Impact of Field Polarization on Radiated Emission Characterization in an Open TEM Cell

This article presents a comprehensive study of the influence of electromagnetic (EM) field polarization on radiated emission characterization in an open transverse EM (TEM) cell, which is usually neither considered in the measurement process nor clearly identified. It also takes into account the influence of the operating frequency with respect to the higher order mode cutoff frequency of the cell, the latter being generally unknown to the experimenter. Commonly used 433-MHz (LoRa) and 2.4-GHz (Wi-Fi) antennas were placed inside an open TEM cell in different orientations in order to investigate the impact of the antenna polarization. For antennas parallel to the septum, a 1.6-8.6-dB difference in power levels collected by the cell (corresponding to a 31%-86% underestimation), for different polarizations, depending on the operating frequency being below or above the higher order mode cutoff frequency, was obtained between orthogonal orientations due to reduction in E- and (mainly) H-field levels, leading to significant uncertainty on power levels. For the 2.4-GHz antenna perpendicular to the septum, the power level was highly dependent on both the antenna orientation and location along the cell's axes, being more dependent along the main axis. This clearly demonstrates that the physical phenomena due to the polarization of the radiating element and the cell's dominant mode frequency must be characterized and estimated, in addition to the generally known tangential H-field coupling to integrated circuit pins or printed circuit board (PCB) traces. This leads to an update of best practices of IEC 61000-4-20 or IEC 61967-2 emission testing.