Study On The Accerlation Mthod Of Incremental Length Diffraction Coefficients

The study of electromagnetic scattering mechanism plays an important role in the field of civil and national defense.Surface reflection,although in many cases,constitutes a major scattering contribution,but in areas such as stealth target design,antenna analysis and other conditions with high precision requirements,edge diffraction,as an important scattering mechanism,still has a very important research value.However,the existing ray-based diffraction theories are with some inherent shortcomings such as caustics and lower accuracy compared with current-based diffraction theories,hence cannot meet the requirements of electromagnetic engineering.Truncated wedge incremental length diffraction coefficients,one of current-based theories,though with high robustness and accuracy,its current formulas are relied on the hypothesis of the parallelogram.When trailing edges and the leading edge are not parallel,in order to satisfy this hypothesis and to ensure the accuracy,it is necessary to divide the leading edge into many small segments by frequency,and from the mid-point of each segment,the truncated length is obtained by tracing in the direction of the grazing diffraction,then the diffraction coefficients of each segments are added to obtain the total coefficient.This is time consuming when dealing with complex targets in high frequency.Therefore,on the basis of edge-mesh method,this thesis made the following improvements:(1)The method of mid-point truncated incremental length diffraction coefficients is proposed.Because of the consideration of partly second-order diffraction,we need to calculate the truncated length of each incremental strip.The mesh density of the edge-mesh method is high,so the total diffraction coefficient can be calculated more accurately.However,from the point of electromagnetic engineering,the truncated length of the subdivision points on the same diffraction edge is often of a linear change.In order to improve the efficiency,the method that the truncated length of all the sample points are approximated by the truncated length obtained by tracking the midpoint of a wedge is proposed by this thesis,which can not only avoid the tracking from all the subdivision points,but can also maintain the equivalence with TWILDC based on the complementary effect of midpoint average truncated length.Hence,the accuracy is close to edge-mesh method with efficiency largely improved.(2)A linear truncated incremental length diffraction coefficients method is proposed.Since the correction current formulas of TW-ILDC assumes that the truncated length of its applicable subscope should be constant,that is,the integral region of the analytic expression is approximately parallelogram,which is the important reason that the traditional method needs to subdivide the diffraction edge.This thesis has derived and obtained the analytic expression of the line integral on the fringe-wave current correction terms among the trapezoidal region.Based on this formula,the trailing edges can be linearly divided into several segments only related to the geometric properties,and only the truncated lengths of the minimum endpoints are required to obtain the equivalent accuracy of the edge-meshed TWILDC.The two accelerating methods proposed in this thesis are frequency-independent,which can greatly improve the computational efficiency of edge-meshed TW-ILDC under the premise of maintaining the calculation accuracy.