High strength electromagnetic field generation for radiated EMI measurements

Abstract

Electro-magnetic compatibility (EMC) is the important parameter to guarantee that electronic and electrical products are electronically compatible with their environments. Radiated immunity (RI) testing, also called radiated susceptibility (RS) testing, requires a high field strength to illuminate the equipment under test (EUT). The crucial and important components of a susceptibility test system are the indispensable and high-cost power amplifier, an anechoic chamber (AC) and a proper antenna. The challenge and topic of this thesis is to make the RS test more effective to lower the cost of equipment, and more efficient by saving test time, while maintaining or improving the quality of the test results. There are three major techniques for creating high field strength for RS testing: an antenna inside the AC, an enclosed coaxial structure like the transverse electromagnetic (TEM) cell, or the reverberation chamber (RC). This thesis studies two methods: the antenna structure as a transmitting transducer inside the AC and the RC structure for creating high field strength for EMI testing. First, the antenna technique has been investigated by exploring and improving our understanding of the power required by the antennas and resulting field obtained in the uniform field area (UFA). By means of measurements (empirical) and numerical simulation it is shown that a double-ridged guide horn (DRGH) is comparable to a double log-periodic dipole array (LPDA) antenna in terms of power-to-field-strength and is slightly better in creating good field uniformity. Additionally, two different RS test methods have been studied showing that the size of the EUT affects the field strength. This research result supported the change in some standards where pre-calibration (or substitution) is now preferred above closed-loop-levelling when testing large EUT. Radiated electromagnetic (EM) field measurements in different environments, using the AC antenna technique, the TEM method, and the RC method have been investigated to analyze the relationship between the receive- radiation-pattern of an EUT and the test technique. Focusing on RS, this research could have been using an EUT with indication of the perceived field strength. In this work, such a fictitious EUT was replaced by a field strength sensor. By adding a box with different holes and hole patterns, a real-life EUT could be replicated. Although using this validation approach is very valid, it is cumbersome and costly, as high-power amplifiers are needed for all test environments. By using the reciprocity concept, the validity of comparing test environments is performed using radiated emission measurements where a broadband emitter was used as source. One major issue in applying the RC technique is the slow readout of electric field strength probes, and a key limiting factor in a widespread use of RCs. Furthermore, the number of sensors is often limited. In some standards, the field strength is monitored and/or measured using a single antenna (polarization). In various measurements described in this thesis fast three-dimensional (3D) sensors which allows time-efficient RS testing. Moreover, the fast and very sensitive field probe are best suited for vibrating intrinsic reverberation chamber (VIRC) measurements, which have rapid and time-varying electric field (E-field) behavior inside the chamber. To prove these advantages, a major automotive company has applied the VIRC and the fast multi-probe system that allows closed-loop EMC evaluation of large and complex systems. Finally, the RS test inside AC and VIRC with the box with field probe inside to represent EUTs were investigated to analyze the EUTs behavior and its susceptibility to the high E-field coming from various directions. The results showed that at higher frequencies, with thirty-six points in rotation increments, the probe inside the box received higher E-field strength than the target field. This implies that there is a high probability that in this frequency range the worst-case interference is coupled into the box and hitting the most susceptible part of the EUT. In the VIRC, with lower power and faster measurement time, the same results are also obtained. In conclusion, the classic antenna technique in the AC method is expensive mainly due to the necessary EM absorbers and expensive power amplifiers. Furthermore, the current AC method has limited robustness, since a few incident directions and field polarizations are tested, and thus does not resemble actual living and working environments. The (VI)RC has much lower costs for equipment, and illuminates an EUT from all directions, thus resembling actual living and working environments. Further research is suggested and could be focused on the behavior of EUT for directivity and in time-varying fields. The directivity becomes important at higher frequencies. Time-varying fields play a role in the dwell time, which is the time an EUT is illuminated.