If a man writes a better book, preaches a better sermon, or makes a better mouse-
trap than his neighbor, the world will make a beaten path to his door.
—RALPH WALDO EMERSON
As mentioned in the preceding chapter, a transmission line can be used to guide EM
energy from one point (generator) to another (load). A waveguide is another means of
achieving the same goal. However, a waveguide differs from a transmission line in some
respects, although we may regard the latter as a special case of the former. In the first
place, a transmission line can support only a transverse electromagnetic (TEM) wave,
whereas a waveguide can support many possible field configurations. Second, at mi-
crowave frequencies (roughly 3-300 GHz), transmission lines become inefficient due to
skin effect and dielectric losses; waveguides are used at that range of frequencies to obtain
larger bandwidth and lower signal attenuation. Moreover, a transmission line may operate
from dc (/ = 0) to a very high frequency; a waveguide can operate only above a certain
frequency called the
and therefore acts as a high-pass filter. Thus, wave-
guides cannot transmit dc, and they become excessively large at frequencies below mi-
Although a waveguide may assume any arbitrary but uniform cross section, common
waveguides are either rectangular or circular. Typical waveguides
are shown in Figure
12.1. Analysis of circular waveguides is involved and requires familiarity with Bessel
functions, which are beyond our scope.
We will consider only rectangular waveguides. By
assuming lossless waveguides (a
— °°, a ~ 0), we shall apply Maxwell's equations with
the appropriate boundary conditions to obtain different modes of wave propagation and the
corresponding E and H
For other t\pes of waveguides, see J. A. Seeger,
Microwave Theory, Components and Devices.
glewood Cliffs, NJ: Prentice-Hall, 1986, pp. 128-133.
Analysis of circular waveguides can be found in advanced EM or EM-related texts, e.g., S. Y. Liao.
Microwave Devices and Circuits,
3rd ed. Englewood Cliffs, NJ: Prentice-Hall, 1990, pp. 119-141.