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

70 5. RADIOFREQUENCY MEASUREMENT
Figure 5.2. Electric pick-up (up) <strong>and</strong> Magnetic loop
(down) for radiofrequency measurement in cavities.
solded on the external conductor. The solder assures the electric
contact. This type of antenna should be used where the
magnetic field is strong <strong>and</strong> the loop can concatenate magnetic
flux. In figure 5.2 an example of realization is shown.
The probe must be introduced in the cavity at a depth which assures
a certain coupling. To avoid perturbation to the cavity field <strong>and</strong> hence
to the resonant frequency, this coupling should not be too large. As a
rule of thumb, we can consider a good coupling for each pick-up when
the minimum of the reflection coefficient is around 0.5 dB below the
reference level outside of resonance.
Once the frequency range for the network analyzer is restricted
around the resonant behaviour, the resonant frequency is that which
corresponds to the maximum of the transmission coefficient, or to the
minimum of the reflection coefficient. The error of the measurement is
mainly due to the resolution of the used instrument 2 .
The measurement of the resonant frequency for an half cavity is
more difficult. In this case the measurement is performed closing the
half cell on an electric mirror which is a good conductor plane 3 . In
this situation, a good electric contact between the conductor plane <strong>and</strong>
the limit surface of the half cavity must be assured, in order to have
a high quality factor <strong>and</strong> a good frequency measurement. But on the
other h<strong>and</strong>, one has to be sure that both the half cavity <strong>and</strong> the plane
are not deformed after the measurement. In conclusion, this type of
measurement has the high risk to produce systematic errors <strong>and</strong> then
a statistical approach is preferred, as it is shown in the next section.
2 For example, a measurement with 100 points over a frequency range of
100 MHz gives an error of ± 1 MHz.
3 From electromagnetic theory it is known that the equivalent circuit of this configuration
has half inductance <strong>and</strong> double capacitance <strong>and</strong> the resonant frequency
does not change.