is reflected by an impedance mismatch with the load.
The voltage potential of the standing waves at the
points of greatest magnitude can become large enough
to break down the insulation between transmission
line conductors.
The dielectric in waveguides is air, which has a
much lower dielectric loss than conventional insulating
materials. However, waveguides are also subject to
dielectric breakdown caused by standing waves.
Standing waves in waveguides cause arcing, which
decreases the efficiency of energy transfer and can
severely damage the waveguide.
Also since the
electromagnetic fields are completely contained within
the waveguide, radiation losses are kept very low.
Power-handling capability is another advantage
of waveguides. Waveguides can handle more power
than coaxial lines of the same size because
power-handling capability is directly related to the
distance between conductors. Figure 3-18 illustrates
the greater distance between conductors in a
waveguide.
Figure 3-18.—Comparison
and a circular waveguide.
of spacing in coaxial cable
In view of the advantages of waveguides, you
would think that waveguides should be the only type
of transmission lines used. However, waveguides have
certain disadvantages that make them practical for use
only at microwave frequencies.
WAVEGUIDE DISADVANTAGES
Physical size is the primary lower-frequency
limitation of waveguides. The width of a waveguide
must be approximately a half wavelength at the
frequency of the wave to be transported. For example,
a waveguide for use at 1 megahertz would be about
700 feet wide. This makes the use of waveguides at
frequencies below 1000 megahertz increasingly
impractical. The lower frequency range of any system
using waveguides is limited by the physical dimensions
of the waveguides.
Waveguides are difficult to install because of their
rigid, hollow-pipe shape. Special couplings at the
joints are required to assure proper operation. Also,
the inside surfaces of waveguides are often plated with
silver or gold to reduce skin effect losses. These
requirements increase the costs and decrease the
practicality of waveguide systems at any other than
microwave frequencies.
DEVELOPING THE WAVEGUIDE
FROM PARALLEL LINES
You may better understand the transition from
ordinary transmission line concepts to waveguide
theories by considering the development of a
waveguide from a two-wire transmission line. Figure
3-19 shows a section of a two-wire transmission line
supported on two insulators. At the junction with the
line, the insulators must present a very high impedance
to ground for proper operation of the line. A low
impedance insulator would obviously short-circuit the
line to ground, and this is what happens at very high
frequencies. Ordinary insulators display the character-
istics of the dielectric of a capacitor formed by the
wire and ground.
As the frequency increases, the
overall impedance decreases. A better high-frequency
insulator is a quarter-wave section of transmission
line shorted at one end. Such an insulator is shown
in figure 3-20. The impedance of a shorted quar-
ter-wave section is very high at the open-end junction
with the two-wire transmission line.
This type of
insulator is known as a METALLIC INSULATOR
and may be placed anywhere along a two-wire line.
3-10
