Review of Zero-Phase-Shift-Line Loop Antennas for UHF Near-Field RFID Readers

The design of electrically large loop-type antennas for ultra-high frequency (UHF) near-field radio frequency identification (RFID) readers is a challenge due to the inherent limitation of loop type antennas. This paper reviews the development of the zero-phase-shift-line (ZPSL) loop antennas for UHF near-field RFID systems. First, the state-of-the-art ZPSL loop antennas are briefed and then the modeling of the ZPSL structures is introduced. After that, some typical electrically large ZPSL loop antennas for UHF near-field RFID readers are demonstrated, including single-loop antenna, dual-loop antenna, grid loop array antenna, and artificial magnetic conductor backed directional loop antenna. All the patentable works have been applied for industry applications.