In-Road Microwave Sensor for Electronic Vehicle Identification and Tracking: Link Budget Analysis and Antenna Prototype

To reduce the cost and increase reliability of the vehicle radio-frequency identification and tracking systems, an alternative placement of the interrogator is investigated. Conventional systems make use of an overhead interrogator that reads a tag in a windscreen or a license plate. The alternative approach is to embed the interrogator in the road and exclusively read license plate tags. In this paper, the link budget of such a system is fully characterized assuming the ISO/IEC 18000-63 UHF Type-C RFID standard. The obtained results indicate that a microwave sensor that has an elevated toroidal radiation pattern at around a 20°-30° elevation angle above the horizon is desired. This is a challenging task as road regulations dictate that the sensor cannot exceed a profile of 2.5 cm above the road surface. As an example of a sensor that meets those requirements, a modified discone antenna with an improved impedance matching method is presented. To reduce the antenna's profile and give the required mechanical strength to withstand the weight of different vehicles on the road, the area between the disc and the cone is filled with Acetal, which has a high dielectric constant. The proposed microwave sensor is fabricated and successfully tested in a real-road environment. The results confirm that the sensor meets the aforementioned strict requirements from the link budget analysis.