Modal Expansion Based Methods For Near-field Measurement Of Antennas Using Amplitude-Only Data

Due to high confidentiality and short-distance testing,the near-field measurement technology has been widely used,which has gradually attracted attentions of experts and scholars at home and abroad.However,the conventional near-field technique only works if acquisition of amplitude and phase information are available over a measured surface.With the rise in frequency,especially at millimetre and sub-millimetre frequency ranges,it becomes very difficult to obtain reliable phase information in near-field.Considering the degree of difficult measurement,low precision and high cost of measurement in conventional near-field technique,to this end,a large number of experts and scholars have began to investigate a new method — the phaseless measurement technology of antenna,which can calculate a very accurate far-field pattern without considering the phase information in near-field.With the rapid development of electronic and computer technology,the phaseless method will be the mainstream trends in the field of antenna test.At the current state of the art,there are mainly two classes of phaseless techniques : one is phase-retrieved method with a fast convergence and high precision,but one presupposition of above-mentioned advantages is bound to obtain a good initial estimate,which is faced with many problems in practical operation;the second phaseless technique which can overcome these disadvantages of phase-retrieved method is based on an unknown source reconstruction.The core idea of this technique is to reconstruct("guess")the electric field or equivalent current distribution over an unknown source domain exploiting a global optimization algorithm from one or more groups of amplitude-only data in near-field.In contrast to reconstruct the electric field distribution,the technique reconstructing the current distribution often takes more iterative time when utilizing source reconstruction method(SRM),besides,the reasonable base function and number of unknown to be optimized must be considered too.Starting from the conventional near-field measurement techniques based on modal expansion theory,this paper makes a deeper investigation on the the application of phaseless method,developing a novel method for unknown source reconstruction using spherical wave expansion(SWE),its applied range is extended from free-space to half-space.The main innovations of this paper include :1.Aiming at the truncated error in planar and cylindrical near-field measurement,this paper gives a more detailed and keen analysis to the causes,in addition,the far field radiation characteristics resulting from the sampling interval and range will be studied quantitatively,which provides a right direction for the further research;in view of the theoretical advantage for spherical near-field measurement,a deeper study in the breadth and depth has been expanded and extended: the weighting coefficient with same modes both includes the information in near-field or far-field,in consequence,the calculation between near-field and far-field can be available using SWE,on this basis,firstly,this paper proposes a solution scheme to test the accuracy of far-field;secondly,this paper presents a new idea to determine the highest order mode N,which can improve the calculation efficiency,and which sets a foundation for further study.2.This paper proposes a new method for unknown source reconstruction based on SWE,which can be available for phaseless near-field measurement.This method can accurately guess the tangential distribution of spherical electric field using amplitude-only data over one or two closed surfaces by SWE,which can avoid a truncation error,and which can be also available over an irregular closed surface,this method avoids making the requirement(shape)for traditional measurement technology.In relative to the case reconstructing the equivalent current distribution over an unknown source domain,this method establishes two kinds of nonlinear relationships between the phaseless data and electric field to be optimized itself,which can accelerate convergence of the global optimization algorithm,and which can greatly reduce the number of unknown to be optimized,and thus lowering the time-cost;the modified genetic algorithm(MGA)has been designed and presented,the MGA is more suitable for optimization of the high-dimensional nonlinear function,which is able to improve the applicability and robustness of GA.3.The phaseless near-field measurement in half-space is addressed.The scattering mechanism in dielectric half-space is quite different from that in the free space,consequently,the key to solve the radiation problem is consider the effect of interface on the field at any point above the upper half space.The basic idea of image principle is stated and some examples are given to verify the rationality for introducing the image theory.After finalizing the theory illustration,the near-field measurement technique using amplitude-only data above a infinite dielectric or PEC plane has been proposed by combining image theory,SWE,equivalence theory and unknown source domain reconstruction theory,which has opened up a new way to solve electric field above half-space,and which broadens the application range of phaseless near-field method.