Concept to Evaluate and Quantify Field Inhomogeneities in Coaxial TEM-Cells

Abstract

The quality of a field probe calibration directly affects the uncertainty statement of radiated immunity tests. Therefore, precise field calibration is required. An often-used field generator is a transversal electromagnetic (TEM) cell. The IEEE Std 1309 provides three methods to calibrate a field probe. Method B, also known as the standard field method, uses a calculated reference field based on the geometry of the measurement environment and its input parameters. However, in reality, mechanical tolerances and misalignment can decrease the field quality. The results of the calculated model and the actual system may differ. Hence, this contribution proposes an approach for computing electromagnetic (EM) fields inside a TEM-cell considering mechanical tolerances and unknown input parameters. It is accomplished by projecting Maxwell’s equations onto eigenfunctions resulting in an infinite-dimensional differential equation system, the so-called generalized telegraphist’s equations (GTEs). Since this method starts with Maxwell’s equations, it can be used for a variety of applications. The proposed concept is applied on a coaxial TEM-cell with a circular cross-section with random imperfections. Based on the semi-analytical method and an input–output model for the uncertainty propagation, the combined uncertainty can be calculated following the guide to the expression of uncertainty in measurement (GUM).