Long-range RFID emitter antennas for passive keyless entry systems

October 10, 2016 // By Jorge Rodriguez
Passive keyless entry (PKE) is among the recent technologies introduced into the automotive sector one of the trendiest. No doubt! Touching the door handle or approaching to it and automatically opening is like dark magic.

Most of us have had the tedious routine of unlocking the doors using a physical key, looking for the right one among a cluttered bunch after feeling a sting in your pocket pierced by a sharp point.

Currently, most of the vehicles are equipped with remote keyless entry (RKE), allowing the driver to control the car from a distance. If the driver presses the button on the fob, then the car doors automatically lock or unlock. But in the near future the trend is to introduce PKE at an affordable price, as it is the most comfortable one. A study from Research and Markets estimate that the global automotive passive keyless entry system market to grow at a CAGR of 10.6% during the period 2016-2020.

The principle of operation can be summarized as follows, when a trigger event occurs such as approaching the car or touching the door handle, a low powered LF wake-up signal is transmitted to the key fob.  Afterwards, an ultra-high frequency (UHF) identification response is sent back to the car. Once the mutual authentication has been processed through the UHF channel, the car door opens.

LF communication is set to 21.8 kHz, 125 kHz or 134 kHz via Binary or Quad Pulse Length Modulation for different purpose: LF Wake-up, Anti-collision and RSSI measurement. LF Wake-up is a predefined string of bits to recognize the key fob in a very low consumption sleep mode. Once the signal is processed, the key answers back to wake up the PEPS controller. Anti-collision is a method to avoid interference from simultaneous fobs signals in case several keys are in use. RSSI measurement, received signal strength indication, is performed by the key to determine it position by measuring the magnetic field strength for the X, Y and Z axes.

Fig. 1: Outdoor emitter antenna placement.

In order to localize the fob, the car is equipped with several emitter antenna (LF driver) strategically distributed. Usually there are three inside to cover the cabin space and another three on each handles for the outside coverage. This creates an isotropic field space all over the car. The final distribution and number of antennas depends on the size and geometry of the vehicle.

Fig. 2: Indoor emitter antenna placement.

The R&D team at Premo has invented a new concept of emitter antennas by introducing two patented flexible and semiflexible magnetic materials that allow making long-range antennas. Based on the reduction on the number of antenna per car, the new design is longer than the standard 120mm door handle, with a length ranging from 300-500mm.