Eigen Modes of the GTEM Cell

The paper addresses problems of TEM cell design used for wireless communication terminals testing with focus on GigaHertz TEM (GTEM) cells. To maximize usable test volume modern TEM cells often have an asymmetric design, where septum is displaced away from loading door of the cell. Doing so will alter electromagnetic field distribution, so quantitative characterization of this effect is required. The paper presents electromagnetic field problem solution for the eigenmodes of a cross-section of a TEM cell as a rectangular coaxial line. Dependencies of the electromagnetic field distribution and the characteristic impedance of the TEM mode on the geometrical parameters of the coaxial line have been obtained. They allow evaluation of the field inhomogeneity effect on the transmission coefficient from the input of the TEM cell to the antenna of the wireless communication device and facilitate matching impedance characteristics of the TEM cell with the input port. Though the usable space of the TEM cell with displaced septum is larger, maintaining nominal characteristic impedance along the cell cannot be achieved with simple trapezoid septum. Field uniformity in wider area degrades as well, which would require special measures to compensate this effect. Knowledge of the cutoff frequencies and distribution of the electromagnetic field of higher modes makes it possible to apply technical measures to improve the frequency characteristics of the transmission coefficient of the TEM cell by reducing the influence of higher modes on the general structure of the field along the direction of propagation of electromagnetic waves.