In this section we would like to introduce you to students at Dutch Technical Universities and Universities of Applied Sciences who are studying our field of expertise. Each newsletter will feature a student or PhD candidate introducing themselves and explaining what their research entails. With this, we hope to build a bridge between the business community and universities. If you would like to get in touch with these students, you can do so via the following email address: info@emc-esd.nl.
Anouk Hubrechsen – Reverberation chambers: The bridge between EMC and the antenna world
Anouk Hubrechsen obtained her Bachelor's degree in Electrical Engineering from the Eindhoven University of Technology in 2017 and completed her Master's degree in 2019, specializing in Electromagnetism and Antennas. As part of this master's degree, she worked for a year as a researcher at the National Institute of Standards and Technology (NIST), a US government agency. Here, she worked on geometry for antennas in reverberation chambers. The section below discusses research that was performed within this project [1].
Since January 2020, she has been working as a PhD candidate at Eindhoven University of Technology in the Electrical Energy Systems group on the Amicable project under Guus Pemen, Anne Roc'h and Sander Bronckers, where she applies techniques from the antenna world to measure electromagnetic interference (EMI) in cable bundles in reverberation chambers. Further publications on the Amicable and NIST projects can be found at research.tue.nl/en/persons/anouk-hubrechsen.
Historically, reverberation chambers have been used primarily in the EMC world, because they can easily generate strong fields with limited input power. In such measurements, the main interest is therefore in the value of the 'peak field' that can be generated. This is a completely different approach than is used in the antenna world, where the reverberation chamber has become increasingly popular in the past decade for measuring the radiation efficiency of antennas, among other things. The radiation efficiency is defined as the ratio between the power with which the antenna is excited via a cable and how much of it is actually transmitted. In order to calculate the radiation efficiency correctly, we always use the measured average field compared to the peak field. The reverberation chamber at NIST can be seen in Figure 1. When an antenna is placed in a reverberation chamber, a distribution of standing waves or 'modes' is created that can be adjusted by moving a metal structure, a so-called 'stirrer'. In this way, all reflections change, and therefore also the positions of the antinodes and nodes of the field. The average field is calculated from the average of multiple stirrer positions.
Measuring the radiation efficiency of an antenna in a reverberation room is a fairly simple process: Two antennas of unknown efficiency are placed in the room and connected to a Vector Network Analyzer that is placed outside the room to measure the phase and magnitude of the incoming and outgoing waves. With this measurement the losses in the room and in the antennas can be determined. The only information missing is the volume of the room and using an equation that depends on the losses we know the efficiency of both antennas, without knowing it in advance [2]. In the EMC world, separate measurements in the room can differ by 3 dB and still meet the standard [3], while in radiation efficiency that translates to values between 50% and 200%, assuming 100% actual efficiency. Such an uncertainty is far too high for these kinds of measurements (usually we try to stay within 2% uncertainty), and that is why we often use other techniques and equipment in the antenna world.
A good example that shows how quickly such a measurement can be influenced can be seen in Figure 2 [1]. There, the same measurement was performed three times, where in the second and third measurements a simple (unconnected) power cable and a few power supplies were placed in the room, where we assume that they are not there. This sounds crazy, but in many institutes it is a standard procedure to determine the losses in the room once in advance, and then place equipment in it that is needed to measure an active antenna, for example. As a result, the losses are actually higher than you would expect. We define these losses in the room using the 'time constant'. This shows for how long a wave continues to reflect in the room, until it has weakened so much that it has ended up in the noise floor. This is often a few microseconds, which is equal to several kilometers in the room [4]. Figure 3 clearly shows that this time constant is very sensitive to the presence of additional equipment. If this is not taken into account, your measured efficiency with 5% can deviate with the presence of a cable, and 10% with a few power supplies (which could not be placed outside the room) used to drive an antenna, as can be seen in Figure 2. This is much more than the standard measurement uncertainty, which is indicated by the error bars on the reference measurement.
The expected reason why a cable has so many losses in the room is that it will show antenna behaviour over a wide frequency band, since many different wavelengths fit in the length of the cable. With better shielded cables, such as a coaxial cable, this effect is less prominent and almost no extra losses were measured. In this way, the time constant, a concept from the antenna world, could be used to determine various aspects of the quality of cables, which in turn is useful in the EMC world. In this way, reverberation rooms can function as the bridge between EMC and the antenna world.
[1] A. Hubrechsen, LA Bronckers, KA Remley, RD Jones, RD Horansky, ACF Reniers, A. Roc'h and AB Smolders, “The Effect of Peripheral Equipment Loading on Reverberation-Chamber Metrics,” 2019 International Symposium on Electromagnetic Compatibility – EMC EUROPE, Barcelona, Spain, 2019, pp. 7-12.
[2] CL Holloway, HA Shah, RJ Pirkl, WF Young, DA Hill and J. Ladbury, “Reverberation Chamber Techniques for Determining the Radiation and Total Efficiency of Antennas,” in IEEE Transactions on Antennas and Propagation, vol. 60, no. 4, pp. 1758-1770, April 2012.
[3] D. Barakos and R. Serra, “Performance characterization of the oscillating wall stirrer,” 2017 International Symposium on Electromagnetic Compatibility – EMC EUROPE, Angers, 2017, pp. 1-4.
[4] LA Bronckers, A. Roc'h and AB Smolders, “Chasing the Wave in a Reverberation Chamber,” 2018 International Symposium on Electromagnetic Compatibility (EMC EUROPE), Amsterdam, 2018, pp. 708-712.
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