Waveguide Terminations

Electromagnetic energy is often passed through a waveguide to transfer the energy from a source into space. As previously mentioned, the impedance of a waveguide does not match the impedance of space, and without proper impedance matching, standing waves cause a large decrease in the efficiency of the waveguide.

Any abrupt change in impedance causes standing waves, but when the change in impedance at the end of a waveguide is gradual, almost no standing waves are formed. Gradual changes in impedance can be obtained by terminating the waveguide with a funnel-shaped HORN, such as the three types illustrated in figure 1-44. The type of horn used depends upon the frequency and the desired radiation pattern.

Figure 1-44A. - Waveguide horns. E PLANE SECTORAL HORN

Figure 1-44B. - Waveguide horns. H PLANE SECTORAL HORN

Figure 1-44C. - Waveguide horns. PYRAMID HORN

As you may have noticed, horns are really simple antennas. They have several advantages over other impedance-matching devices, such as their large bandwidth and simple construction. The use of horns as antennas will be discussed further in chapter 3.

A waveguide may also be terminated in a resistive load that is matched to the characteristic impedance of the waveguide. The resistive load is most often called a DUMMY LOAD, because its only purpose is to absorb all the energy in a waveguide without causing standing waves.

There is no place on a waveguide to connect a fixed termination resistor; therefore, several special arrangements are used to terminate waveguides. One method is to fill the end of the waveguide with a graphite and sand mixture, as illustrated in figure 1-45, view (A). When the fields enter the mixture, they induce a current flow in the mixture which dissipates the energy as heat. Another method (view (B)) is to use a high-resistance rod placed at the center of the E field. The E field causes current to flow in the rod, and the high resistance of the rod dissipates the energy as a power loss, again in the form of heat.

Figure 1-45A. - Terminating waveguides.

Figure 1-45B. - Terminating waveguides.

Figure 1-45C. - Terminating waveguides.

Figure 1-45D. - Terminating waveguides.

Still another method for terminating a waveguide is the use of a wedge of highly resistive material, as shown in view (C) of figure 1-45. The plane of the wedge is placed perpendicular to the magnetic lines of force. When the H lines cut through the wedge, current flows in the wedge and causes a power loss. As with the other methods, this loss is in the form of heat. Since very little energy reaches the end of the waveguide, reflections are minimum.

All of the terminations discussed so far are designed to radiate or absorb the energy without reflections. In many instances, however, all of the energy must be reflected from the end of the waveguide. The best way to accomplish this is to permanently weld a metal plate at the end of the waveguide, as shown in view (D) of figure 1-45.

Q.37 What device is used to produce a gradual change in impedance at the end of a waveguide?
Q.38 When a waveguide is terminated in a resistive load, the load must be matched to what property of the waveguide?
Q.39 What is the primary purpose of a dummy load?
Q.40 The energy dissipated by a resistive load is most often in what form?