and electric transverse planes and forms an equivalent parallel-LC circuit across the waveguide. At the resonant frequency, the iris acts as a high shunt resistance. Above or below resonance, the iris acts as a capacitive or inductive reactance.
POSTS and SCREWS made from conductive material can be used for impedance-changing devices in waveguides. Views A and B of figure 3-45, illustrate two basic methods of using posts and screws. A post or screw that only partially penetrates into the waveguide acts as a shunt capacitive reactance. When the post or screw extends completely through the waveguide, making contact with the top and bottom walls, it acts as an inductive reactance. Note that when screws are used, the amount of reactance can be varied.
Figure 3-45.—Conducting posts and screws.
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 3-46. The type of horn used depends upon the frequency and the desired radiation pattern.
Figure 3-46.—Waveguide horns.
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.
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 3-47, view A. When the fields enter the mixture, they induce a current flow in the mixture that 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.
Still another method for terminating a waveguide is the use of a wedge of highly resistive material, as shown in view C of figure 3-47. The plane of the wedge is placed perpendicular to the magnetic lines
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