Practical_Antenna_Handbook_0071639586

546 P a r t V I I : T u n i n g , T r o u b l e s h o o t i n g , a n d D e s i g n A i d
quency, calculation of feedpoint voltage and current, currents in all wire segments, etc.,
are also available.
The core for most of the programs in use today was originally developed for the
U.S. Navy. Called the Numerical Electromagnetic Computation (NEC) program, it has had
a number of updates over the years; the most recent that is available to users and software
authors is NEC-Â4. The government still charges for an NEC-Â4 license, and it cannot
be exported without a permit from the U.S. government, but an earlier version,
NEC-Â2, is available free of charge and is currently the version most commonly used by
hobbyists and as the core of free or inexpensive NEC-Âbased applications. When PCs
became popular, a smaller version known as miniNEC was made available. Initially it
ran under BASIC, but faster FORTRAN versions became available later. You can still
download a number of miniNEC variants from several Internet sites.
Despite its computational power, the core NEC software is not particularly user-Â
friendly, with an input/output format analogous to the old IBM punched card decks of
a half-Âcentury or more ago. In particular, it lacks any pretense of a GUI (graphical user
interface), such as those enjoyed by modern-Âday PC and Macintosh users. However, a
few vendors offer compound products consisting of the core miniNEC program
wrapped in shells that provide decent GUIs for the user. Four excellent examples at the
time of this writing are EZ-ÂNEC, NEC-ÂWin Plus, and 4nec2—all for the PC—and
cocoaNEC for the Mac. The latter two are free. (Vendor information for these and other
modeling packages can be found in App. B.) Some vendors have more than one NEC-Â
based product, with pricing and performance set by the maximum complexity of the
antenna system that can be analyzed and whether or not it employs NEC-Â4 (with that
core’s added requirement for separately purchasing a government license). In addition,
there are other free products—often with somewhat less versatility or capability—that
may well be perfectly suitable for a specific antenna design, analysis, or optimization
task.
NEC-Âbased programs model wire antennas using the standard three-Âdimensional
cartesian coordinate system, as shown in Fig. 25.1. In normal practice, the z axis is vertical,
and the x and y axes are at right angles (orthogonal) to it and to each other. F and q
are the azimuth and elevation angles, respectively. The user “places” the antenna to be
examined in this space by identifying the x, y, z coordinates of both ends of each straight
wire element that is part of the antenna, along with similar representation of any nearby
conducting surfaces likely to affect the performance of the antenna being modeled. (No
model of an attic antenna would be complete or yield accurate results, for instance,
without including any proximate house wiring in the attic or between the attic floor and
the ceiling of the story below.) If the antenna is located in free space, the orientation of
the antenna with respect to this x, y, z coordinate system is not particularly important,
but if the antenna is near a conducting or dielectric medium (such as ground, seawater,
etc.) having characteristics different from those of free space, orientation of the antenna
properly with respect to the z-Âaxis becomes important, since most of the inexpensive or
free programs available assume a flat surface (earth, a car top, the ocean, etc.) spanning
the xy-Âplane that corresponds to z = 0.
Figure 25.2 depicts a single-Âwire antenna in free space. It has been laid out in model
space centered on the origin (0,0,0) and extending from x = x 1 (expressed as a positive
number) to x = x 2 (expressed as a negative number). In the more common case of an
antenna close (in wavelengths) to an underlying ground such as earth, z at all points
along the wire would be a positive, nonzero number if the wire represented a horizontal