In this paper the validity of coupling impedance bench measurements is theoretically demonstrated for the standard model of a uniformly distributed wall impedance. Integral equations expressing the magnetic field at the wall are found for the two cases where the device is excited either by a beam or a central wire. The integral equations are solved by a perturbation method with the coupling impedance as the expansion parameter, leading to expressions for the coupling impedance of the beam and the forward scattering coefficient in the bench measurement. Conditions for the validity of bench measurements are indicated, and the interpretation of wire measurements via the scattering coefficient by the conventional formulas is discussed.
