Every year, consumers are dazzled by the latest smartphones and wireless devices that hit the market. Before these upgraded gadgets reach the shelves, there is an extensive design and testing process that goes into developing them. Antennas, a paramount component of wireless devices, are consistently updated to keep up with advancing technology such as 5G and the Internet of Things (IoT). They are expected to have greater bandwidth, meet safety regulations, and be small enough to fit into micro-designs.

To help engineers working with wireless equipment, Bluetest (based in Gothenburg, Sweden) has developed easy-to-use reverberation test systems (RTS) that measure the performance of wireless devices and antennas. Today, Bluetest is a market leader in over-the-air, multiple-input-multiple-output (MIMO) testing. The company uses simulation to ensure that the components of their RTS designs are optimized for performance.

Evolution of Reverberation Testing

Since the early 1940s, antenna performance has been tested in anechoic chambers, or microwave-absorbing rooms. In this type of chamber, an antenna is rotated and its radiated intensity is measured in different directions. The data gained from this testing method is relatively easy to interpret but anechoic chambers tend to be expensive and their large size makes them unwieldy.

In the 1960s, a different type of chamber was developed — the reverberation chamber — that was originally used for electromagnetic compatibility (EMC) testing. Unlike anechoic chambers, reverberation chambers reflect electromagnetic waves (or sound for the acoustic equivalent) instead of absorbing it. “You can generate very high field intensities inside this kind of chamber, which is a great feature for testing immunity and how sensitive a device is when it gets radiated with high-power electromagnetic fields,” said Robert Rehammar, chief technology officer at Bluetest.

In the late 1990s, people learned that reverberation chambers can also be used to test certain antenna parameters. For example, a small antenna’s most important property is its efficiency, or the quotient between the power you put into the antenna compared to how much power is actually radiated (typically measured in dB). “What was realized is that you can measure antenna efficiency in reverberation chambers and it turned out that for a small antenna, you can do it very fast and accurately,” said Rehammar.

Toward the beginning of the reverberation testing system’s popularity, PerSimon Kildal, a professor of antenna systems at Chalmers University of Technology in Sweden, started a research project on reverberation chambers and their ability to analyze antennas. After studying these chambers, Kildal was inspired to start a company based on his findings and Bluetest was born. Around 2010, 4G — the fourth generation of mobile systems (also known as LTE) — was introduced, along with MIMO. As a result, said Rehammar, “A lot of very complicated questions popped up like, ‘How are we going to test the performance of these systems?’”

Measuring Antenna Performance

Bluetest’s reverberation systems (Figure 1) perform passive and active tests to determine whether or not a device is optimized. Passive tests predominantly measure antenna efficiency, while active tests measure the total radiated power and total isotropic sensitivity in the device under test’s (DUT) transmitter and receiver, respectively. During active tests, the transmitter and receiver in the DUT are powered on. Active measurements help give an overview of how the DUT performs as a whole. Both tests help ensure that the device, such as a mobile phone, meets regulations and customer requirements.

All of Bluetest’s reverberation test systems and products are designed and produced in Gothenburg. The RTS contains a wide variety of components, such as walls made of reflective material, a reference antenna, four to 16 measurement antennas with different polarizations, mode stirrers, RF interfaces, and more. When the production process is complete, the system is packaged in a large wooden cradle and sent to customers around the world.

Design, Fabrication, Test, and Validation

Bluetest is in the process of designing new technology to use in their RTS for millimeter-wave applications, including the 5G mmWave band, where the center frequency is an order of magnitude higher than that of conventional microwave applications. High-speed communication relies on a wide bandwidth, which is provided by a high carrier frequency.

One of the most popular antenna designs for wideband applications is the Vivaldi antenna — a tapered slot antenna. “When it comes to antennas, we need to be able to test anything from the low cellular bands around 650 MHz up to over 40 GHz,” said Rehammar.

The wavelength in millimeter-wave device designs is much smaller than the microwave wavelength and any minor physical distortion due to thermal-structural effects or fabrication tolerance error would undesirably impact its performance. Therefore, it is critical to validate the performance of such devices using simulation. Bluetest used COMSOL Multiphysics® simulation to optimize their antenna and circuit designs including the Vivaldi antenna.

Figure 2. Bluetest’s first prototype of a Vivaldi antenna, simulated in COMSOL Multiphysics®. The S-parameter plot (right) shows the impedance matching characteristics up to 3 GHz.

The first prototype of the Vivaldi antenna design was modeled in FR4 substrate (a composite material made up of woven fiberglass and epoxy resin) with a thickness of 1.6 mm. Simulating the first iteration of this antenna allowed Rehammar and his team to see that there were a few issues relating to its mounting, size, stability, and efficiency while operating at low frequencies. Thanks to these findings, they were able to simulate an improved Vivaldi antenna by implementing Bézier curves into their model (Figure 2).

Figure 3. Fabricated antennas designed using simulation.

Bluetest also simulated, designed, and tested the efficiency of a wideband monopole antenna for ultra-wideband operating from 6 GHz to 67 GHz. This type of antenna is used in their RTS for 5G measurements; it also helps provide the system with more versatility because it can be used during a measurement without switching the standard test antenna.

The use of simulation is not limited to antenna designs. To enhance the performance of the reverberation chamber, Bluetest not only investigated the resonance eigenmodes of a customized cavity but also developed circuit-to-waveguide transitions.

Keeping Up with the Advancement of Technology

At Bluetest, Rehammar believes that simulation technology and measurement technology complement each other completely. “At the beginning stages of building a design, you need simulation, and to confirm your physical device is working properly, you have to do measurements,” said Rehammar. Bluetest’s systems are consistently being updated to keep up with the advancement of wireless technology, especially within the mobile phone development industry.

“Before 5G, mobile systems operated up to about 2.6 GHz and now you have 5G systems that can run up to 40 GHz,” said Rehammar. To stay on track with this advancing field, Bluetest has been working on supporting as many frequency bands as possible. With the help of simulation, Bluetest can focus on improving their RTS test time and measurement accuracy, while keeping the testing complexity at a high level.

This article was written by Rachel Keatley, Content Writer at COMSOL, Burlington, MA. For more information, visit here .

COMSOL Multiphysics is a registered trademark of COMSOL AB .



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This article first appeared in the November, 2021 issue of Tech Briefs Magazine.

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