Practical_Antenna_Handbook_0071639586

CHAPTER 3
Antenna Basics
An antenna is an example of a transducer—a device that converts one form of
energy to another. Other examples include audio loudspeakers (electronic signals
→ sound waves), thermocouples (temperature changes → electrical signals),
and woodstoves (stored chemical energy → heat).
An antenna converts time-varying electrical currents that are confined and guided
within a circuit or transmission line to a radiated electromagnetic wave varying at
the same rate and propagating outward through space—completely independent of the
circuit that produced it. It is not unreasonable to visualize this process as “freeing” the
electromagnetic waves created by the time-varying currents and “launching” them into
space—much like a slingshot launches a projectile. As we shall see in future chapters,
the efficiency with which an antenna performs this conversion is one of the key measures
of its “goodness”, and devices that accomplish this conversion efficiently share
special physical and geometrical attributes.
In this chapter we shall summarize just enough basic electronics and electromagnetic
theory to allow us to develop a feeling for the physics of antennas. That’s about all
we can do, because an in-depth knowledge of antenna theory requires a command of
various advanced mathematics techniques, including calculus, coupled with completion
of specialized antenna courses usually not taken until graduate school following
the completion of a four- or five-year bachelor’s degree in electrical engineering or
physics.
Circuit Fundamentals
In this chapter and throughout this book, we will assume (without deriving or
Âproving)—and frequently use—some fundamentals of physics and electronics engineering:
• The superposition principle. Unless otherwise noted, all components, circuits,
systems, antennas, transmission lines, and propagation media discussed in this
book are linear in their operation. That is to say, if an input voltage of 1 V applied
to some device or system of devices results in an output of, say, 1 A (ampere)
elsewhere in the system, an input of 5.32 V will result in an output of 5.32 A. In
other words, output is directly and exactly proportional to input. Further, we
assume the devices are bilateral. That is, if we switch things around and swap
what we call “input” and “output”, applying 5.32 A at the old output (now the
new input) will result in a voltage of 1 V at the original input (now the new
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