Universal UHF RFID reader antenna based on the ferrimagnetic slab with tunable operating frequency

A circularly polarized (CP) patch antenna based on ferrimagnetic substrate is discussed in this paper, for universal ultra-high-frequency (UHF) RF identification (RFID) applications in the Internet of Things (IOT). Such antenna is composed of top dielectric layer printed with rectangular radiation patch, middle ferrimagnetic layer, and bottom dielectric layer printed with ground plane. The ferrimagnetic layer under an applied dc magnetic bias provides the CP radiation characteristic, and most importantly, tunes arbitrarily the operating frequency band in a wide range covering all the worldwide operating frequency bands of UHF RFID, by adjusting the magnetic bias. Numerical results demonstrate that the proposed antenna, under conditions of different dc magnetic biases ranged from 330 to 700 Oe, can be arbitrarily operated from 840 to 960 MHz with a narrow impedance bandwidth of about 7 MHz, a tunable gain level ranged from -1.9 to 2.5 dBic, and a wide 3-dB AR beamwidth of 120° over the impedance bandwidth.