Research On Near-field To Far-field Transformation Algorithm Of Spherical Near-field Measurements

With the arrival of the era of 5G communications,the antennas used in various base stations and terminal equipment are also facing upgrading.In the future,the requirements for antenna performance will be higher and higher,which poses new challenges to the design of antennas.The accurate measurement of antenna radiation is particularly important,because it can make in-depth analysis of the parameters of the antenna according to the measurement results,and can diagnose the problems of the antenna,which can improve the design of the antenna and improve the performance of the antenna.Spherical near-field measurement has been widely applied because it is suitable for most antennas,and has the advantages of high measurement accuracy,high measurement efficiency,and strong confidentiality.In this thesis,the near-field to far-field transformation algorithm of spherical near-field measurement is studied.Different from the traditional near-field to far-field transformation algorithm based on mode expansion,the scattering matrix is used in this paper to derive the near-field to far-field transformation algorithm.The method is based on the idea of microwave network,and the measured antenna and probe are considered as an open two-port network respectively.Through the transmission formula between the received signal of the probe and the excitation signal of the measured antenna,and substituting the near-field data of the measured antenna into the transmission formula,the transmission coefficient of the measured antenna is obtained.Since the transmission coefficients of the near field and the far field are the same,the obtained transmission coefficient can be substituted into the transmission formula of the far field to obtain the far-field characteristics of the measured antenna.Therefore,the key to this algorithm is to find the transmission coefficient of the measured antenna.So this paper first introduces the theory of scattering matrix,which focuses on the scattering matrix of electric dipole.Then using the scattering matrix to derive the transmission formula between the antenna and the probe.Using the transmission formula,we can derive the near-field to far-field transformation algorithm,that is,substituting the near-field sampling data of the probe into the near-field transmission formula,obtaining the transmission coefficient of the measured antenna,and then substituting the transmission coefficient into the far-field transmission formula,the far region radiation field of the antenna is obtained.Finally,we introduced how to implement these algorithms by writing programs,and verified a near-field to far-field transformation program without probe compensation through an example.That is,using HFSS software to simulate a standard gain pyramid horn antenna,the near-field data of the antenna is taken as the sampling data of the electric dipole probe,and then it is substituted into the near-field to far-field transformation program written in MATLAB,and the normalized far-field patterns of the antenna is obtained.The patterns were compared with those patterns simulated by HFSS and drew by far-field analytical solutions,and it was found that they were highly consistent,thus verifying the correctness of the algorithm.Finally,error contrast and analysis were performed for different sampling intervals,different truncations of the sampling sphere,and different measurement distances.Since the actual measurement uses a rectangular waveguide probe,the probe compensation algorithm needs to be further studied.