Design Of Wideband Bidirectional/Multi-Band Directional Circularly Polarized Antennas

With the rapid development of modern electronics information technology,the antennas has a critical influence on the performance of communication system as a key component of radio communication equipment.Due to the advantages of flexible orientation of both the transmitting and receiving antennas and effectively restraining the interference,the circularly polarized(CP)antenna is widely applied in the fields of radar,navigation,satellite positioning and radio communications system and so on.In consideration of the applied requirements of modern radio communication systems,this thesis mainly concerned with the dual-band CP antenna fed by coaxial probe and wideband CP antennas fed by microstrip line.The author's major works and contributions are as follows:1.A dual-band CP microstrip antenna fed by coaxial probe is designed.Dual-band CP operation is abtained by adopting a two-layer structure.The disk patch centrally cut with an ellipse-shaped slot is located on the upper side of the lower layer while the parasitic square rings are arranged on the bottom side of the upper layer.The proposed antenna works at 4.65 GHz and 5.42 GHz,with the characteristics of simple structure,unidirection radiation and a small frequency ratio of 1.17.The disk patch contributes to the right-handed circularly polarized(RHCP)radiation at the lower band.Similarly,the square rings also generate left-handed circularly polarized(LHCP)radiation at the upper band.The proposed antenna is designed by regulating the gap distance between the inner ring and the outer ring,solving the mismatch problem between the impedance bandwidths and axial ratio bandwidths in the higher band.The experimental results show that the 3 d B axial ratio bandwidths are 2.6%(4.59~4.71 GHz)for the lower band and 1.5%(5.38~5.46 GHz)for the higher band,within which the return losses better than 10 d B.The measured results agree well with the simulated ones.2.A wideband CP monopole antenna fed by microstrip line is proposed.The printed antenna is mainly composed of two parts,a ground plane with C-shaped branch and a deformed monopole with asymmetric arrangement.Firstly,the deformed monopole is designed to attribute a line-polarized wave.Secondly,by loading a C-shaped branch in the ground plane,surface current flows are alternated to stimulate CP.The C-shaped branch in the square ground plane has a great impact on the CP radiation.Test results show that the proposed antenna can provide an AR bandwidth of 65.5% and an impedance bandwidth of 57.3%,respectively.The measured results agree well with the simulated ones.In addition,based on Ant I,we present a coaxial probe-fed CP monopole antenna with C-shaped loaded branch.An I-shaped slit is embedded into the ground plane to improve the impedance matching.The presented antenna has a 10 d B return loss bandwidth from 1.83 to 4.1 GHz(75.6% centered at 2.96GHz),and a wide 3 d B axial-ratio(AR)bandwidth from 1.84 to 3.86 GHz(71% centered at 2.85 GHz).The measured gains are greater than 2.0 d Bic in the AR band,and the gain variation within the AR band is less than 1d Bic,which means a good flat gain feature in the broad frequency band.Because of its broad AR bandwidth and simple structure,the designed antenna has various applications,such as worldwide interoperability for microwave access(Wi MAX),the wireless local area network(WLAN)and satellite digital audio radio(SDAR)service system.3.A novel broadband circularly polarized square slot antenna(CPSSA)with low axial ratios in broadband is proposed.The antenna is fed by an L-shaped microstrip line with tapered section for impedance matching.Stubs and slots are utilized as perturbations to motivate other resonance points of circularly polarization.All of loaded strips and slots placed on the ground plane can provide additional current paths.Therefore,the impedance bandwidth and AR bandwidth can be markedly enhanced by controlling the current rotation direction in these important parts.The load structure consists of two rectangular-shape notches,two loaded spiral slots,two square-shaped notches,two inverted-L strips,two I-shaped stubs,two loaded L-shaped slots and a rectangular-shaped slot.Measured results show that the 2 d B AR bandwidth is approximately 56% and the 10 d B return loss bandwidth is about 87%.Good consistency between the simulated and measured results data is obtained.