A Research On Plane-Polar Near-Field Measurements

It has attracted the considerable attention of many researchers since plane-polar near-field measurements wave presented.Because the aperture surface of antenna under test points to the vertical direction and sampling points present a radial shape,this method of measurements has advantages of simple mechanical operation,gravitational equilibrium in the vertical direction,etc.So this method of measurements has important applications for the measurement of large-scale space antennas.To apply plane-polar near-field measurements to the engineering practice better,firstly this thesis expounds the mechanical sampling of plane-polar near-field measurements,and discusses the mechanical features and the computational features.Then,the relation between antenna radiation field and near-field electromagnetic current density is derived by applying antenna radiation theory based on Maxwell equations and boundary conditions.And this thesis presents the near-field to far-field transformation formula without probe compensated.In order to make plane-polar near-field measurements be suitable for the engineering practice,this thesis presents the relation between the probe output and the near-field of antenna under test by applying the reciprocity theorem between the antenna under test and the probe.And this thesis derives the expression of apparent induced electromagnetic current in the scan plane by applying the convolution theorem.Then by applying antenna radiation theory,the radiation integral with probe compensated and the relation between antenna radiation field and the radiation integral are derived.So the near-field to far-field transformation formula with probe compensated is given in this thesis.To calculate the radiation integral in the near-field to far-field transformation formula,this thesis discusses the Lagrange interpolation algorithm,the Jacobi-Bessel series expansion algorithm and the application of these algorithms for the calculation of the radiation integral in detail.By comparing these algorithms,the advantages of the Jacobi-Bessel series expansion algorithm are presented,including high precision on calculation and the characteristics that the near-field data can be used directly without interpolation algorithms.Then,the calculation formula of the radiation integral is got in this thesis.In addition,this thesis computes the expansion coefficients using a trapezoidal rule and Simpson's rule,and presents the complete transformation formula of plane-polar near-field measurements.Then,the codes for near-field to far-field transformation are programmed using Matlab.This thesis creates a 43?31 elements array of the half-wave dipole and a horn antenna.Near-field data and the radiation far-field of antennas are computed by the theory formula.This thesis compares the theoretical radiation far-field with the far-field which is got by substituting near-field data in the transformation program to prove if the plane-polar near-field measurements theory is true.It should be noted that this thesis computes near-field and far-field of the horn antenna by using both theory formula and electromagnetic simulation software HFSS.It shows that the far-field transformed fits the theoretical far-field well,which proves that the theory is true.To apply plane-polar near-field measurements to the engineering practice better,this thesis also analyzes measurement errors about some sensitive factors,such as the size of the scan plane,the radial sampling interval,the angular sampling interval,the accuracy about Jacobi-Bessel series expansion,and so on.In addition,to make near-field to far-field transformation theory impeccable,this thesis makes a discussion on other parts of plane-polar near-field measurements in detail,including the improvement of the algorithm for computing the expansion coefficients,the optimization of the sampling interval and the fast algorithm for the calculation of the radiation integral.To apply plane-polar near-field measurements better to the scan plane in which the field value changes by a wide margin,the Cotes integral algorithm is presented in this thesis.This thesis presents the double half-wavelength sampling method,which reduces the number of sampling points greatly and saves plenty of time for practical measurements comparing with traditional plane-polar sampling method.Because the two-dimensional FFT algorithm cannot be directly applied to the procession of data in the polar coordinates,this thesis applies the one-dimensional FFT algorithm to calculate the radiation integral by means of the formula's equivalent transformation,which provides a new method for fast calculation of the radiation integral.