A Reader Antenna for UHF Near-Field RFID Applications Based on the Segment-Line Oppositely Directed Currents

A reader antenna based on the segment-line oppositely directed currents is proposed in this letter for the ultrahigh frequency near-field (NF) radio frequency identification applications. The proposed antenna consists of two identical electrically large dipole elements without additional impedance matching network. To keep the surface currents along the dipole elements in-phase, the segmental and loaded method is adopted. Through the dispersion analysis, the maximum length of the dipole elements is one and a half guide wavelength with 12 segments and 0.7 pF lumped capacitors at 920 MHz. Subsequently, adjusting an appropriate distance between the dipole elements can form the oppositely directed currents, resulting in a strong, uniform, and large NF magnetic field distribution. The measured results show that the bandwidth is 53 MHz from 884.5 to 937.5 MHz covering the FCC standards. The interrogation zone is approximately equal to 480 mm × 250 mm under the observing height of 50 mm above the antenna surface. Thus, the proposed antenna is suitable to be used for the smart showcases and shelves.