Multi-probe Compensation Technology In Near Field Measurement

With the rapid advance of communication technology,the engineers often require measuring the performance of the antenna under test(AUT)with high accuracy and speed.Among various near-field antenna measurement methods,the spherical near-field antenna measurement technology has certain advantages over the others.Since the spherical scanning surface is a closed surface surrounding the AUT,there is no truncation error,and its accuracy is much better than that of the planar and the cylindrical near-field measurements technology.In addition,the spherical near-field measurement can acquire a far field pattern covering the entire space.Of course,spherical near-field measurements also have some disadvantages.On the one hand,the mechanical structure of the spherical scanning system is relatively complex.Furthermore,the latest multi-probe spherical scanning systems are extremely complex and expensive.On the other hand,the corresponding near-field to farfield transform algorithm is rather complicated,and the correction algorithm for multiprobes is still a bottleneck.As for the existing multi-probe spherical near-field measurement system,its near-field to far-field transform algorithm is probably a version developed for only one probe more than40 years ago,which cannot accurately compensate the measurement error introduced by the multi-probe system.This paper attempts to develop a versatile correction algorithm,which can be applied to compensate any probes,no matter how many probes are used to scan,no matter which type of antenna is the probe,and no matter which band the probe is operating.The content of this thesis is as follows:1.Based on the electromagnetic field equation,we deduce the spherical wave expansion algorithm for an antenna,and study the algorithms of several special functions involved in the spherical wave expansion coefficient,and introduce the simple spherical near-field to far field transform algorithm without compensation.2.We study the high-order probe correction algorithm for the single-probe scanning system.We derive the spherical wave transmission equation for the single-probe system,and the calculation techniques of the rotation function and the probe response constant in the transmission equation.The proposed correction algorithm applies some techniques to accomplish the deconvolution.Firstly,the receiving signals are processed making use of the orthogonality of the exponential function with the angleφ,and the fast Fourier transform(FFT)is used to speed up the evaluation.Secondly,the receiving signals are extended inθdirection so that they are also the exponential functions with orthogonality,and calculated by FFT.Thirdly,the transmission equation in converted into a series of matrix equations,which are solved by the least square method to obtain the spherical harmonic coefficient of the AUT.And finally,other parameters of the AUT are obtained readily.A virtual example shows that the proposed high-order probe correction algorithm is more accurate and stable than the existing algorithms,and the former is significantly faster than the latter.3.A multi-probe correction algorithm is established.The multi-probe correction algorithm uses a technique similar to the higher-order probe correction algorithm,except that the probe response constants of the multi-probe system need to be carefully computed by a special approach.First of all,the probe response constants of each probe,suppose it is operating alone,are calculated by the commonly used method.And then,the port signal of each probe,suppose they are operating together,is simulated by computational electromagnetic methods.Finally,the probe response constants of the entire multi-probe system are accumulated by the response constants of each probe and the port signals.Note that the response constants corresponding to the two polarization directions are calculated separately.Through several simulation examples,it is found that the proposed multi-probe correction algorithm can accurately obtain the pattern of the AUT.Therefore,the proposed algorithm is a well solution to the multi-probe spherical near-field measurement system.