Connection in the car: RF communication

Today's passenger cars, especially in the United States, have become the driving communication center. A typical General Motors (GM) includes AM, FM, satellite radio, two GPS receivers, and cellular radio. In the passenger seat, there may be Bluetooth; in addition, DSRC (dedicated short-range communication, 5.9GHz) for vehicle-to-vehicle and vehicle-to-infrastructure communication will be added in the future, and there may also be anti-collision radar, but this is usually A separate system.

Most of the electronic circuits used in all these devices are located in the "center console"-the area between the driver and the front passenger where the screen and main control knob are installed. Satellite broadcasting, GPS, AM/FM radio and cellular signals all arrive in the car from the outside, and these signals must be captured by an antenna, and then coupled to the electronic circuit through a cable. In order not to make cars full of antennas (which is different from police cars), OEMs (Original Equipment Manufacturers) are developing multi-functional modules containing multiple antennas and circuit boards (containing more than 100 components). Figure 1 is a recent example.

Figure 1: The picture on the left shows a GM "shark fin" antenna used in the 2011 model year. The picture on the right shows the shark fin antenna installed on the 2013 GMC Yukon. The antenna module is installed directly above the windshield. (Source: General Motors)

The module in Figure 1 includes GPS antenna and low noise amplifier (gain>25dB, noise figure 1dB-black line), XM satellite broadcasting antenna and low noise amplifier (yellow-brown cable), and cellular antenna (blue line). Three coaxial cables terminated with special SMB connectors are secured in a housing to simplify installation. 3 The plug connector is fixed together with the cable assembly inside the vehicle. It has a locking function to prevent vibration and temperature cycling from separating these connectors. The length of the entire coaxial cable can reach 20 feet or more. Generally, the cable is similar to RG-174. For 2.4GHz satellite receivers and 1.575GHZ GPS antennas, it will introduce a lot of loss.

In the center of the module, you can see a head bolt and a red clip, which are used to fix the module on the roof of the car. The cable and the mounting hardware share a mounting hole, and the diameter of the mounting hole is about 20mm. Because so many vehicles currently use the design of installing modules on the roof, this hole on the roof becomes a "sweet" channel for any incoming and outgoing vehicle communication.

There are also modules that integrate a "short FM" antenna. They include special matching networks to allow shorter antennas to also achieve impedance matching with RF feeds. These designs are completely targeted to specific vehicle platforms. Figure 2 is an example of such a module used in the 2013 Chevrolet Cruze.

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