1-4
Skin effect was discussed in NEETS, Module 10, Introduction to Wave Propagation, Transmission
Lines, and Antennas, Chapter 3. Skin effect tends to increase the effective resistance of the conductor.
Although energy transfer in coaxial cable is caused by electromagnetic field motion, the magnitude of the
field is limited by the size of the current-carrying area of the inner conductor. The small size of the center
conductor is even further reduced by skin effect and energy transmission by coaxial cable becomes less
efficient than by waveguides. DIELECTRIC LOSSES are also lower in waveguides than in two-wire and
coaxial transmission lines. Dielectric losses in two-wire and coaxial lines are caused by the heating of the
insulation between the conductors. The insulation behaves as the dielectric of a capacitor formed by the
two wires of the transmission line. A voltage potential across the two wires causes heating of the
dielectric and results in a power loss. In practical applications, the actual breakdown of the insulation
between the conductors of a transmission line is more frequently a problem than is the dielectric loss.
This breakdown is usually caused by stationary voltage spikes or "nodes" which are caused by
standing waves. Standing waves are stationary and occur when part of the energy traveling down the line
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 1-4 illustrates the greater distance between conductors in a waveguide.
Figure 1-4.—Comparison of spacing in coaxial cable and a circular waveguide.
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.
