Near-Field Focused Phased Array And Its Applications

Phased arrays are widely used in Radar,astronomy and communication systems due to their superiorities in high gain,ease of beam forming and fast beam scanning ability,etc.However,most of the reported phased arrays have only explored their far-field radiation properties.Recently,the near-field zone(Fresnel zone)properties of phased arrays have attracted more and more attention in multiple applications,such as radio frequency identification(RFID)systems,noncontact sensing and industrial inspection,microwave hyperthermia,gateway control system,wireless power transmission and microwave imaging.In these applications,the targets are in the near-field region of antennas,and therefore such antennas are required to concentrate their radiated power in the near-field for higher system efficiency and less interuption.Researchers have come up with different solutions to near-field focusing,and different antennas have been explored,e.g.,Fresnel lens,reflectarray,modulated impedance surface and phased array antenna.However,most of them have only realized fixed focuses,and research on focus scanning is limited.In this dissertation,near-field focused phased arrays and their potential applications are explored.Contributions are made in the following aspects:1.Simplified modeling and theoretical analysis of near-field focused phased array.Different to phased arrays used in far-field beam scanning applications,a focused phased array does not have periodicity in excitation phase distribution on the array aperture,and different elements show different radiation properties(gain and polarization)to the same space point.As a result,periodic boundaries are not applicable and time-consuming array simulation is required.Therefore,geometric optics and vector analysis are used in array modeling and theoretical analysis,and the method to calculate near-field electric field distribution is proposed,which significantly increases array design efficiency.2.Investigations on near-field focused array elements.(a)Near-field focused array antenna based on subarrays is proposed.The multiple-port equivalent circuit model of antenna is used to design subarrays,and each subarray consists of one active element and three parasitic elements.The active element spacing is enlarged in this way,and the resultant grating lobes are surpressed by subarray radiation patterns.The complicated corporate feed network in conventional focused array antenna is simplified in this way,and mutual coupling between subararys is lower than that between full-fed arrays.Furthermore,series-feed networks are used in subararys,which leads to low loss.(b)A near-field focused slot array antenna is proposed.Near-field focusing is achieved by properly sampling the positions of radiating slots,so as to obtain in-phase superposition of radiated fields at the desired focus.The slot array features simple structure,and shows lower loss in feed networks and higher radiation efficiency compared to conventional focused array antennas,while keeping high focusing performance.Furthermore,the slot array has frequency-tunalbe focal distance.(c)Study on dual-polarized microstrip antenna.Dual-polarized microstrip antenna element is designed for focused array antenna,with low active reflection coefficient in wide band in a large scan range.For a near-field focused array,the radiation direction to focus varies with different array elements,therefore low active reflection coefficient in wide scan range are required even though the focus is fixed.3.Research on steerable near-field focused beams.(a)Phased array with two-dimensional(2-D)steerable near-field focus is proposed for the first time.2-D steerable focus is achieved on the desired focal plane by a combination of one-dimensional(1-D)frequency scan and 1-D phase control,and thanks to the use of frequency scan,the number of required beam-forming components and the resultant cost are reduced by a large amount.(b)An array antenna with frequency-tunable focal distance is proposed.The near-field radiation performance is similar to fully-phased focused array antenna,however,the cost is greatly reduced without any phase control components.4.Applications of focused phased array antenna.(a)Applications in microwave imaging.A near-field focused phased array features narrow beam waist and low sidelobe level on the focal plane,which leads to high image quality when applied to a near-field imaging system.Furthermore,by the combination of 1-D phase control and 1-D frequency scan,the number of required beam-forming components or swiches can be reduced by a large amount,which leads to a lower cost.(b)Applications in wireless power transmission.By properly designing the excitation distribution on the aperture of a transmitting phased array,maximum power transmission efficiency from the array to a receiving antenna can be achieved.The difference between this work and the reported ones is that,real-time phase control is discussed here to fit movable objects,and guidelines for array design in some special cases,such as long-distance transmission and fixed objects,are given.(c)Applications in near-field measurement system.A phased array solution is given to meet the demands on beam waist,electric field intensity and focus scan range on the required spherical plane,in which the array scale,inter-element spacing and input power are determined.