Page 471 - Engineering Electromagnetics, 8th Edition

13

CHAPTER

Guided Waves

n this chapter, we investigate several structures for guiding electromagnetic
waves, and we explore the principles by which these operate. Included are trans-
I mission lines, which we first explored from the viewpoint of their currents and
voltages in Chapter 10, and which we now revisit from a fields point of view. We
then broaden the discussion to include several waveguiding devices. Broadly defined,
awaveguide is a structure through which electromagnetic waves can be transmitted
from point to point and within which the fields are confined to a certain extent. A
transmission line fits this description, but it is a special case that employs two conduc-
tors, and it propagates a purely TEM field configuration. Waveguides in general depart
from these restrictions and may employ any number of conductors and dielectrics—or
as we will see, dielectrics alone and no conductors.
The chapter begins with a presentation of several transmission line structures,
with emphasis on obtaining expressions for the primary constants, L, C, G, and R, for
high- and low-frequency operating regimes. Next, we begin our study of waveguides
by first taking a broad view of waveguide devices to obtain a physical understanding
of how they work and the conditions under which they are used. We then explore the
simple parallel-plate structure and distinguish between its operation as a transmission
line and as a waveguide. In this device, the concept of waveguide modes is developed,
as are the conditions under which these will occur. We will study the electric and
magnetic field configurations of the guided modes using simple plane wave models
and the wave equation. We will then study more complicated structures, including
rectangular waveguides, dielectric slab waveguides, and optical fibers. ■

13.1 TRANSMISSION LINE FIELDS
AND PRIMARY CONSTANTS

We begin by establishing the equivalence between transmission line operations when
considering voltage and current, from the point of view of the fields within the line.
Consider, for example, the parallel-plate line shown in Figure 13.1. In the line, we

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