3. Remote feeding of RFID tags

For the most common applications, most RFID tags have no embedded power source. The interrogator's first task is to remotely power the tag's microchip. Depending on the frequencies used and the desired remote powering distances, this energy transfer will take place either via a magnetic field, or via an electromagnetic wave. The antennae used will therefore be mainly loops in the first case, and electrical dipoles in the second. In the case of LF and HF RFID systems, the size of the electrical antennas to be deployed is incompatible with application constraints. Closed antennas, which mainly create a magnetic field in the Rayleigh zone, are therefore preferred. On the other hand, for RFID applications in the UHF range or beyond, the system will operate more in the far field, and the size of the electric antennas is generally compatible with the geometric constraints of the applications....