between the sample energy and the energy in thewaveguide is constant. Otherwise, the sample wouldcontain no useful information. The ratio is usuallystamped on the coupler in the form of an attenuationfactor.and the probe are in opposite positions from thedirectional coupler designed to sample the incidentenergy. This positioning causes the two portions ofthe reflected energy to arrive at the probe in phase,providing a sample of the reflected energy. Thetransmitted energy is absorbed by the absorbentmaterial.Figure 3-56.—Incident wave in a directional couplerdesigned to sample incident waves.The effect of a directional coupler on any reflectedenergy is illustrated in figure 3-57. Note that thesetwo waves do not travel the same distance to thepickup probe. The wave represented by the dottedline travels further and arrives at the probe 180degrees out of phase with the wave, represented bythe solid line. Because the waves are 180 degreesout of phase at the probe, they cancel each other andno energy is induced into the pickup probe. Whenthe reflected energy arrives at the absorbent material,it adds and is absorbed by the material.Figure 3-57.—Reflected wave in a directionalcoupler.A directional coupler designed to sample reflectedenergy is shown in figure 3-58. The absorbent materialFigure 3-58.—Directional coupler designed to sampleretlected energy.A simple bidirectional coupler for sampling bothtransmitted and reflected energy can be constructedby mounting two directional couplers on opposite sidesof a waveguide, as shown in figure 3-59.Figure 3-59.—Bidirectional coupler.Resonatorsdefinition, a resonant cavity is any spaceCavityBycompletely enclosed by conducting- walls that cancontain oscillating electromagnetic fields and possessresonant properties. The cavity has many advantages3-26
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