Theory, Design and Tests on a Prototype Module of a Compact ...

7. MEASUREMENT OF THE WAVEGUIDE-MODULE COUPLING 93
Figure 5.24. Coupling through an iris. Sketch of a
resonant cavity coupled with a transmission line.
Figure 5.25. Coupling through an iris. The generator
is referred to the secondary.
These set of parameters are involved in a equivalent circuit that represent
the cavity in one mode <strong>and</strong> can be used in most applications.
7.2. Equivalent circuits, definitions of Q0, QL, Qext <strong>and</strong> coupling
coefficient β. Consider a signal generator connected to a transmission
line, those characteristic impedance Z0 is equal to the internal
impedance of the generator. The transmission line is coupled to a resonant
cavity through a magnetic coupling mechanism. The resonant
cavity is represented by a lumped circuit as it is shown in figure 5.24.
The magnetic coupling mechanism is represented as a transformer,
where L1 is the auto-inductance <strong>and</strong> M is the mutual-inductance of
the transformer. The coupling mechanism is supposed to be without
loss.
We can either refer the cavity impedance to the primary or to refer
the generator to the secondary. The latter case leads to the definition
of matched generator, whose internal impedance is
<strong>and</strong> this leads to the circuit of figure 5.25.
Z = (ωM)2
, (5.16)
jωL1 + Z0