  The coaxial wire method is a common and appreciated technique to assess the
beam coupling impedance of an accelerator element from scattering parameters.
Nevertheless, the results obtained from wire measurements could be inaccurate
due to the presence of the stretched conductive wire that artificially creates
the conditions for the propagation of a Transverse ElectroMagnetic (TEM) mode.
The aim of this work is to establish a solid technique to obtain the beam
coupling impedance from electromagnetic simulations, without modifications of
the device under test. In this framework, we identified a new relation to get
the longitudinal resistive wall beam coupling impedance of a circular chamber
directly from the scattering parameters and demonstrated that it reduces to the
exact theoretical expression. Furthermore, a possible generalization of the
method to arbitrary cross-section chamber geometries has been studied and
validated with numerical simulations.
