Electrical Communication is a free textbook on the basics of communication technology. See the editorial for more information....


Wave Guides

The transmission lines and cables considered in the preceding pages are, in a sense, wave guides. They guide the electromagnetic waves that contain the electric energy transmitted. Lines and cables are seldom treated as guides, however.

A solid dielectric material such as wood, glass, or polystyrene will guide electromagnetic waves. These materials have dielectric constants greater than air, and electromagnetic waves will follow a stick or rod of such material in preference to air, but the attenuation is high.

A hollow metal tube of circular, rectangular, or other cross-sectional shape, also will guide energy, in the form of electromagnetic waves, from one point to another. Such hollow metal tubes are what is usually meant by the term wave guide. Wave guides are used at ultrahigh and superhigh frequencies largely because the radiation loss and attenuation loss in open-wire lines and coaxial cables become excessive at such frequencies.

The electromagnetic wave that transmits the electric energy is established at the sending end of the wave guide by an arrangement of one or more conductors that is similar to a radio antenna. Once the energy is transferred to the hollow wave guide, it will be transmitted along the guide until the energy is dissipated in the walls of the guide, or absorbed by other means; the high-frequency energy cannot escape through the conducting walls. In fact, the currents that are established in the walls of the wave guide by the passing electromagnetic waves flow so completely in the surface layers that silver plating the inner surfaces materially reduces the transmission losses. At the distant end of the wave guide the received electromagnetic energy is extracted by an arrangement of conductors constituting a receiving antenna, or the energy is radiated by an electromagnetic horn.

From the engineering viewpoint, it is possible to study wave guides by analogy with transmission-line theory. Wave guides are at present (1949) of limited practical importance, relatively speaking, in commercial communication systems. The use of wave guides in commercial communication systems undoubtedly will grow. Perhaps it is not absurd to envision a transcontinental wave-guide system. References 8 and 33 to 36 are recommended for information on wave guides.



Last Update: 2011-05-30