Design of band-notched characteristics for compact UWB monopole antennas

Abstract



This thesis focuses on three research topics on the design about planar ultrawide-band (UWB) monopole antennas, namely, the design of band notches for UWB monopole antennas, the ground-plane and cable effects on the measurement of compact UWB monopole antennas, and the design of a chipless UWB radio-frequency-identification (UWB-RFID) system.

The designs of single, dual, triple and quadruple band-notched UWB monopole antennas using coplanar waveguide (CPW) resonators, quarter-wavelength (λ/4)-resonators and meander lines (MLs) are presented. The center frequencies and bandwidths of the individual notches in all these designs can be adjusted independently by varying the dimensions of the resonators. Studies of the designs are carried out by computer simulations using the EM software tool, CST MWS. For verification of the simulation results, these antennas are fabricated and measured using the antenna measurement system, Satimo Starlab. The frequency-domain performances, in terms of return loss, peak gain, efficiency and radiation pattern, and the time-domain performances, in terms of pulse responses and fidelity, are investigated by simulation and measurement. Results show that these UWB antennas have approximately omnidirectional radiation patterns with good band-notched characteristics and fidelities of more than 85% in the pulse responses.

Results of studies show that, using a small ground plane in the design of the compact UWB antennas, there will be larger discrepancies between the measured and simulated radiation patterns, radiation efficiencies and peak gains at low frequencies. The discrepancies are due to diffraction of the electric fields at the edges of the small ground plane, which leads to currents flowing back to the measuring cable and hence secondary radiation. Computer simulation and measurement are used to study the ground-plane effects using a group of nine UWB antennas. These antennas have the same radiator but with rectangular ground planes of different sizes. Results show that the width of the ground plane affects the efficiency more than the length, while the length affects the lower cut-off frequency. The cable effects are further studied by modeling the measuring cables. Results show that, by using the cable model, the simulation and measurement efficiencies agree extremely well.

The design of a novel chipless UWB-RFID system is presented. The system employs uniplanar chipless tags and a pair of high-gain reader antennas. The chipless tag is composed of two UWB monopole antennas connected by a CPW. Tag identification (ID) is represented by a spectral signature in the UWB and created by using a multi-resonator embedded on the CPW. Detection of spectral signature is based on only the amplitude of the spectral signature. Vertically and horizontally polarized signals are used to reduce mutual coupling between the uplink and downlink signals. Further reduction of the mutual coupling is achieved by using a copper plate in the reader to separate the uplink and downlink signals. Results of studies in an anechoic chamber show that the proposed RFID system can achieve a read range larger than 30 cm, indicating that the proposed system has great potentials for short-range item tracking at low-cost.