Three-Axis Coil Array for Null-Free Near-Field Tag Reading with UHF RFID

Tiny radio-frequency identification (RFID) tags with a 1 mm square scale can be attached to small insects such as ants and used to detect their passage at RFID reader antennas in a laboratory arena. Although the movement paths of ants can be restricted inside a narrow tube, the ants pass through the tube in various postures. A single small coil antenna attached to the tube generates nulls, where the tags become unreadable due to specific tag orientations. This letter proposes a tag reading system with three antennas generating major magnetic field vectors in three orthogonal directions, to eliminate nulls within the tube, regardless of the tag orientation.