RF coupling impedance measurements versus simulations

Bench measurements nowadays represent an important tool to estimate the coupling impedance of any particle accelerator device. The well-known technique based on the coaxial wire method allows to excite in the device under test a field similar to the one generated by an ultra-relativistic point charge. Nevertheless the measured impedance of the device needs comparisons to numerical simulations and, when available, theoretical results.We discuss the basics of the coaxial wire method and report the formulae widely used to convert measured scattering parameters to longitudinal and transverse impedance data. We discuss typical measurement examples of interest for the LHC. In case of resonant structures, impedance measurements and comparison with simulations become easier. The bead-pull technique may be used in this case.