In antenna applications it is necessary to use some form of transmission line to connect the antenna to a transmitter or receiver. The purpose of this chapter is to provide the essential characteristics of propagation of the most common forms of transmission lines.
A waveguide is a special form of transmission line consisting of a hollow metal tube. The wall of the tube provides distributed inductance, while the empty space between the walls of the tube provides distributed capacitance: Figure below
Waveguides are practical only for extremely high frequency signals, where the wavelength approaches the cross-sectional dimensions of the waveguide. Under such frequencies, waveguides are useless as electric transmission lines.
However, when they function as transmission lines, waveguides are considerably simpler than two-conductor cables - especially coaxial cables - in their manufacture and maintenance. With only one conductor (the "helmet" of the waveguide), there is no concern with the appropriate conductor-conductor spacing or the consistency of the dielectric material, since the only dielectric in a waveguide is air. Humidity is not as serious a problem in waveguides as it is inside coaxial cables, either, and therefore waveguides will save the need for "filling" of gas.
Waveguides can be thought of as conduits for electromagnetic energy, the waveguide itself is nothing more than a "director" of energy and not as a signal conductor in the normal sense of the word. In a sense, all transmission lines function as conduits of electromagnetic energy by transporting pulses or high-frequency waves, directing waves while the banks of a river direct a tide. However, because the waveguides are single-conductor elements, the propagation of electrical energy by a waveguide is very different in nature from the propagation of electric energy by a two-conductor transmission line.