| Recently,antennas designed for near-field coupling have been widely concerned with the rapid development of near-field applications such as near-field communication(NFC),near-field radio frequency identification(RFID),near-field wireless power transmission(WPT)and near-field imaging.Consequently,the investigation of antenna near-field becomes a hotspot in antenna and propagation society.In near-field WPT,the transmit coil antenna is expected to generate a uniform magnetic field distribution to transfer power,simultaneously,to several receivers that located at different places.In near-field RFID,the reader antenna is expected to generate a uniform magnetic field distribution to enlarge the interrogation area as well as to improve the reading rate.The biggest challenge in WPT coil antenna and RFID reader antenna design is to achieve a uniform magnetic field distribution within a relative large area.To solve the problem,a series of investigations is done and presented in this thesis.The main contributions in this thesis are:(1)The concept of achieving uniform magnetic field distribution by changing the current strength distribution is proposed to achieve a uniform magnetic field distriburion in near-field WPT coil design.To improve the uniformity of field distribution of conventional spiral coil,the whole coil is divided into three parts that connected in series,and each parts is composed by several indentical spiral coils that connected in parallel.By property adjusting the number of coils in each parts and their turn number,the current strength is changed,which eventually improved the uniformity of magnetic field distribution.The experiment results show that,the proposed method is capable of achieving a uniform magnetic field distribution within a big ciol area,which is very helpful in improving the freedom of receiver device in near-field WPT.(2)The concept of "Inter-digital oppositely directed currents"(Int-ODCs)is proposed in this thesis to improve the uniformity of antenna near-field in UHF band.To eliminate the"dead zone" in the interrogation area of antennas that designed according the concept of conventional oppositelly directed currents(ODCs),a new current is added to the ODCs to form the Int-ODCs.By properly adjusting the phase of the newly added current,the uniformity of magnetic field is improved.In this thesis,two type of linear antennas,ie.folded dipole and dipole,are employed to realize the concept of Int-ODCs.The simulation and experiment results show that,by applying the concept of Int-ODCs,the uniformity of magnetic field distribution can be improved,and the "dead zone" can be eliminated effectively.(3)The Yagi-Uda antenna is employed in this thesis to enlarge the area of uniform magnetic field distribution.To enlarge the area of uniform magnetic field distribution,a new kind of Yagi-Uda antenna is proposed in this thesis,to achieve big area uniform magnetic field distribution by employing the travelling wave in its near-field region.The cover area of antenna can be improved by increasing the number of directors in the proposed antenna,a load and an additional reflector is also added to the end of directors to prevent the travelling wave to go beyond the antenna area.The simulation and experiment results shows that,the proposed new kind of Yagi-Uda antenna is capable of achieving a uniform magnetic field distribution over a big area,so as to enlarge the interrogation area of near-field UHF RFID.(4)The near-field properties of series-fed dipole array antenna are investigated in this thesis.The relationship between geometric parameters and magnetic field distribution is found by constructing an equivalent circuit model of serie-fed dipole array.Then,parameter studies are carried out.The influence on near-field distribution of lengths of each element and the distances between them are then investigated.After numerious study,a new kind of series-fed dipole array that can achieve uniform magnetic field distribution within the antenna area is designed and verified by commercial software.The simulation results show that,by properly adjusting the lengths of dipoles in a serie-fed dipole array and distances between the elements,a uniform magnetic field distribution can be achieved. |
