Higher order mode field distribution in loaded TEM cells-numerical approach

The TEM cell is a piece of rectangular coaxial 50 ohm transmission line with a transverse electromagnetic (TEM) field propagated inside. Putting some device into the work volume of the cell (this device is usually called EUT-equipment under test) entirely changes the electromagnetic environment by introducing some irregularity into the structure. It is assumed that if the size of EUT is small compared to the dimensions of the cell, this irregularity may be neglected. The problem is, that this is not always true, especially when EMC susceptibility tests are taken into account. Very often dimensions of EUT are near one third of the corresponding values of the inside dimensions of the cell (one third is a commonly used limit because of the field irregularity)