10.3 WAVEGUIDES

So far, we have considered transmission media in which signals are conveyed as voltages and currents on metallic conductors. At higher frequencies, however—particularly in the super-high-frequency (SHF) range and above—the losses in conventional cables become excessive, and transmission by guided electromagnetic waves becomes more efficient. Waveguides support the propagation of electromagnetic energy as a self-contained field pattern within an enclosed structure rather than as conduction current along wires.

As frequency increases, conductor losses rise due to skin effect and dielectric losses increase in insulating materials. At microwave frequencies, these losses become significant enough that hollow metallic structures, which guide energy primarily in the field rather than through bulk conduction current, offer lower attenuation and superior power handling.

The most common type is the metal waveguide, in which the wave is confined within a hollow metallic tube, usually rectangular or circular in cross-section. Another, much more familiar form of waveguide is the optical fiber, where electromagnetic energy at optical frequencies is guided along a dielectric core rather than by metal walls. Although operating in vastly different frequency ranges, both metal waveguides and optical fibers rely on boundary conditions that confine electromagnetic fields—metallic walls enforcing tangential electric-field conditions in one case, and refractive-index discontinuities enforcing total internal reflection in the other.