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 500 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.
Q-3. Why are coaxial lines more efficient at microwave frequencies than two-wire transmission lines?
Q-4. What kind of material must be used in the construction of waveguides?
Q-5. The large surface area of a waveguide greatly reduces what type of loss that is common in
two-wire and coaxial lines?
Q-6. What causes the current-carrying area at the center conductor of a coaxial line to be restricted to
a small layer at the surface?
Q-7. What is used as a dielectric in waveguides?
Q-8. What is the primary lower-frequency limitation of waveguides?
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 1-5
shows a section of 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 characteristics 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 1-6. The impedance of a shorted quarter-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. Note that quarter-wave sections are
insulators at only one frequency. This severely limits the bandwidth, efficiency, and application of this
type of two-wire line.