USING NEC
                         R. P. Haviland, W4MB


	The following information is developed from the definitive
documentation for NEC:

	NUMERICAL ELECTROMAGNETIC CODE (NEC) - METHOD OF MOMENTS
	  Part 1: Theory
	  Part 2: Code          Both in NTSC report AD - A075 289
	  Part 3: Users Guide        in NTSC report AD - A075 460

Available from the National Technical Information Service
				Springfield, VA 22161
				Phone (703) 487-4650

	These reports are available as micro-fiche at reasonable
cost, and in paper at higher cost. The suggested approach is to
get the fiche copy, and to prepare paper copy of most used
material, now possible at most city and county libraries.

	Parts 1 and 2 are needed if changes to the program are
contemplated. Although a purpose of this issue of NEC has been to
make program features available in a simplified form, Part 3 is
probably needed for full use of all features of the program.

GENERAL FEATURES OF NEC

	NEC is for the solution of radiation problems involving
antennas and metal structures. These are modelled by thin wires
or by smooth surfaces, in free space or over ground. Non-
radiating networks and transmission lines may be used to join
wires and structure elements. Perfect or lossy conductors and
lumped element loads may be included. Perfect or lossy earth may
be specified. Antennas and structures may be excited by voltage
or current sources, or by incident waves.

WIRES

	Wires are specified by the geometric position X,Y,Z of the
two ends, and by the radius (a): thus only straight wires or wire
sections are allowed. Each wire can be identified by a tag number
(n), and each can be divided into (m) equal length (Ç) segments.
Identification is by tag #n, segment #m.

	In general the following rules for wire definition in terms
of wavelength (L) should be followed:
	Segment length > L/1000
	Segment length < L/10
	Radius         a/L << .06
	Radius         L/a > 8    standard kernel
		       L/a > 2	 extended kernel
These limits can be relaxed somewhat for long straight wires, or
if reduced accuracy is acceptable. 

	Wires will be assumed to be connected if the gap between
them is less than 1/1000 of the length of the shortest segment,
but it is recommended that the two wire ends to be joined have
exactly the same coordinates. The maximum number of wires which
can connect together is 30.

	There is no restriction on the angle between two wires, but
accuracy will be lost if the center of a segment falls within the
volume of the wire the segment connects to. The risk of this
reduces as the angle between wires approaches 180 degrees.

	Wires which intersect away from their ends are not
connected, but errors will occur if one wire occupies the space
of another one. For accuracy, separate wire centers by several
radii of the largest wire.

	Two wires of different radii may be connected end to end.
Avoid large steps by introducing short sections of stepped wire
diameter.

	Parallel wires must have their segments exactly aligned or
accuracy is lost. If a curved wire is modelled by short straight
segments, their length should be chosen to avoid marked changes
in the angle between adjacent wires.

EXCITATION

	Excitation can be by voltage sources, current sources or an
incident plane wave. Wires must be continuous across feed points:
a voltage source is modelled as a voltage drop, rather than by
true two-terminal "voltage feed". If such a source is specified,
the segments on either side of the feed point should be in a
line, and of the same radius and length, or, for ground
connection, should be vertical.

SURFACES

	Surfaces are modelled as small flat surface patches,
described by the coordinates of the patch center, the angles of
the vector normal to the surface, and the patch area. Patches may
be square, rectangular, quadrilateral or triangular. Large flat
areas may be automatically divided into appropriate patches.

	Patches must not overlap, but must completely cover the
area, and completely enclose a volume: for example, a flat plate
must be modelled as an upper surface, a lower surface, and
edges. The two surfaces must be separated by the same rules as
for wire separation. Accuracy reduces if the two dimensions of a
patch differs markedly, so use a number of patches for edges. In
the same way, model a sphere by more patches for the zones near
the equator, say 4 for each 15 degrees of latitude near the
poles, and 24 near the equator. 

	For good accuracy use 25 patches per square wavelength,
i.e., approximately square patches near 0.2 wavelength per side,
or less.

	Wires may connect to surfaces, but only at their center.
Only a single wire may connect to a given patch, and only one end
of a segment to the patch. Best accuracy is secured if the
patches in the general area of connection are nearly square.

GROUNDS

	Wires and structures may be in free space, over an ideal
ground, or over a lossy earth. Ideal ground is modelled by an
image simulating the ground reflection source. Vertical wires and
structures may end at the surface. for horizonal elements, there
should be several radii separation between element and earth, and
the mean distance SQR(a*a+h*h) should be greater than about 1E-6
wavelength.

	For wires and structures higher than 0.25 wavelength, the
reflection coefficient model may be used: this modifies the
radiation from the image. It is not accurate for antennas having
a large horizonal extent, such as the Beverage. A radial-wire
ground screen can be modelled. Differing ground elevations and
ground conditions (cliffs) can be used with wires only:
reflection is from horizonal surfaces, so a slope is not
modelled. Wires ending on conducting earth may have the charge
set to zero, but souce impedance will depend on segment length.

	For wires only, ground may also be modelled by relations
derived from the Sommerfield equations. Height limit is the same
as for ideal ground. The method requires an auxilary
interpolation table generated separately. A new table is required
for each new frequency or ground condition. If structures are
also present, structure-structure and wire-structure interactions
use reflection coefficient approximations.

	Execution time is about 4 times longer with the Sommerfield
model. The auxiliary table takes a long time to prepare, but can
be used for any wire or structure if the frequency and ground
conditions are appropriate.

	This model can be used with ground screens raised above the
surface. A ground stake cannot be modelled.

GENERAL COMMENTS

	Because of the limitations and interaction complexities of
this (and other) electromagnetic models, modelling must be
considered to be an art. Practice with antennas and structures of
known and experimentally verified characteristics is needed.
Results must be evaluated for reasonableness.

	One method of evaluation is to vary some paramater, and to
observe the result. For example, the number of segments may be
increased in steps. If the result varies smoothly, and tends to
converge to a value, that value is likely to be correct. Similar
steps may be used with such factors a wire diameter, angles,
patch size, and so on.

	When reporting results, be careful to specify the input
conditions, as well as presenting the output.