Measuring 5G antennas in the reverberation chamber

New generations of wireless networks are being developed at a rapid pace. The move from 4G to 5G is currently ongoing, but the next generation, the even faster 6G, is already waiting in the wings. Ever more devices are communicating over these wireless networks, using small antennas – as many as hundreds of them – to send and receive high frequency signals.

While the integration of chips makes it difficult to create antennas for 5G or 6G systems, their performance testing has been beyond virtually everyone. Normally, when testing a chip to see whether it meets quality requirements, you'd connect up the chip. But when it's working in tandem with an antenna, it's not designed for a cable connection. This is a wireless system, working with airborne electromagnetic waves.

And so, the chips, too, must be tested ‘over the air’. What's more, all those hundreds of antennas have to be tested, in various settings, and there's always a chance they will – inadvertently – affect each other.

 “I do like a challenge,” says Anouk Hubrechsen with a smile. And she achieved her goal. By adopting a new measurement method, she succeeded in testing numerous aspects of high-frequency antennas, both accurately and fast. On Thursday October 26^th, she defended her doctoral thesis cum laude at the Department of Electrical Engineering.

Test capacity bottleneck

As a rule, today's antennas are tested in what is called an anechoic chamber. This is a room in which the walls absorb all echoes, explains Hubrechsen. But both the set-up and the interpretation of the measurement data are time-consuming activities, requiring a great deal of expertise.

“Many companies spend more than a third of their development time on performance testing their antennas,” she emphasizes. “5G applications are mushrooming and every device needs testing. We've got a serious bottleneck in terms of test capacity.”

Thinking that a totally new approach was needed, Hubrechsen set to work in TU/e's integrated antenna systems group, researching the switch to a different type of measurement chamber. Initially, she and her colleagues were viewed with some mistrust. After all, their vision was novel; whereas in the anechoic chamber everything is absorbed, in her ‘reverberation chamber’ Hubrechsen uses metal walls to do precisely the opposite: reflect every wave.

“The concept of the reverberation chamber is found in acoustics and it isn't at all new. We're working with electromagnetic waves instead of sound waves, and applied existing lower-frequency concepts to higher frequencies. The reflections enable us to simulate reality much more closely. What's more, this greatly reduces the time needed to capture all the power the antenna emits. In conventional measurement systems you have to scan the room mechanically, point by point, before eventually compiling everything. In a reverberation chamber everything is measured all at once. It also allows for a simpler way of measuring whether a device is emitting at the right frequency and whether the maximum transmitted power isn't being exceeded, an important issue with, say, cell phones. All in all, this method is much more precise, more scalable, and up to a hundred times faster when it comes to the testing of certain aspects of antenna quality.”

Open mind

In order to measure the performance of, specifically, high frequency antennas, a prototype reverberation chamber was built, and then Hubrechsen conducted an extensive validation process. When pushed, she admits this was pioneering work.

“When I started my PhD project, I was working in the first reverberation chamber capable of handling such a high frequency. I saw a great many aspects of this chamber and all the measurement work I did enabled me to root out a great many problems. Gradually, I discovered how they could be solved. By adopting an open-minded approach and being less tied to a stochastic process in which models are sometimes used blindly, we were able to translate our findings into an actual product.”

Practical guide

Although the results of Hubrechsen's work demonstrate that the reverberation chamber offers many advantages over current measurement techniques, she still encounters doubtful researchers. This, she thinks, is because it is more difficult to determine antenna direction in a reverberation chamber than it is in an anechoic room, an issue considered significant when direction is highly relevant for high frequency antennas, which cluster their radiation to create a strong signal.

Hubrechsen, however, demonstrates that for many measurements antenna direction is less important than has been supposed. For those measurements where it is important, she and her colleagues developed a new methodology for use with the reverberation chamber. To convince people of the value of the reverberation chamber in measuring the properties of the latest antenna systems – antenna plus electronics – she decided to write her doctoral thesis in the form of a practical handbook, sharing the ins and outs of the new approach. “The way the reverberation chamber works is more difficult to understand. If a thing works intuitively, it's easy to trust. It just means I have to demonstrate it, that's all. Because when people experience for themselves how well it works, they're persuaded.”

Not automatic

Increasingly, she is seeing this among users, including when she is in her role as CEO of TU/e spin-off ANTENNEX, the first company to market a portable high-frequency reverberation chamber for commercial use. Her PhD route is far from conventional, she admits reluctantly. Last year she was asked to become CEO of ANTENNEX, the company founded in 2021 by the university's integrated antenna systems group.

Increasingly, the company were receiving requests from large companies to performance test their antennas. Similarly, Hubrechsen has been providing measurement services since her bachelor's internship. “After spending a year as a PhD candidate, I decided to quit my project due to circumstances at work and started a new PhD research study in the Electromagnetics group. In terms of timing, becoming CEO of a startup eighteen months later wasn't a really practical move,” she laughs.

After a short silence she continues in a serious tone: “I've had to fight hard to get everything done on time while running a company, and it's certainly taken its toll. And I've been lucky. Good students, a good many people supporting and helping me, and now a fresh team and investors keen to take ANTENNEX in a new direction. It's been a fabulous challenge, but racing to complete a PhD thesis in your evenings and weekends is something I wouldn't recommend to anyone.”

Source: Cursor.