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

CHAPTER 5
Radiofrequency Measurement
In this chapter a detailed description of the radiofrequency (RF)
measurement performed on LIBO prototypes is presented. RF measurement
are very important on this type of structures, since they are
the only way to correct the unavoidable errors that are mirrors of the
mechanical machining tolerances. Note that the errors we have dealings
with can big enough to need a correction to get the desired design value
for the main electromagnetic parameters, even if they are very small
(for example, the frequency error of each cavity is typically included
within the 0.1%).
In the following, each section begins with an overview on the technique,
the instruments <strong>and</strong> the tools used for the specific measurement
explained: single cavity <strong>and</strong> coupled cavities measurement are presented,
<strong>and</strong> for each type of measurement, specific tasks for LIBO are
illustrated. The tuning procedure to obtain a flat longitudinal electric
field for the whole LIBO module is also presented.
1. Introduction
In the analysis of structures where the geometrical dimensions are
comparable with the wavelength of the electromagnetic signals, it is
correct to use the concept of reflected <strong>and</strong> transmitted waves rather
then currents <strong>and</strong> voltages, since propagation effects become very important.
In this frame, the parameters of interest are the scattering parameters,
namely the reflection <strong>and</strong> the transmission coefficients [33].
The sequence of RF measurement to perform is quite the opposite
of the one used for the design. Indeed, the tiles with two half cavities
are normally the first pieces built <strong>and</strong> then the first measurement is
on the single cavities. In figure 5.1 it is shown a flux diagram of the
procedures adopted with LIBO to build up the module, involving both
measurement <strong>and</strong> machining on the copper pieces.
First, as we said, one starts from the measurement on the tiles,
as they come out from the mechanical workshop. On these pieces
three types of measurement are performed: coupled cells measurement
giving coupling coefficients <strong>and</strong> resonant frequencies, single frequency
measurement <strong>and</strong> stacked cell measurement that gives the π/2 mode
frequency <strong>and</strong> the stop-b<strong>and</strong> amplitude. If the values are not correct
<strong>and</strong>/or present a large dispersion around the design values, one can
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