UHF near-field RFID antennas

Near-field radio frequency identification (RFID) technology has been traditionally used at low frequency (LF) and high frequency (HF) bands; now developing this technology at ultra high frequency (UHF) band is of increasingly interest. Designing a UHF near-field RFID reader antenna is one of the most challenging tasks. Traditional loop antennas cannot operate properly at the UHF band when the antennas tend electrically large so that they are unable to generate strong and uniform magnetic field in an adequate interrogation zone. A number of techniques have been presented to design UHF near-field RFID reader antennas, which enables the electrically large loop-type antenna generating desired magnetic field distribution. In this paper, the challenges of the UHF near-field antenna design are addressed. The state-of-art UHF near-field antenna techniques are reviewed and discussed.