Modeling and Characterization of Zero-Phase-Shift Lines and Optimization of Electrically Large ZPSL Loop Antennas for Near-Field Systems

A methodology for modeling and characterization of a zero-phase-shift line (ZPSL) structure is presented, and a ZPSL-based electrically large loop antenna is optimized for near-field wireless systems. The ZPSL loop is first analyzed with a full-wave driven-mode solver to obtain the dispersion curve. An equivalent circuit model is then presented for characterizing the ZPSL structure. Based on the dispersion analysis, a design guideline is proposed for the ZPSL loop antenna to enlarge its interrogation zone, where a uniform magnetic field distribution is desired. A design example at 915 MHz shows that the perimeter of the ZPSL loop antenna with a desired uniform magnetic field distribution can be optimized up to 2.5λ0, which is much larger than those reported with 2λ0, achieving a 56% increase in the area of the interrogation zone. The proposed method can be applied in the antenna design for near-field wireless systems such as wireless charging, radio-frequency identification, nearfield communications, and magnetic resonance imaging.