Figure of merit for determining the usable test volume of the GTEM cell

A simple closed-form solution for local ratios between the TEM mode and the higher order TM11 mode at different frequencies is given. This formula can be used as a figure of merit to estimate and/or define the maximum usable test volume during the design phase of the GTEM cell and measurements within the Cell but may also be applied as enlargement strategy when applying performance optimization approaches in the GTEM cell. This is an important tool for both the EMC engineer seeking to use the GTEM cell for testing purposes as well as the cell's designers who would like to design a GTEM cell with a maximum possible working volume.