Design Of Scanning System For Antenna Near Field Measurement Based On Current Detection

With the rapid development of microwave antennas in satellites,radars and other fields,there are higher and higher requirements on performance indicators of antennas.Traditional far-field antenna measurements can no longer meet the test requirements in some occasions.Therefore,Nanjing University of Information Science and Technology launched a research on the antenna near-field measurement system in July 2016,so as to improve the accuracy of antenna measurements.It shall be noted that the measuring principle of antenna planar near field,the core algorithm of the near-and-far field transformation as well as the moving platform and the control system of antenna scanning frame are key technical problems in the antenna planar near-field measurement system.Focusing on the core algorithm of the near-field measurement system,the moving platform and the control system of the scanning frame,main research contents in this paper are as follows:(1)Firstly,it thoroughly researches the basic principle of the antenna planar near-field measurement and briefs the theory of plane wave expansion,the selection of parameters of the scanning surface,the electric field integral equation of the equivalent magnetic current,as well as the principle of measuring dipole.(2)The method of implementing near-and-far field transformation of antenna based on current detection is researched through Matlab values.Also,the Green equation of current integration is converted into a matrix equation with the Galerkin-type moment method.Moreover,a Toeplitz block matrix can be constructed by means of reasonably designing the scanning moment matrix.Meanwhile,the matrix equation is calculated using the Fast Fourier Transform conjugate gradient algorithm,greatly reducing the calculated amount and the operation time.The dipole array antenna and the horn antenna model were established by the HOBBIES simulation software.The far-field pattern was obtained.At the same time,the induced current in the near-field area of the antenna was acquired with a dipole probe,and an equivalent magnetic-field near-and far-field transformation algorithm based on current detection was obtained.Besides,comparing with the far-field pattern obtained directly from the HOBBIES software,the method researched is feasible,since the near-and-far field transformation theory of the equivalent magnetic current method based on current detection researched in this paper is completely consistent within the range of 120 degrees on both sides of main lobes of the antenna radiation.(3)In order to obtain accurate antenna near-field data,a high-precision near-field scanning frame structure system is designed.The scanning frame system is designed with an inverted "T" plane as its main structure,with structural steel and glass steel as main materials,with lead screws as transmission gears,and with a stepping motor as its drive mode.Ultimately,a scanning frame moving platform that meets the design indicators is developed through using the finite element analysis software of COMSOL Multiphysics to simulate and to optimize the design structure of the scanning frame.(4)A control system for the antenna planar near-field scanning frame is designed.STM32F407 is selected as a processor chip to carry out detailed design on hardware circuit such as peripheral serial communication,data acquisition and storage,three-dimensional motor motion control and power switching,etc.In terms of the software part,modular ideas are adopted separately to design the main control unit and sub-module program of each function.Finally,the function module is called in the main program cycle to realize the control system function.(5)At last,the hardware and software of the control system are commissioned.The results show that functions of each module of the control system meet the design requirements and can be operated reliably.