IMPEDANCE OF A HOLE IN COAXIAL STRUCTURES

We derive the impedance of a circular hole in the inner tube of a coaxial beam pipe. The method used differs from the classical Bethe’s theory since, in the calculation of the electric and magnetic dipole moments, we take into ac-count also the scattered fields in the aperture to match the power conservation law. The low frequency impedance shows a real contribution accounting for the TEM waves propagating within the coaxial waveguide. The method is also applied to the case of the outer tube closed at both ends by conducting plates, thus forming a coaxial cavity. The resistive part of the longitudinal impedance obtained can be predominant near the cavity resonances.