Transmission Line Metamaterial-Inspired Circularly Polarized RFID Antenna

A novel wideband (11.1% -10 dB bandwidth) and low-profile (0.03 λ0 height) metamaterial transmission line (TL)-inspired circularly polarized (CP) antenna for radio-frequency identification (RFID) application is proposed. A capacitor-loaded TL (CL-TL) is realized by inserting multiple series metal-insulator-metal (MIM) capacitors into the TL. The MIM capacitor works as a series left-handed (LH) capacitor as well as a radiator. First, the CL-TL is studied and analyzed through circuit model and dispersion curves. Then, a linearly polarized (LP) antenna and a CP antenna based on the CL-TLs are implemented. Compared with the conventional right-handed (RH) TL-based antennas, here the size reduction is achieved by pushing down the dispersion curve to lower frequency using multistage structures and larger LH capacitance. The measured -10 dB impedance bandwidth, 3 dB axial-ratio (AR) bandwidth, and radiation efficiency for this CP antenna is 859-959 MHz (11.1%), 902-923 MHz (2.3%) and 83.9%, respectively. It features a compact size, low cost, wide bandwidth, and good radiation performance, which could be conveniently used for RFID reader.