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

3. RF MEASUREMENT ON COUPLED CAVITIES 75
Δ f [MHz]
0
−2
−4
−6
−8
−10
−12
0 2 4 6 8 10
[half turns]
Figure 5.5. The change in frequency due to a metallic
plunger for the measurement of r over Q.
the calculation it is only necessary to use known relative steps advancement
for the plunger, rather than exact measurement of how much the
plunger penetrates into the cavity.
Consider, for example, the figure 5.5. It shows the typical change
in frequency due to a 2.3 mm diameter cylindrical plunger which penetrates,
through a conductor plane, an half pill-box like cavity with a
resonant frequency of 3.0359 GHz. The plunger is moved through a
series of positions at which the changes of resonant frequency are measured.
The slope is determined from a numerical interpolation of the
curve.
3. RF Measurement on coupled cavities
From the electromagnetic theory it is known that two or more resonators
coupled together lose their single resonant frequencies <strong>and</strong> meet
in a number of modes equal to the number of resonators [17]; those frequencies
mainly depend from the single resonant frequencies <strong>and</strong> from
the coupling between the resonators. In few words, the resonant modes
of the coupled cavities correspond to all the possible ways to satisfy
the new boundary condition.
From the single cavity point of view, also the own resonant frequency
is modified from the coupling mechanism. For example, an
aperture between two cavities in a zone where the magnetic field is
strong changes the stored magnetic energy of the cavities <strong>and</strong>, as a
consequence, the single resonant frequencies.