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. Wave-guides are practical only for extremely high frequency signals, where the wavelength approximates the cross-sectional dimensions of the waveguide. Under such frequencies, the 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 "shell" of the waveguide), there is no problem with adequate spacing from conductor to conductor or with 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 in coaxial cables, so waveguides are often spared from the need to "fill" the gas.
It may be thought that the waveguides are conduits for electromagnetic energy, and that the waveguide is no more than a "director" of the energy rather than a signal conductor in the normal sense of the word. In a certain sense, all the transmission lines function as conduits of electromagnetic energy when pulses or waves of high frequency are transported, directing the waves when the banks of a river direct a tidal wave. However, because the waveguides are elements of a single conductor, the propagation of the electrical energy down a waveguide is of a nature very different from the propagation of electrical energy in a transmission line of two. drivers