 TL connected from wire 1, segment 11, to wire 2, segment 2.
The impedance of TL line is 50Ohm.
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3.1 TL 1 11 2 2 50 0
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4.1 Wire conductivity =5.8e7
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4.1 LD 5 0 0 0 5.8e7 0
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5.1 Excitation: voltage source(0) on tagnr=2,segment=2, 0=no action,voltage (real)=1.0
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5.1 EX 0 2 2 0 1 0
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6.1 Ground parameters: 0=finite ground,0=no ground screen,0,0=blank,dielectric constant=13, conductivity of ground=0.005

6.1 GN 0 0 0 0 13 0.005
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7.1 frequency, 0=linear, nr=1(no of steps),0,0,start freq=14.15MHz,step=0
7.1 FR 0 1 0 0 14.15 0

NEC modeling

1. Wire - defined by two of its end points and radius(P1,P2,r)
Wire modeling
1.1 Divide wire into segments using piece-wise linear fit on curves.
*Typically the segment length should be less than 0.1*lamda of the desired frequency.
For critical section, 0.05*lamda or less may be needed. However, extremely short
segment(<10^-3 lamda) should be avoided as it may lead to numerical inaccuracy.
1.2 wire radius-- only currents in the axial direction of the segment are considered.
two appx in NEC are available:
* thin-wire kernel--- The current on the surface of the wire is
reduced to a filament of current on the segment axis.
* extended thin-wire kernel ---assume the current uniformly distributed around
the segment.
* Delta/a>8 for thin-wire kernel, delta/a>2 for extended thin-wire kernel.

antenna pattern

1.E and H-plane patterns(note:we're taking about far-field pattern)
The 3-D antenna pattern is useful for illustrative purpose. The cross-section/plane diagrams of this 3-D pattern is more useful for normal application. Two cross-section planes:
1.E-plane
for a linearly-polarized antenna, this is a plane made up of E-field vector and the direction of max. radiation. The E-plane determines the polarization of the EM wave.
For vertically- polarized antenna, E-plane is elevation plane/vertical plane. For horizontally-polarized antenna, E-plane is azimuth plane.
For dipole antenna, E-plane contains the dipole antenna and
2.H-plane
made up of H-field vector and direction of max. radiation. H-plane perpendicular to E-plane as in far field (E perpendicular to H).

E and H-planes share one axis: direction of max. radiation.

antenna pattern