Study Of Cylindrical Near-field Measurement Method For Predicting Of Bistatic Rcs Of Antennas

As an important equipment to effectively radiate and receive electromagnetic waves,the antenna's scattering characteristics have been used as an important design parameter in the military field.The traditional far-field antenna scattering measurement method has higher requirements on the measurement environment.But also the confidentiality and acquired information of it also has more disadvantages than near-field measurement.Based on the derivation of a three-dimensional algorithm for bistatic cylindrical near-field measurement,a near-field to far-field transformation program with bistatic cylindrical near-field scattering measurement is designed in this paper.The corresponding near-field scattered data file can be input in this near-field to far-field transformation program and it will compute the corresponding far-field scattered data.At the same time,according to the specific mode term of antenna scattering,a specific algorithm for separating the mode terms of the antenna is built.By comparing with the far-field RCS corresponding to ideal plane wave incidence the RCS obtained by near-field to far-field transformation,the correctness of three-dimensional algorithm for bistatic cylindrical near-field scattering measurement is verified in this paper.The main contents of this paper are as follows:Firstly,the related theory of electromagnetic field under cylindrical coordinates is introduced.The three-dimensional bistatic cylindrical near-field to far-field transformation algorithm is derived.and the theory of the Radar Cross Section and antenna scattering are introduced and analyzed.In conclusion,a three-dimensional program for cylindrical near-field to far-field transformation was designed.Secondly,the far-field bistatic RCS of the typical target ideal conductor sphere is simulated by FEKO,and compared with the analytical solution RCS of the ideal conductor sphere,which prove the accuracy of FEKO in simulating the target RCS.In a conclusion,the results of FEKO simulation can be used as theoretical values to compare with the results of near-field to far-field transformation.Next the near-field data of the ideal conductor sphere is input the near-field to far-field transformation program and then compared with the theoretical value.In addition,an ideal conductor cylinder is used as the target under test,and the ideal plane wave is incident from a certain direction.The near-field data and the far-field RCS of the ideal conductor cylinder are obtained by FEKO.By the three-dimensional program of bistatic cylindrical near-field to far-field transformation,the actual RCS after near-field to far-field transformation is obtained and compared with the theoretical RCS.In this way,the correctness and feasibility of the algorithm is verified.Then,with a helix antenna as the target under test,the feeding port of the helical antenna is set and the ideal plane wave is set to incident in a certain direction,and the FEKO is used to simulate the far-field bistatic RCS and the corresponding near-field data.By substituting the obtained near-field data file into the three-dimensional program of the bistatic cylindrical near-field to far-field transformation,the RCS of antenna after the near-field to far-field transformation,containing the mode item and the structure item,can be obtained.It is known that when the antenna is perfectly matched,it only has a corresponding structure term when it scatters.The antenna impedance of the feed port at this time is also obtained by computer simulation,and the corresponding matching load is added to the feed port of the corresponding antenna,so that the far-field RCS,containing only the structure term RCS,can be obtained.By the relevant procedure,the mode term of the antenna can be separated by the method of vector addition and subtraction.In addition,the helix array antenna is also used as the target under test,and the near-field to far-field transformation results and the theoretical far-field RCS of the helix array antenna are compared.The port impedance of the array antenna element is simulated.Due to the mutual coupling between the array antenna elements,the port impedance of the array antenna element is different from the port impedance of a single antenna element.Therefore,when the mode item RCS of the array antenna is separated,the conjugate matching load,applied to the antenna element port,is also different from that of the single antenna element.Finally,the near-field data of the scanning surface in partial region is taken,and the near-field data is weighted by adding the cosine window function.The calculation results show that the error caused by the truncation of scanning surface can be effectively reduced by applying the cosine window function in bistatic cylindrical near-field scattering measurement.