Designing and manufacturing a multi-channel antenna dramatically impacted its testing as well. As early as 2016, HUBER+SUHNER engineers designed a semi-automated system to test all channels and ensure the quality of the delivered products.

Finally, the simultaneous use of several channels called for higher integration between the antenna and the active electronics. A dedicated, and proprietary, RF interface solution was developed to directly mount the antenna onto the printed circuit board (PCB) with no waveguide flanges required to increase overall product compactness and achieve higher performance.7

77 GHZ AUTOMOTIVE MIMO RADAR: A SWEET SPOT

After the implementation of HUBER+SUHNER metallized plastic antennas into the communication market, the use of this technology was introduced for automotive radar applications in 2016,8 with its first demonstration in 2018.9 Since then, several antenna solutions and sensors have been designed, manufactured and validated, focusing on both product development and their integration into the ecosystem.

As most original equipment manufacturers (OEMs) and Tier-1 suppliers require dedicated and customized antenna designs protected under confidentiality, this article describes antennas and systems that are the result of internal HUBER+SUHNER development for different radar applications (long-, mid- and short-range, corner and side-looking radars).

THE 3D ANTENNA AND ITS ECOSYSTEM

The antenna developed for the first system demonstrator—‘Demorad’ (see Figure 6)—comprised four 3D-printed layers, standard microstrip-to-hollow waveguide launchers on a low loss RF PCB substrate and an almost uniformly corporate-fed antenna array. Lambda over two and lambda over four spacings were selected for receive (Rx) and transmit (Tx) elements, respectively, to establish a virtual linear array (8 Rx, 4 Tx).

Figure 6

Figure 6 Demorad radar developed with Infineon.

Figure 7

Figure 7 Measured Demorad normalized antenna pattern from 76 to 81 GHz (a) and |S11| of the Rx and Tx antenna elements (b).

Figure 7 shows some of the measured characteristics of the manufactured prototype. The technology demonstrates a broadband behavior (12 percent relative bandwidth), enabling use of the entire 76 to 81 GHz automotive radar frequency band. Such performance is matched by a stable radiation pattern over frequency with -15 dB sidelobe levels and a high efficiency of 90 percent (0.5 dB loss).

These characteristics outperform the state-of-the-art PCB-based antennas traditionally used within the industry.10 While this initial demonstration was developed in collaboration with Infineon, the technical solution finds application with all MMIC suppliers.

Today, the design and the technology have taken several steps forward. The first step is product miniaturization. ‘Demorad’ offered exceptional RF performance but, due to the use of four plastic layers, the product was bulky. With technological evolution, the number of layers is now reduced to two, with the overall thickness reduced to less than two lambdas.

The second step is the introduction of advanced feeding techniques with both amplitude and phase tapering for complex radiation pattern shaping. With just two plastic layers different performance characteristics are achieved, from broad azimuth pattern coverage through a tilted beam to narrow elevation patterns with extremely low sidelobe levels, as shown by the measured radiation characteristics in Figure 8. All the typical automotive radar requirements for long-, mid- and short-range radar can now be fulfilled. As a further result of modern advancements, diverse polarizations such as horizontal, vertical, slant or circular can be easily obtained, allowing for polarimetric radar applications as well.

Figure 8

Figure 8 Measured automotive radar antenna patterns: elevation (a) and azimuth (b) planes.

The third step comes in the form of integration with the rest of the sensor and the MMIC. The first demonstrator employed a low loss RF substrate to route and launch signals from the MMIC to the antenna; however, this requires the presence of high performance/high-cost RF substrate material that still generates noticeable losses within the substrate and fails to use the full potential of the low loss waveguide technology.

Thanks to the latest joint development with Texas Instruments,11,12 a highly integrated sensor has been realized (see Figure 9a). Direct coupling from the MMIC, through plated holes in the PCB, to the antenna dedicated RF interface enables efficient power transmission without the need for a low loss substrate. This provides an RF substrate independent solution and a dramatic benefit in terms of both performance (because there is no need for PCB launchers that easily add 2 to 3 dB loss to the link budget) and cost (by avoiding the need for a high performing RF material).

Figure 9b is another example of integration showing a HUBER+SUHNER 3D metallized plastic antenna as part of the next generation of digital imaging radars. Its low loss characteristics are crucial to providing an antenna array with a highly sparse location of elements over large apertures.

Finally, using a 3D antenna with a large area allows for novel design features to be added to mitigate—if not cancel—radiation pattern distortion introduced by the radome and/or the bumper placed in front of the radiator.13

Figure 9

Figure 9 Mid-range demonstrator developed with Texas Instruments (a) and Uhnder digital imaging radar using HUBER+SUHNER 3D antennas (b).

Figure 10

Figure 10 Simulated effect of a metasurface to minimize antenna-bumper reflections: azimuth (a) and elevation (b) planes.

The orange traces in Figure 10 show the result of multiple reflections when simulating a simple case of antenna-bumper radiation interaction using a flat bumper model with dielectric constant of 3.0, 3.5 mm thickness and an antenna to bumper distance of 18 mm. By introducing periodic elements (i.e., a metasurface) on the antenna top layer, the desired main beam radiation performance is substantially recovered and pattern ripples are noticeably reduced (blue traces).14,15