Research On Near-field Measurement Method For Coupling Scattering Targets

Radar cross section(RCS)is an important parameter to represent the electromagnetic properties of targets,which is often used to identifying the echo of targets.Moreover,it is also a key indicator to evaluate the target’s stealth properties.The RCS of target is usually obtained from the calculation and the measurement,and the test result is used for verifying the calculation result.Generally,RCS measurement need to satisfy the far field condition,that isR≥2D~2/λ,where R,D andλare the testing distance,the maximum size of target and the wavelength respectively.However,because of the limitation of the testing distance,the maximum size of target and the working frequency,it is difficult to meet the far field condition in practice.A new method of the RCS near-field measurement method is presented to solve this problem,by testing the scattering data with near testing distance,the far field RCS can be obtained by the near-field to far-field transformation.This method decreases the cost of test system and improves the work efficiency,which is applicable for the equipment inspection in producing and the equipment maintenance.According to the difference of scattering mechanism,targets can be divided into the single scattering targets and the coupling targets.Currently,most near field measurement methods are applied to the single scattering targets,nevertheless few research about the near field measurement method for the coupling targets.This dissertation focuses on several typical coupling targets consisting of plane-plane structure(dihedral angle and rectangular cavity),and investigates the near field measurement method of RCS.It is important to research the near field measurement method for the coupling targets because these typical coupling targets are the strong scattering sources on the weapon equipment,and the near field measurement method of single transmission and single reception(STSR)has major testing error.To solve this problem,the main work and innovation of this dissertation are as follows:1.A time-frequency analysis method is proposed for identifying the single scattering targets and the coupling targets.First,the reason why it is difficult to obtain the far-field RCS of the coupling scattering targets using the method of STSR is analyzed theoretically.Then the simulation for the coupling scattering sources of the targets is performed,which verify the results of the theoretical analysis.By using the short time Fourier transform(STFT)with the monostatic/bistatic data for several typical targets,the 2-D scattering characteristic results with a different window function can be obtained.It can be found that the spectrum component of single scattering target is very low,and the coupling targets has obvious spectrum component within certain angular region.To obtain the better time-frequency resolution,the pseudo Wigner-Ville distribution(PWVD)has been used for analyzing bistatic scattering data for further identifying the discrepancy between the single scattering and coupling targets,and it can be a help for choosing the systems.2.To solve the problem of calculating the far field RCS of coupling targets requiring full bistatic scattering data,a multiple transmission and multiple reception(MTMR)method is proposed.The mathematical relationship of scattering response between near-field and far-field for the coupling targets is derived firstly.Then,according to the rule that the near-field to far-field transformation of coupling targets,which requires full scattering data at near field,the extrapolation is performed with the near field scattering data twice,by combining the reciprocity theorem between transmitting antenna and receiving antenna,the far field RCS of coupling targets can be obtained.At last,some typical coupling scattering targets are used for verifying the method in simulation.Compared with the near-field to far-field transformation method with STSR,the proposed method is more accurate,and the mean error within the coupling scattering region is about 0.5d B,which improves the accuracy of the near field measurement.3.To solve the problem of too much cost using MTMR method with full bistatic near field measurements in practice,a method of choosing the truncation region is researched.It is proved that the scattering field is almost space bandlimited,and the effective bandwidth is related to the size of the target.Moreover,the effective region of transfer function in the method of near-field to far-field transformation is related to field angle.According to this theory,the effective reception region of measurementing the coupling scattering targets is further verified by simulation.The simulation is performed with different reception region when the target is fixed,compared with the far field RCS under different truncation region,the test error with different truncation region can be obtained.Then the proper truncation region can be chosen empirically by the statistical analysis of the test error,that is,the test error is less than 1 d B while the reception region is approximately two times than the field angle of target.4.Because the system with full bistatic measurement is hard to construct and takes too much test time,a measurement method of“single transmission and multiple reception combining the rotation of platform”is proposed.Based on the rule for choosing the truncation region,the arc scanning array is composed of one transmitting antenna and several reception antennas,where the interval between the reception antennas is determined by the sampling theorem.By chooseing the microwave switches to activate the reception antennas with different path and combing the rotation of target,the near field scattering data is collected by these reception antennas,and some typical coupling scattering targets are verified.By the near-field to far-field transformation,the extroplated far-field RCS is obtained,and the error between the extroplated result and the measured RCS is less than 1d B,which proved the effectiveness of proposed measurement method.