Physical relationship between resonant frequency and structure parameters of a normal mode helix dipole antenna for an ultra-high frequency radio frequency identification tag thread

Abstract

The thread-based ultra-high frequency radio frequency identification (UHF RFID) tag with a normal mode helix dipole antenna (NMHDA) shows great potential in anti-theft and implant wireless sensors; for the engineered and digitized design of the high-performance tag, it is necessary to know the physical relationship between the resonant frequency and structure parameters of the NMHDA. Previous work for the design of the NMHDA structure is based on the self-resonant principle, that is, zero port reactance, while the port impedance of the NMHDA in the UHF RFID tag thread at resonant frequency is ideally conjugate matching to the chip. Under the requirement of conjugate matching, this study built the physical relationship between the resonant frequency and structural parameters of the NMHDA (helical radius, helical pitch, and single arm length) by linking the chip impedance with the NMHDA impedance. Meanwhile, the reported equivalent impedance model of the NMHDA, consisted of a lumped inductor and a linear dipole antenna, was utilized and modified by considering the distributed capacitance deviation from the NMHDA geometry simplification as part of the lumped inductor. Finally, the physical relationship was put into practice for the design of an UHF RFID tag thread with expected resonant frequency, and the long reading range of the prototype (over 14 m) demonstrated the rationality of the modification. In addition, the discussion on the structure parameter of the NMHDA clarified the efficiency of our proposed method. Generally, the built physical relationship explicitly gives the physical effects of the structural parameters and provides a parametric method for the structure design of a NMHDA for the UHF RFID tag thread.