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

5. COUPLING BETWEEN WAVEGUIDE AND BRIDGE COUPLER 39
Zo
d1 d2
jXi
Figure 3.13. The equivalent circuit of the structure
simulated with MAFIA. Two transmission lines <strong>and</strong> a
purely reactance.
on the waveguides lengths. These waveguide lengths are only a tool we
use to explore the behaviour.
X i [kΩ]
40
20
0
−20
−40
−60
−80
3 3.05 3.1 3.15
f [GHz]
Zo
35x10x2mm iris
Figure 3.14. An example of reactance coming from
MAFIA. The iris has dimensions 35 × 10 × 2 mm (model
made in Naples).
In figure 3.14 the reactance crosses the frequency axes, i.e. has zero
value, after it has a resonant behaviour <strong>and</strong> asymptotically goes to zero
for frequencies approaching infinity. The behaviour is always of this
type, in spite of different irises <strong>and</strong> bridge coupler cavity dimensions.
Such a behaviour is well fitted by the equivalent circuit shown in
figure 3.15 on the left: the parallel elements give the resonant behaviour
<strong>and</strong> at high frequency the reactance is mainly due to the capacitance
C1. The correct values for these elements can be found by a least
square fitting. But, more important is the interpretation of the lumped