A Loop Antenna Array With an Enlarged Near-Field Interrogation Zone for UHF RFID Near-Field and Far-Field Applications

This letter presents a loop antenna array for ultrahigh frequency near-field and far-field radio frequency identification (RFID) reader applications. The antenna array consists of several microstrip collinear loop antennas, and each loop antenna is composed of several periodic radiating units. By deploying microstrip collinear radiating units, a novel loop antenna with in-phase radiating currents is realized, which generates a strong and uniform magnetic field over a large interrogation area and a horizontally polarized omnidirectional radiation. By configuring the array antenna, the magnetic fields distribute on the entire antenna array and create an enlarged near-field reading area. The far-field coverage is enlarged by the superposition of omnidirectional radiation. The antenna array is simulated and validated with a four-element prototype made on FR-4 substrates. The antenna array is measured to have a -10 dB impedance bandwidth from 802 to 963 MHz with an enlarged near-field reading volume of 280 x 280 x 540 mm and a far-field gain of 6.66 dBi at the UK RFID band.