Near-field coupling in UHF-RFID printer-encoders

In the framework of Near-field (NF) UHF-RFID applications, printer-encoder antennas represent an interesting scenario to investigate. In commercial RFID printer-encoders, the distance between the encoder and transponder antennas is typically a few millimeters only, and the encoder antenna has to reliably communicate with only one targeted tag among a number of tightly spaced transponders in very close proximity. In this paper, a number of encoder antenna layouts have been numerically analyzed in terms of power delivered to the transponder chip, by considering two commercial RFID tags. Moreover, the spatial selectivity of the encoder antennas is estimated. As a result, a reader antenna identical to the tag antenna is suggested as an optimal solution for meeting the severe requirement on the spatial selectivity of the encoding process.