Research On Novel UHF Near-field Antennas

With the propose of item-level radio frequency identification(RFID), ultra-high frequency(UHF) RFID technology has gained much attention due to its excellent performance in itemlevel RFID applications. As an essential component of item-level RFID systems, UHF nearfield RFID reader antenna directly affects the performance of the system. This dissertation has carried out systematically studies for the technical difficulty of how to generate strong and uniform magnetic field in the design of item-level UHF near-field RFID reader antennas. And the main innovative contributions are summarized as follows:1. Proposed a novel UHF near-field RFID reader antenna based on double-sided parallel strip line(DSPSL). This antenna is composed of DSPSL, a phase inverter, four dipoles, and a microstrip-DSPSL transition. The phase inverter which is composed of a offset DSPSL and two metal pins plays an important role in impedance matching. Four dipoles form a square dipole loop, outside the dipole is DSPSL structure which is used to fed dipoles. The structure of the antenna makes the current on the dipole loop in-phase, thus generates strong and uniform vertical magnetic field within the dipole loop. The measured results indicate that the antenna has a 10-d B impedance bandwidth of 51 MHz(887-938 MHz). With the input power of 30 d Bm, the maximum reading distance of the antenna is 200 mm, and within the height of 30 mm the antenna can obtain 100% read percentage.2. Proposed a novel center-fed broadband UHF near-field RFID reader antenna. This antenna is composed of four tapered double-sided parallel strip line(TDSPSL) fed dipoles. Four dipoles form a circular dipole loop, TDSPSLs are connected with each other at the center of the dipole loop where the antenna is fed. The current on the dipole loop is kept in-phase,thus generates strong and uniform vertical magnetic field within the dipole loop. TDSPSL is used to expend impedance bandwidth of the antenna, moreover it doesn’t affect the magnetic field distribution of the antenna. The measured results show that the antenna has a 10-d B impedance bandwidth of 236 MHz(814-1050 MHz). With the input power of 30 d Bm, the maximum reading distance of the antenna is 200 mm, and 100% read percentage can be achieved within the height of 135 mm.3. Proposed a novel UHF near-field RFID reader antenna based on folded dipole array. This antenna is composed of four unequal conductor folded dipoles. Four folded dipoles are connected by coplanar striplines(CPS) and form a folded dipole loop. The antenna is fed by a microstrip-CPS transition. The structure of the antenna ensures that the current on the folded dipole loop is kept in-phase, thus generates strong and uniform vertical magnetic field within the folded dipole loop. The measured results show that the antenna has a 10-d B impedance bandwidth of 237 MHz(828-1065 MHz). With 30 d Bm input power, the maximum reading distance of the antenna is 212 mm, and within the height of 145 mm the antenna can achieve 100% read percentage. The antenna has a wide bandwidth.4. Proposed a novel UHF near-field RFID reader antenna based on coupling structure. This antenna is composed of a folded monopole structure and a split ring resonator(SRR) which printed on the top side of a substrate and a ground plane printed on the bottom side of the substrate. The ground plane covers parts area of the substrate. The folded monopole feeds the SRR by coupling structure. The currents on folded monopole, SRR and ground plane form an in-phase current loop, thus generates strong and uniform vertical magnetic field.Because the antenna is a strong resonant structure, it has a narrow frequency band. So that an impedance matching network is used to expand the impedance bandwidth of the antenna.The measured 10-d B impedance bandwidth of the antenna without impedance matching network is 27 MHz(899-926 MHz). With 30 d Bm input power, the maximum reading distance is 188 mm, and within the height of 130 mm the antenna can achieve a 100% read percentage. The measured 10-d B impedance bandwidth of the antenna with impedance width is 64MHz(900-964 MHz), which is about 2.4 times than the antenna without impedance matching network. With 30 d Bm input power, the antenna with impedance matching work can still achieve a 100% read percentage within the height of 115 mm. When the input power is 10 d Bm, the maximum reading distance of the antenna is 38 mm, and the antenna can achieve a 100% read percentage within the height of 10 mm. This means that the antenna has good performance at very low input power. The size of the antenna is 93.11 × 76.82 × 1 mm3, it has a compact structure and suitable for low power consumption handheld UHF near-field RFID devices.