Characteristic impedance variation of the TEM cell caused by the introduction of the equipment under test

This paper firstly describes the basic principle of using the finite element method (FEM) to calculate the characteristic impedance. Then characteristic impedance variations of two typical TEM cells (one symmetric and the other asymmetric) with the presence of the equipment under test (EUT) are calculated. Relatively low error of the numerical results obtained by the FEM program developed by us makes the results meaningful. It is found that both size and grounding condition of the EUT influence the characteristic impedance