Q-32. Loose or inefficient coupling of energy into or out of a waveguide can be accomplished by the use
of what method?
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
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. Figure 1-43A and 1-43B, 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.