Design Of Vortex Wave Antennas In The Electromagnetic Imaging System

In recent years,the electromagnetic wave imaging technology as a non-destructive physical detecting method has been widely used in biomedical and other fields.In order to improve the performance of the imaging detection system further,a new high resolution electromagnetic wave imaging technology is needed urgently.Orbital angular momentum(OAM)is one of important characteristics of electromagnetic wave.Electromagnetic waves carrying OAM are called vortex waves.In recent years,orbital angular momentum has been proposed to be applied in electromagnetic wave imaging system technology.Due to the unique phase characteristic of vortex waves,more target information can be obtained.So OAM has significant values and good application prospects to study.As the important element in the electromagnetic wave imaging system,the radio frequency(RF)antenna has the effect on the imaging performance directly.The proposed RF antennas must have good radiation performances to obtain accurate imaging information.In this thesis,the antennas are proposed and optimized to generate vortex waves.The simulated and experimental results validate the rationality and accuracy of the antennas design.The main researched contents in this thesis are as follows:(1)A single-arm Archimedean antenna is designed and analyzed.According to the circular travelling-wave antenna theory and the higher-mode theory of the Archimedean antenna,the Archimedean antenna has the potential ability to generate OAM.The simulated results demonstrate that the proposed antenna can generate vortex waves whose OAM modes are reconfigurable by changing the working frequencies.(2)An inner-and outer-fed dual-arm Archimedean antenna is designed and analyzed.The proposed antenna can generate vortex waves carrying multiple OAM modes.The generated OAM beams are symmetrical due to the symmetric antenna structure.The gains of the proposed antenna reach more than 6.5 dBi by loading the conical metal cavity.The theoretical analysis,simulated and experimental results demonstrate that the proposed antenna can generate vortex waves of which the OAM mode is reconfigurable by changing the operating frequency.In addition,the proposed antenna can generate two OAM modes in the single frequency by changing the feeding positions.(3)A nested dual-arm plane spiral antenna is designed and analyzed.The antenna can generate three OAM states by controlling the feeding ports.Meanwhile,the metal reflector which is added on the outside of the antenna can not only enhance the directivity of the antenna,but also isolate the radio frequency(RF)coaxial cables form the top microstrip spiral to reduce the effect of the RF coaxial cables on the performance of the antenna.In addition,the OAM-modes detecting methods are investigated based on the proposed antenna.Two methods which are based on the passive structure carrying the opposite topological charge to the OAM mode to be detected and based on the transmission system of transmitting and receiving OAM modes are proposed to detect OAM modes.The two methods can detect the OAM modes.