The discussion of waveguide devices, up to this point, has been concerned only with the transfer of energy from one point to another. Many waveguide devices have been developed, however, that modify the energy in some fashion during transit. Some devices do nothing more than change the direction of the energy. Others have been designed to change the basic characteristics or power level of the electromagnetic energy.
This section will explain the basic operating principles of some of the more common waveguide devices, such as DIRECTIONAL COUPLERS, CAVITY RESONATORS, and HYBRID JUNCTIONS.
The directional coupler is a device that provides a method of sampling energy from within a waveguide device for measurement or use in another circuit. Most couplers sample energy traveling in one direction only. However, directional couplers can be constructed that sample energy in both directions. These are called BIDIRECTIONAL couplers and are widely used in radar and communications systems.
Directional couplers may be constructed in many ways. The coupler illustrated in the figure below is constructed from an enclosed waveguide device section of the same dimensions as the waveguide in which the energy is to be sampled. The "b" wall of this enclosed section is mounted to the "b" wall of the waveguide from which the sample will be taken.
There are two holes in the "b" wall between the sections of the coupler. These two holes are 1/4 wavelength apart. The upper section of the directional coupler has a wedge of energy-absorbing material at one end and a pickup probe connected to an output jack at the other end. The absorbent material absorbs the energy not directed at the probe and a portion of the overall energy that enters the section.
The next figure below illustrates two portions of the incident wavefront in a waveguide. The waves travel down the waveguide in the direction indicated and enter the coupler section through both holes. Since both portions of the wave travel the same distance, they are in phase when they arrive at the pickup probe. Because the waves are in phase, they add together and provide a sample of the energy traveling down the waveguide.
The sample taken is only a small portion of the energy that is traveling down the waveguide. The magnitude of the sample, however, is proportional to the magnitude of the energy in the waveguide. The absorbent material is designed to ensure that the ratio between the sample energy and the energy in the waveguide is constant. Otherwise the sample would contain no useful information
Incident wave in a directional coupler designed to sample incident waves.
The ratio is usually stamped on the coupler in the form of an attenuation factor. The effect of a directional coupler on any reflected energy is illustrated in the figure below. Note that these two waves do not travel the same distance to the pickup probe. The wave represented by the dotted line travels 1/2 wavelength 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 in the pickup probe. When the reflected energy arrives at the absorbent material, it adds and is absorbed by the material.
Reflected wave in a directional coupler.
A directional coupler designed to sample reflected energy is shown in the next figure below. 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 sampled transmitted energy, however, is absorbed by the absorbent material.
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 the last figure below.
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