What are the challenges in designing V2X and radar antenna connectors?

Vehicle-to-everything (V2X) connectivity and automotive radar are important technologies required to support higher levels of autonomous operation. Antennas are critical components in both types of systems, and the connectors needed with the antennas can present engineering challenges related to solution size and performance.

This FAQ begins by reviewing common V2X connectors, looking at how compensators can be used to improve connectivity performance, and at some proposed solutions to simplify V2X connectivity. It closes by looking at the unique challenges with connectors for automotive radar for advanced driver assistance systems (ADAS).

Rosenberger initially developed FAKRA (Fachkreis Automobile in German or Working Group Automobile) connectors, a variation of subminiature B (SMB) coaxial RF connectors. They have become an industry standard, are available in a wide array of mechanical formats, and are widely used in telematics and antenna applications (Figure 1).

The latest version is a mini FAKRA developed primarily for automotive applications. Mini FAKRA supports up to 80% smaller solutions than basic FAKRA connectors.

Figure 1. FAKRA connectors can support a variety of system design needs (Image: Rosenberger).

Compensating to cope with distance
Most V2X systems can function with a single antenna. However, one antenna solution has limitations and may not always provide optimal coverage. As a result, antenna diversity solutions based on two antennas are often implemented. The two antennas may be placed on the front and rear, on the left and right sides, or in other dispersed locations on the vehicle. Antenna diversity requires using a compensator on at least one of the antennas to balance the power loss between the locations.

The power loss results from a combination of losses in the coaxial cable, for example, about 0.12 dB per 10 cm at 5.9 GHz and about 0.4 dB per connector. The signal in a 2-meter cable can experience attenuation up to 3 dB. A compensator is used to balance the power losses (Figure 2). Elevated temperatures can increase losses. Some compensators also include diagnostic connections that can send information on temperature, transmission power, and other parameters to the controller for further performance optimization.

Figure 2. Compensators are used to balance antenna performance in antenna diversity solutions (Image: TE Connectivity).

Farms replacing fins
A shark fin on the roof of a vehicle is a common way to house multiple antennas for functions like cellular connectivity, global navigation satellite system (GNSS), and other V2X systems. The telematics control unit (TCU) is located nearby, often in the trunk, and connected with a coaxial wiring harness.

However, shark fin designs are limited to a maximum height of 70 mm, and the outer dimensions are limited by aesthetic considerations, reducing the number of antennas the fin can accommodate.

As a result, some car makers are moving to a new solution called an antenna farm, which is a box of antennas placed under the roof. Being under the roof means the farm can spread out to accommodate more antennas. Being under the roof can reduce antenna performance, but the ability to spread out the antennas can somewhat compensate for that.

Initially, antenna farms simply replaced the fins, and the TCU was still remotely located and connected with a coaxial wiring harness. In some newer designs, the TCU is integrated with the farm, eliminating the losses from the wiring harness and helping to improve performance, making farms more comparable with fin-based solutions.

Radar is different
Radar can be an important system in ADAS. Its operation at mmWave frequencies results in signal integrity (SI) challenges in the RF signal chain between the monolithic microwave IC (MMIC) and the antenna. Transitions between the transmission lines on the chip, circuit board, and antenna can significantly degrade SI.

The use of a ball grid array (BGA) package for the MMIC enables the RF signals to be efficiently transferred to the circuit board. When onboard antennas like patch arrays are used, the signal can be delivered to the antenna using microstrip lines, coplanar waveguides (CPW), or substrate-integrated waveguides (SIW).

Some newer radar designs use 3D waveguide antennas that can provide higher performance. In those designs, the transmission line modes carrying the MMIC signal need to be converted to waveguide modes. A waveguide launcher on board (LoB) is used to make that conversion. The geometry of the LoB is optimized to match the impedances and maximize the coupling efficiency (Figure 3).

Figure 3. Conceptual drawing showing a LoB connecting a microstrip line with a waveguide antenna in a radar system (Image: Renesas).

Summary
Effective connectivity is necessary to ensure optimal V2X antenna systems and automotive radar performance. In both cases, new solutions like antenna farms replacing shark fins in V2X and 3D waveguides replacing patch array antennas demand new approaches to connectivity.

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