Practical_Antenna_Handbook_0071639586

CHAPTER 25
Antenna Modeling Software
One of the significant contributions of computer technology to the field of antennas
is the availability of modeling and simulation tools that eliminate most of
the drudgery associated with the complex calculations involved in analyzing
and optimizing antenna designs. For most readers of this book, the best part of this is
the extension of those tools to personal computers at very modest prices and even for
free! Today, any individual interested in experimenting with novel antenna configurations
can do so, and in the process spend little or nothing on modeling software.
Modeling and simulation are used in a wide variety of applications, including management,
science, and engineering. We can now model just about any process, any device,
or any circuit that can be described mathematically. The purpose of modeling, at
least in engineering, is to validate the design quickly and cheaply on the computer before
“bending metal”. Modeling and simulation make it possible to look at alternatives
and gauge the effect of a proposed design change before the change is actually implemented.
The old “cut and try” method works, to be sure, but it is costly in both time and
money—two resources perpetually in short supply. If performance issues and problems
can be solved on a computer, then we are time and money ahead of the game.
Nowhere is this more true than in the field of antennas, where each design iteration
historically involved lowering the antenna from its support, adjusting its dimensions,
reinstalling it many feet up in the air, and conducting a new round of test measurements.
Alternatively, towers with sizeable nonconducting work platforms were utilized,
and all the physical adjustments and fine-Âtuning accomplished “up topside”—in
situ, as it were. A half-Âcentury ago or less, antenna test ranges with these capabilities
were found only at government research centers and commercial antenna manufacturers’
facilities.
Under the Hood
Virtually all commonly available antenna modeling software for personal computers is
based on a numerical analysis technique known as the “method of moments”, in which
each wire or antenna element in the system is broken into many short segments, and the
current in each segment is calculated based on the rules of electromagnetism and the
boundary conditions seen by each segment. The fields generated by the antenna are determined
by summing the contributions of all wire segments throughout model space,
varying both azimuth and elevation angle to compute, tabulate, and graphically plot
relative field strength compared to an isotropic radiator as a function of angular position
from the antenna for a constant RF drive power applied to the antenna feedpoint(s).
Ancillary data, including plots of voltage standing wave ratio (VSWR) versus fre-
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