Wondering about the application of reverberation chambers in industry, I took the opportunity to interview Frank Leferink about the topic:
Prof. dr. ir. Frank Leferink (LinkedIn, UT Homepage) is the Director Electromagnetic Compatibility (EMC) EMC at Thales, the Netherlands and Manager of the Network of Excellence of the Thales Group, and (part-time, full research) Professor at the University of Twente were he holds the Chair for EMC. He is the former President of the Dutch EMC-ESD Association, Chair of the IEEE EMC Benelux Chapter, IEEE fellow, and a member of the Board of Directors of the IEEE EMC Society. He also is Associate Editor for the IEEE Transactions on Electromagnetic Compatibility and IEEE Letters on Electromagnetic Compatibility Practice and Applications.
In the transcript of the shortened interview below, clarifying comments can be found in parenthesis. For the curious reader, I contained references for further reading in square brackets. If you are looking for a general introduction to the topic of reverberation chambers, I can recommend articles [7], [8] from the IEEE EMC Magazine. Enjoy the interview!
Marc Kopf
Frank, you are the inventor of the VIRC (Vibrating Intrinsic Reverberation Chamber), but before speaking about your well-known invention let’s talk about reverberation chambers in general, and how they are used in industry from your perspective?
Frank Leferink
It depends on what kind of industry: People in communication use reverberation chambers for multi-path channel testing. But when we focus on EMC then there are two key advantages of reverberation chambers: One is the generation of high field strengths and two that you can illuminate your object from various angles. And both of advantages are being used when we apply reverberation chambers for real life equipment testing.
Marc Kopf
How are you using reverberation chambers at your company, Thales?
Frank Leferink
We use reverberation chambers nearly on a daily basis for evaluating materials, for instance shielding effectiveness of composite panels, the shielding effectiveness of cables or shielding effectiveness of the structure of bigger equipment. But we also use reverberation chambers to illuminate a part or bigger parts of systems.
And the ultimate use of the Reverb (short for “reverberation chamber”) is when we build a VIRC or a (conductive) tent around the system (e.g. for large DUTs like trucks [5] or radar systems [6]). So when a radar system is being installed in the test environment (in the normal functional test environment), then we can bring the reverberation chamber to that test environment and perform an EMC test with it.
Marc Kopf
How would you see the advantages of reverberation chambers compared to traditional techniques? For example, like scans in anechoic or semi-anechoic chambers?
Frank Leferink
The semi-anechoic chambers have been developed as a an in-house environment to replicate an open area test site. And when we look to an open area test site, this was developed 100 years ago as an easy way to perform tests.
Nowadays we cannot do this anymore (due to a higher utilization of the radio spectrum and other electromagnetic disturbances in the environment [1]) and we have to shield our environment. But when we look back 100 years ago, the source of emission was small equipment and to make sure that we could catch all the direction of the radiation, we put our object on a turntable and scan it with an antenna. And that is still the reference in standards, which is completely nonsense because the actual environment is semi reverberant.
In the office I am in, the fields are coming from all directions and are transmitted in all directions, for example by my computer. And thanks to that I have reception of a signal all over the building. And we should replicate those environments in our test environment (introduction to over-the-air-tests with reverberation chambers: [2]).
When we are using a mobile phone in a car, that car is causing a semi resonant environment (in the inside [3]). It’s some kind of a cavity with holes and we should replicate such environments. Otherwise, we are testing an electrical module in a free space (testing) environment but then bring it inside a car and that electrical module will see completely other electromagnetic fields. So, a reverberation chamber is replicating an actual environment (which matches the one inside of the car better).
Marc Kopf
Do you see a point (in time) where reverberation chambers are fully replacing these traditional test sites?
Frank Leferink
It depends for what kind of application: I think for testing above a few gigahertz it will be completely stupid to do conventional testing with an antenna mast and a rotating table. If you look at 10 gigahertz or 20 gigahertz, it would be insane to think about such a test method. You have to use a reverberation chamber. When you go to lower frequencies, there is some point where it might be easier to use anechoic chambers because then the multi reflection effect of reverbs doesn’t give any advantage anymore: And that is around 100 to 300 megahertz range. It’s in the range where the size of our living environments is not really resonant anymore.
And then at very low frequencies, you might consider other test technique than using antennas. So why use an antenna when we would like to evaluate the effect of medium wave radiation? Why using an antenna test setup to see what is the unwanted emission of a solar panel array of 100 meter length? Maybe we have to use something like current probes to see what are the common mode currents on the interconnecting cables. So, when do you want look to reverberation chambers? I think it is the preferred test site in the frequency range above 100 megahertz and above one gigahertz it’s the only possible test site.
Marc Kopf
Speaking of that, I talked to a lot of people from industry at the last two (big) conferences, IEEE EMC+SIPI and also IEEE EMC Europe and large parts of industry are still using conventional test sites and are not using reverberation chambers except for immunity tests on a limited scale. How do you explain this?
Frank Leferink
There are two reasons for that: The standards and the publications are written by scientists. They only put a very small object into a reverb, and then they write 100 pages about it. But in industry, we are interested in the behavior of actual products. There are nearly no papers and no academic research about the behavior of actual products in a reverb: So, the standards are far away from what industry expects from standards.
One typical example is to IEEE 299.1: It is nearly 100 pages and it’s mostly academic nonsense and I don’t say nonsense in a sense that it is really nonsense, but it is not of any value for industry. They (the industry) just want to use a test technique which gives reliable test results and they don’t want to have all the academic burden around it.
This is a main reason why the reverb is not being used a lot. The other reason is that there is a misconception of what the European directives are requiring from industry, many people think, and I think it’s more than 99%, that you have to perform a test as described in the harmonized standards, but that’s not correct: The directive, which is the law, is not requiring a test to be performed using test techniques which have been developed 100 years ago. The directive is requiring industry to provide equipment which is not causing interference or being interfered with. And this is the issue: Because of the misconception that you have to use harmonized standards and because the default test setup is an anechoic chamber, people think that is the law and they are following the majority of people who are doing this.
On top of that, the test labs are also thinking that: In the past, until five years ago a competent body was a test house. So, if you want to have a competent body assessment you came to a test house and what is the business of a test house? That is to perform tests. And what was the biggest investment? That was the anechoic chamber! So why not put all those customers (DUTs) in those expensive anechoic chambers? So that is actually happening. And that is something we have to fight against!
Marc Kopf
Let’s talk about reverbs again: You are the inventor of the VIRC [4]. How would you describe a VIRC compared to a conventional, mode-stirred reverberation chamber?
Frank Leferink
The VIRC gives exactly the same type of field as a mode-stirred chamber: You’re mixing the modes continuously and it means that your object will be illuminated from all directions; it will see the same field strength (on average, within the working volume). It is like you had a monkey with an antenna which is dancing around your object / your equipment under test. When you compare it to a mode tuned chamber, a mode tuned chamber is a monkey with every second illuminating your object or measuring or your object.
If you believe that you need a long dwell time, then mode tuned would be better. However, for emission measurements, mode stirred is excellent and the VIRC is also mode stirred, but we used the VIRC for all kind of testing for susceptibility and immunity testing as well as for emission testing: And it gives excellent results in in terms of reproducibility.
Marc Kopf
Do you see if in future, that the VIRC will replace conventional reverberation chambers as we know them now?
Frank Leferink
I don’t know, I never pushed for using the VIRC in standards, I never was an active member of standard committees. My job at Thales is to make sure that our systems operate in their intended environment, and that’s where we use the VIRC. If people want to use it in standardized test methods, it’s free to use them. For me there’s a big advantage of using the VIRC. There are many advantages. The main advantage is that by moving the walls your fields (in a sense of field statistics) are already much better at lower frequencies compared to mode stirred chambers.
So why not use the VIRC? I never pushed for it and we never marketed it as we are just using it because our objective is not to sell a test method but to sell radar systems which operate in their intended operational environment.
Marc Kopf
Let’s stick with talking about your perspective on the future: Generally speaking about reverberation chambers, if you go to conferences (nowadays), you can see that there is an increasing interest into the topic. What do you think about that? Do you see reverberation chambers being used in industry more often in future?
Frank Leferink
In 2018 I published a paper about the next five years and that means that in 2023, all my predictions should be true. I predicted that time domain EMI measurements would be the reference, and I also predicted that for higher frequencies, reverberation chambers would be the reference. Maybe we need a few more years, but I’m convinced that at higher frequencies you cannot use absorber-lined chambers anymore. You need to use reverberation chambers unless you just want to do a test to pass the test. But if you want to know the risk of EMC or EMI then you have to use reverberation chambers.
Marc Kopf
Thank you for the interview!
Frank Leferink
You’re welcome!
References:
[1] Leferink, F., Silva, F., Catrysse, J., Batterman, S., Beauvois, V., & Roc’h, A. (2010). Man-made noise in our living environments. URSI Radio Science Bulletin, 2010 (334), 49–57. https://doi.org/10.23919/URSIRSB.2010.7911080
[2] Hubrechsen, A., Remley, K. A., & Catteau, S. (2022). Reverberation Chamber Metrology for Wireless Internet of Things Devices: Flexibility in Form Factor, Rigor in Test. IEEE Microwave Magazine, 23(2), 75–85. https://doi.org/10.1109/MMM.2021.3125464
[3] Herbert, S., Loh, T. H., Wassell, I., & Rigelsford, J. (2014). On the analogy between vehicle and vehicle-like cavities with reverberation chambers. IEEE Transactions on Antennas and Propagation, 62(12), 6236–6245. https://doi.org/10.1109/TAP.2014.2363681
[4] Leferink, F., Boudenot, J. C., & van Etten, W. (2000). Experimental results obtained in the Vibrating Intrinsic Reverberation Chamber. IEEE International Symposium on Electromagnetic Compatibility, 2, 639–644. https://doi.org/10.1109/isemc.2000.874695
[5] Izzo, D., Rommel, A., Vogt-Ardatjew, R., & Leferink, F. (2020). Validation and Use of a Vibrating Intrinsic Reverberation Chamber for Radiated Immunity Tests. 2020 IEEE International Symposium on Electromagnetic Compatibility and Signal/Power Integrity, EMCSI 2020, 56–60. https://doi.org/10.1109/EMCSI38923.2020.9191603
[6] Leferink, F. B. J., Groot Boerle, D. J., Sogtoen, F. A. G., Heideman, G. H. L. M., & Van Etten, W. C. (2002). In-situ EMI Measurements Using a Vibrating Intrinsic Reverberation Chamber. International Symposium on Electromagnetic Compatibility – EMC Europe. https://ris.utwente.nl/ws/portalfiles/portal/13490501/Zurich_VIRC.pdf
[7] Serra, R., Marvin, A. C., Moglie, F., Primiani, V. M., Cozza, A., Arnaut, L. R., Huang, Y., Hatfield, M. O., Klingler, M., & Leferink, F. (2017). Reverberation chambers a la carte: An overview of the different mode-stirring techniques. IEEE Electromagnetic Compatibility Magazine, 6(1), 63–78. https://doi.org/10.1109/MEMC.2017.7931986
[8] Serra, R. (2017). Reverberation chambers through the magnifying glass: An overview and classification of performance indicators. IEEE Electromagnetic Compatibility Magazine, 6(2), 76–88. https://doi.org/10.1109/MEMC.0.7990003