Fraunhofer Institute for High Frequency Physics and Radar Techniques FHRRadar is the most important sensor for autonomous driving. A hHigh spatial resolution is achieved, according to the multiple-input/multiple-output (MIMO) principle, by a large number of antennas, each connected to individual gates ports of integrated radar ICs. When designing the complex high-frequency interconnect networks, the previously used printed circuit board technologies reach their limits.
For cost reasons, in the past the antenna elements were mostly printed radiator elements such as microstrip antennas or surface-integrated (SIW), slotted waveguides, because these can be monolithically integrated with the radar MMICs and possibly other electronic components on the same printed circuit board. Fraunhofer FHR has extensive experience with these standard approaches, also in combination with newer technologies such as structured absorbers and electromagnetic bandgap (EBG) or meta-surfaces. With a very large number of antenna elements as used in the current generations of radar sensors, the routing of the high-frequency lines between MMICs and antennas becomes a major challenge. A solution based on printed multilayer circuit boards seems unrealistic due to the required large number of layers, layer changes, and especially the conductor losses.
Fraunhofer FHR develops innovative antenna solutions
In recent years, Fraunhofer FHR has engaged with various alternative interconnect and antenna technologies. In the beginning, milled metallic hollow waveguides and sector horn antennas were used. The complex feeding network between MMICs and antennas was initially realized as a layered aluminum structure. A specially adapted radar-absorbing cover for the unused areas on the antenna side of the hollow-waveguide circuit should simplify the installation of a radar and its integration into a vehicle. Full-wave simulations and measurements of the antenna properties yielded high radiation efficiency, low mutual antenna coupling, and very low distortion radiation patterns. Radar experiments by the industrial customer were very positive. In a next step, multilayer feeding networks based on rectangular hollow waveguides made from metallized plastic parts were investigated. They showed no disadvantages compared to the purely metallic predecessors. However, since the individual rectangular waveguides in multilayer networks must consist of two parts with a good electrical contact between them, this approach is susceptible to large manufacturing tolerances. This is often mitigated by using conductive adhesives.
Gap waveguide approach shows promising results
A novel approach for several years now has been based on so-called gap waveguides, i.e., waveguides whose side walls are replaced by periodic structures with specially designed band gaps, so that no electrical contact between the two parts from which a waveguide is assembled is necessary. With this new technology, Fraunhofer FHR has already amassed extensive theoretical and practical experience, which also includes transitions between conductors on different layers and standard waveguides.
