Waveguide Impedance Matching

Waveguide transmission systems are not always perfectly impedance matched to their load devices. The standing waves that result from a mismatch cause a power loss, a reduction in power-handling capability, and an increase in frequency sensitivity. Impedance-changing devices are therefore placed in the waveguide to match the waveguide to the load. These devices are placed near the source of the standing waves.

Figure 1-42 illustrates three devices, called irises, that are used to introduce inductance or capacitance into a waveguide. An iris is nothing more than a metal plate that contains an opening through which the waves may pass. The iris is located in the transverse plane.

Figure 1-42. - Waveguide irises.

An inductive iris and its equivalent circuit are illustrated in figure 1-42, view (A). The iris places a shunt inductive reactance across the waveguide that is directly proportional to the size of the opening. Notice that the edges of the inductive iris are perpendicular to the magnetic plane. The shunt capacitive reactance, illustrated in view (B), basically acts the same way. Again, the reactance is directly proportional to the size of the opening, but the edges of the iris are perpendicular to the electric plane. The iris, illustrated in view (C), has portions across both the magnetic and electric 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 1-43, illustrate two basic methods of using posts and screws. A post or screw which 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 1-43A. - Conducting posts and screws. PENETRATING

Figure 1-43B. - Conducting posts and screws. EXTENDING THROUGH

Q.33 What is the result of an impedance mismatch in a waveguide?
Q.34 What is used to construct irises?
Q.35 An iris placed along the "b" dimension wall produces what kind of reactance?
Q.36 How will an iris that has portions along both the "a" and "b" dimension walls act at the resonant frequency?