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Exploring RF communications in automotives

19 Feb 2014  | Blaine Bateman

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A typical General Motors (GM) vehicle houses AM, FM, and Satellite Radios, two GPS receivers, and a cellular radio. There may also be Bluetooth in the passenger compartment, and in the future, DSRC (dedicated short-range communication, 5.9GHz) will be added for vehicle-to-vehicle and vehicle-to-infrastructure communications. (There may also be anti-collision radar, but that is typically a separate system.)

Most of the electronics for all these devices are located in the "centre stack" – that dense area of screens and knobs between the driver and the front-seat passenger. The signals for satellite radio, GPS, AM/FM radio, and cellular all arrive at the vehicle from outside, and must be captured by an antenna, then coupled to the electronics by cabling. To keep cars from bristling with antennas (in contrast to police cars) the OEMs (Original Equipment Manufacturers) have developed multi-function modules containing multiple antennas and boards with over 100 components. A recent example is shown in figure 1.

Figure 1: On the left, a GM "shark fin" antenna from a 2011 model year, available on eBay. On the right, the shark fin antenna on a 2013 GMC Yukon. The antenna module is on the right above the windshield. (Source: General Motors)

The module in figure 1 includes the GPS antenna and low-noise amplifier (gain >25dB, noise figure < 1dB – black cable), the XM Satellite Radio antenna and low-noise amplifier (tan cable), and the cellular antenna (blue cable). The three coaxial cables terminate in special SMB connectors that are held in one shell to streamline installation. The 3-plug connector is mated to a cable assembly inside the vehicle, and has locking features to prevent vibration and temperature cycling from causing the connectors to come apart. The total coaxial cable length can be 20 feet or more. Typically, the cable is similar to RG-174. For the 2.4GHz and 1.575GHz Satellite Radio and GPS antennas, respectively, this can introduce a lot of loss.

In the centre, you can see a bolt-head and a red-coloured clip; that's what fixes the module to the roof. The cables pass through the same opening as the mounting hardware. The hole is around 20mm in diameter. Because so many vehicles are now designed for roof-mount modules, that one hole in the roof has become prime real-estate for any communications entering or leaving the vehicle.

There are also modules integrating "short FM" antennas. These include special matching networks to allow the shorter antenna to be impedance matched to the radio feed, and are very specific to a given vehicle platform. Figure 2 shows one of these modules on a 2013 Chevy Cruze.

Figure 2: 2013 Chevy Cruze RS-010 with short FM antenna module mounted just over the rear window in the centre. (Source: General Motors)

Depending on the vehicle and features, the overall architecture for these signals and the user interfaces might look like figure 3.

Figure 3: Typical vehicle communications architecture circa 2013.

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