Computer simulation as a way of usable volume estimation in TEM waveguides

Striplines are common waveguides used for susceptibility tests. Standards as 61000-4-20 describe procedures for finding volumes with a uniform, strong electric field, but only for frequencies below the first cut-off frequency of the stripline. For higher frequencies modes other than TEM start to propagate and an analytical determination of the field between a stripline's plates is hard or even impossible (i.e. for non-homogenous interior of the stripline). The solution of electric field in a stripline could be found by means of a numerical approximation (e. g. using Finite Element Method) of Helmholtz equation. In this paper such a numerical simulation is presented. The numerical model is verified against the measurements in the frequency range above the first cut-off frequency of a stripline. On the basis of numerical simulation an electric field is determined in the stripline and a usable volume for an extended frequency range is estimated. Such a numerical simulation can be used to take into account also large metallic objects in the vicinity of the stripline.