Study On Imaging By Broadband Iuneburg Lens Technique

Multi-beam antenna(MBA)is a kind of narrow beam that uses the same antenna aperture to produce multiple different pointing and overlapping each other,which can achieve the best beam coverage.After more than 20 years of development,the multi-beam antenna has been developed into a variety of types for different environments,so far widely used in satellite communication system.Multi-beam lens antenna is a typical branch in the development of multi-beam antenna,which is composed of focusing lens and focal plane array antenna array.It has the advantages of no feed shielding and good wide-angle scanning performance,and can be used for antenna radiation and lens imaging.Passive imaging of multi-beam lens antenna is studied in this paper.The design of multi-beam lens antenna is mainly based on focal plane array imaging technology.The whole process is composed of three parts: objects in the scene,optical focusing lens or antenna and receiving imaging display.Multiple receiving antennas work simultaneously on the focal plane,and each antenna can receive microwave radiation signals of small scenes within the range of 3d B beam,so as to achieve beam coverage.The superposition multi-beam mode and multi-channel simultaneous operation are adopted,belonging to the "staring" mode,which has strong real-time performance and does not require a lot of signal processing.The structure of Luneburg lens has spherical symmetry and its relative dielectric constant gradually changes from spherical center 2 to spherical 1,which has great advantages for focusing lens used in multi-beam lens antennas.The Luneburg lens has the following advantages :(1)ultra-wideband characteristics of the Luneburg lens;(2)good focusing characteristics.When used as a focusing antenna,it has the advantages of high gain,narrow beam and low sidelobe.(3)With multi-beam scanning characteristics and good consistency of each beam,so as to ensure the imaging quality of targets in different directions.The transformation optics method is used to compress and transform the Luneburg lens to change its shape,which can reduce the volume and weight while maintaining good focusing performance,and change the spherical focusing surface into a plane,which is easier to feed system integration.In this paper,the principle of quasi-conformal transformation is used to design an optical transform Luneburg lens with broad band,focal plane and good focusing performance.Based on the principle of quasi-conformal transformation,the design method of the transformation of Luneburg lens is studied.The permittivity of the transformation Luneburg lens with unilateral compression is obtained by MATLAB programming and PDE partial differential solver.The relationship between the degree of compression and the degree of isotropy of dielectric constant is analyzed.Due to large size of lens,according to meet the equivalent medium theorem of minimum unit design is unrealistic,just this article grope for the Angle of transformation of luneberg lens discrete layers,first by a good Angle transformation design method according to the step 5 mm discrete distribution of dielectric constant,and based on the dielectric constant,such as stratified hierarchical method for discretization of the unit.The field distribution of the focal area of the lens with different layers is simulated and analyzed by using the method of plane wave incidence.When the lens is divided into 9 layers or more,the performance of the lens improves slowly.The discrete layered transform Luneberg lens has good imaging performance in the scanning range of-30 to 30 degrees at 2-15 ghz,and angular resolutions of 25 degrees,10 degrees,6.5 degrees,5 degrees,and 3.5 degrees at 2GHz,5GHz,7.5GHz,10 GHz,and 15 GHz respectively.In order to verify the focusing performance of the designed transform Luneburg lens,a standard Luneburg lens with the same aperture is designed.The standard Luneburg lens is divided into five layers by minimax layering method.At 2GHz,5GHz,7.5GHz,10 GHz and 15 Ghz,the angular resolution is 27.7 degrees,10.5 degrees,7 degrees,5.2 degrees and 3.4 degrees,respectively.In this paper,compression transform Lombe lens and cut transform Lombe lens of different sizes are designed to achieve higher imaging resolution by cutting large aperture lombe lens along the radius.The results show that under the same aperture,the cutting transform Luneburg lens has no better imaging performance and the scanning characteristics decrease.When the multi-beam lens antenna is used for imaging,in fact,the antenna array on the focal plane is used for imaging.The antenna array is placed on the focal plane,and the antenna is arranged as the receiving array element according to the resolution.When the phase center of the antenna coincides with the image point,the antenna receives the power of the image point,which is difficult to do in fact.Multi-beam technology is similar to focal plane array technology.After focusing lens and focal plane array antenna are added,the angular resolution of multi-beam lens antenna is the angular resolution of the whole imaging system.In this paper,a Wideband Vivaldi antenna is designed as the focal plane array antenna element.The antenna width is 21 mm,and the impedance bandwidth is 2-12 GHz in an infinite array simulated by one-dimensional periodic boundary.One-dimensional periodic arrangement of Vivaldi array elements on plane E was carried out and placed on the focal plane of cutting Luneburg lens.The radiation direction diagram of multi-beam antenna was obtained when the array elements were fed separately from the center to the edge,and the central frequency was 5GHz.The maximum direction of antenna beam is 0 °,10 °,20 °,30 ° and 40 ° respectively.The beam width is 9.8 degrees,10 degrees,9.7 degrees,9.7 degrees,9.3 degrees respectively.The antenna angular resolution is approximately the same as the imaging angular resolution,and the error is mainly caused by the phase center.The shortcoming is that the size of the designed antenna is larger than the focal spot size when the lens is applied to the highest frequency band,which cannot meet the imaging when the antenna is larger than the center frequency,and a smaller size of the feed antenna is needed.A good Angle transformation design method makes the transform lens can realize full dielectric material,and based on the realization of low dielectric constants of low loss material difficulties,and transform luneberg lens realization of different dielectric constant material,we adopt the existing mature perforating technology,the existing typical dielectric materials for punching,equivalent to the dielectric constant value,In this chapter,two different hole-changing Luneberg lenses are designed with 10 mm hole-punching technology,which are vertical and horizontal hole-punching respectively.The imaging performance of the hole-punching lens is simulated and analyzed.The results show that the horizontal holepunching lens has a 3d B focal spot diameter similar to that of the non-hole-punching lens,and the imaging resolution of the vertical hole-punching lens decreases significantly.With the development of 3D printing,the punching structure can be realized by 3D printing.Compared with mechanical work,the cutting and punching steps required by the traditional method can be avoided.The complex punching structure can be realized more easily,and can reduce the waste of materials,which has great advantages.Relevant work is under way.