A Zeroth-Order Slot-Loaded Cap-Shaped Patch Antenna With Omnidirectional Radiation Characteristic for UHF RFID Tag Design

The zeroth-order resonance of a compact patch structure is explored for designing a metal-mountable tag antenna in the United States (US) ultrahigh-frequency (UHF) radio frequency identification (RFID) passband. This zeroth-order resonator (ZOR) structure comprises a cap-shaped patch loaded with two L-shaped slots. Also, the patch is coupled to the ground through two inductive stubs which provide enough parasitic reactance to support the zeroth-order resonance. The proposed single-element ZOR can generate sufficient antenna impedance for achieving a conjugate match with the RFID chip impedance. Good omnidirectional radiation characteristic with vertically polarized fields can be attained in the azimuth plane when the tag antenna is placed on a metallic surface. The proposed tag antenna’s realized gain at 915 MHz is −1.382 dB [effective isotropic radiated power (EIRP) 4 W]. A stable read distance of ~15 m is obtainable with even spatial coverage in the entire azimuth plane. The resonance frequency of the proposed tag does not vary much with the object being tagged.