Practical_Antenna_Handbook_0071639586

C h a p t e r 2 0 : M i c r o w a v e W a v e g u i d e s a n d A n t e n n a s 449
a
Figure 20.2 Rectangular waveguide (end
view).
b
The internal walls of a waveguide
are not mirrored surfaces, as in
our optical analogy, but are, rather,
electrical conductors—typically aluminum,
brass, or copper. In order to
further reduce ohmic losses, the internal
surfaces of some waveguides
are electroplated with either gold or
silver, both of which have lower resistivities than the other metals mentioned.
Waveguides are hollow metal pipes and can have either circular or rectangular
cross sections (although the rectangular are, by far, the more common). Figure 20.2 is a
cross-sectional view of a typical rectangular waveguide; a is the wider interior dimension,
and b is the narrower. These letters are considered the standard form of notation
for waveguide dimensions and are used throughout this chapter.
A
B
Fields not confined
in this direction
Field confined
in this direction
Rectangular Waveguide Operation
One way of visualizing how a waveguide
works is to develop the theory
of waveguides from the theory of
elementary parallel-wire transmission
lines. Figure 20.3A shows the basic
parallel transmission line that was introduced
in Chap. 4. The line consists
of two parallel conductors separated
by an air dielectric. Because air won’t
support the conductors, spaced ceramic
or other rigid insulators are
used as supports.
There are several reasons why the
parallel transmission line per se is not
used at microwave frequencies:
• Skin effect increases ohmic losses to
an unacceptable level.
• Insulators supporting the two conductors
are significantly more lossy
at microwave frequencies than at
lower frequencies.
End view of two wire line
Figure 20.3 Parallel transmission line and
fields.