DIRECTIONAL COUPLERS are devices that permit the sampling of the energy in a waveguide.
Directional couplers may be constructed to sample energy in one direction only or in both directions. The
energy removed by the directional coupler is a small sample that is proportional to the magnitude of the
energy in the waveguide. An example of a directional coupler is shown in the illustration.
A RESONANT CAVITY is any space completely enclosed by conductive walls that can contain
oscillating electromagnetic fields and can possess resonant properties. Several cavity shapes are shown in