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 between the sample energy and the energy in the waveguide is constant. Otherwise, the sample would contain no useful information. The ratio is usually stamped on the coupler in the form of an attenuation

Figure 3-56.—Incident wave in a directional coupler designed to sample incident waves.

The effect of a directional coupler on any reflected energy is illustrated in figure 3-57. Note that these two waves do not travel the same distance to the pickup probe. The wave represented by the dotted line travels 1/2λ further and arrives at the probe 180 degrees out of phase with the wave, represented by the solid line. Because the waves are 180 degrees out of phase at the probe, they cancel each other and no energy is induced into the pickup probe. When the reflected energy arrives at the absorbent material, it adds and is absorbed by the material.

Figure 3-57.—Reflected wave in a directional coupler.

A directional coupler designed to sample reflected energy is shown in figure 3-58. The absorbent material and the probe are in opposite positions from the directional coupler designed to sample the incident energy. This positioning causes the two portions of the reflected energy to arrive at the probe in phase, providing a sample of the reflected energy. The transmitted energy is absorbed by the absorbent material.

Figure 3-58.—Directional coupler designed to sample reflected energy.

A simple bidirectional coupler for sampling both transmitted and reflected energy can be constructed by mounting two directional couplers on opposite sides of a waveguide, as shown in figure 3-59.

Figure 3-59.—Bidirectional coupler.

Cavity Resonators

By definition, a resonant cavity is any space completely enclosed by conducting walls that can contain oscillating electromagnetic ﬁelds and possess resonant properties. The cavity has many advantages 3-26