A Switchable UHF RFID Reader Antenna for Near-Field and Far-Field Applications

In this letter, we present a switchable near-field (NF) and far-field (FF) antenna designed for ultrahigh-frequency (UHF) radio frequency identification applications. The antenna consists of a four-dipole array and a feeding network. By adjusting the current distribution through the multiport feeding network, the antenna can either create a strong, uniform magnetic field in the NF region with low FF gain or produce circularly polarized radiation in the FF region. Through the use of segmented lines and parasitic elements, the antenna achieves a flat magnetic field distribution over a large area. The NF and FF modes can operate independently or in polling. A prototype was fabricated and tested, showing that the antenna generates a strong, uniform magnetic field over an interrogation zone of 270 mm × 270 mm in NF mode, while delivering a 3.7 dBic gain in FF mode. These results confirm the antenna's excellent performance in both NF and FF regions, with an isolation of over <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$-$</tex-math></inline-formula>22 dB between the two modes.