Study Of Spherical Near-Field Antenna Measurements And Diagnosis Methods

Compared with planar near-field measurement and cylindrical near-field measurement,spherical near-field measurement has the advantage of a wider scope of angular range measurement,which is fit for more types of antennas,including the narrow beam antenna,the wide beam antenna and the omnidirectional antenna and so on.This thesis begins from the basic Maxwell equations,with emphasis on spherical near-field to far-field transformation based on the theory of mode expansion.The spherical wave mode expression and the spherical wave mode coefficients are derived.The recursive relations of some special functions,such as joint Legendre function,ball Hankel function and so on are analyzed,which are convenient for programming and calculation.In the process of solving the spherical wave mode coefficients,fast Fourier transform(FFT)can be utilized to improve the speed of running the program.With the planar and curved phased array antenna as calculation mode respectively,the correctness of the formulas of near-field to far-field transformation derived in this thesis is verified by processing the near-field data analytic solution by spherical near-field to far-field transformation.The calculation results show that the far-field pattern obtained by spherical near-field to far-field transformation agree well with the theoretical far-field pattern.With the development of near-field antenna measurement technology,near-field measurement has also been used to inversely solve the aperture field of antennas and the excitation current of elements,which is near-field diagnosis of antennas.Especially for phased array antenna,the working status of each radiating element can be resolved through diagnosis,which has important practical engineering significance for antenna design and modulation.Based on the plane wave spectrum of the antenna radiation field,the expression of the aperture field of antennas based on plane wave spectrum is derived and the plane wave spectrum can be obtained from the far-field pattern calculated by spherical near-field to far-field transformation.With taking the planar half-wavelength dipole array antenna as an example,the distribution of aperture magnetic field is analyzed,which can be equivalent to the current distribution of each dipole element.Through the analysis of the results,it can be obtained that although the presence of the attenuation mode is neglected in the process of solving the plane spectrum with the far field pattern function,it does not have much influence on the specific diagnosis and the normalized wave abdominal current is in good agreement with the theoretical value.The correctness and feasibility of the diagnostic method are furtherly validated by the introduction of fault radiating elements.There exists errors in any measurement technique,and near-field antenna measurement is no exception.The error sources in spherical near-field antenna measurement are summarized,with emphasis on the truncation error introduced in the actual measurement process when the data on the entire sampling sphere cannot be completely obtained.With the phased array antenna as an example,the size of the truncation error is closely related to the distance between the maximum radiation direction of the antenna and the edge of the sampling surface.For this instance,the mode filtering method and cosine window function weighting method are presented to reduce the truncation error.The basis of mode filtering is that the number of spherical wave modes of antenna radiation field is determined strictly according to the smallest ball that surrounds the antenna under test.Finally,the combination of mode filtering and cosine window function weighting is studied in this thesis.The near-field data in unsampled area can be reconstructed by mode filtering,and then the cosine window function weighting can be utilized to reduce the truncation error further more.The feasibility of this method is verified by an example calculation.