Practical_Antenna_Handbook_0071639586

448 P a r t V I : A n t e n n a s f o r O t h e r F r e q u e n c i e s
resistance with increasing frequency. Dielectric losses are caused by the electric field
acting on the molecules of the insulator, thereby causing heating through molecular
agitation, and tend to be largest when the wavelength is comparable to the interatomic
resonances of the dielectric. Radiation losses result from incomplete shielding (in a coaxial
line) or from imbalance and incomplete cancellation of RF in the two sides of the
line—usually because at some frequency the spacing between the wires becomes too
large a percentage of the RF signal wavelength. All these losses are proportional to both
frequency and transmission line length and serve to limit both the maximum power
that can be applied at the transmitter end of the line and the maximum frequency for a
given line length.
To circumvent these problems, waveguides are used. What is a waveguide? Probably
the simplest response is to say that a waveguide is to radio waves what an optical fiber
is to light waves. Each is a hollow tube or tunnel through which an electromagnetic
wave is “encouraged” by the tube’s characteristics to pass. Consider the light pipe analogy
depicted in Fig. 20.1. A flashlight serves as our RF source, which (given that light is
also an electromagnetic wave) is not altogether unreasonable. In Fig. 20.1A the source
radiates into free space and spreads out as a function of distance. The intensity per unit
area falls off as a function of distance (D) according to the inverse square law (1/D 2 ).
But now consider the transmission scheme in Fig. 20.1B. The light wave still propagates
over distance D but is now confined to the interior of a mirrored pipe. Almost all
of the energy coupled to the input end is delivered to the output end, where the intensity
is practically undiminished. Although not perfect, the light pipe analogy illuminates
(pardon the pun . . . again) the value of microwave waveguides. Similarly,
fiber-optic technology is waveguide-like at optical (IR and visible) wavelengths. In fact,
the analogy between fiber optics and waveguide is a more rigorous comparison than
the simplistic light pipe analogy.
D
A
Large diffused beam
"Light pipe"
B
Small intense beam
Figure 20.1 Waveguide analogy to light pipe.