Research On Analytical Calculation Methods For The Shielding Performance Of Cavity Structures

The electromagnetic compatibility(EMC)of electrical and electronic products has now been rising as a critical issue because of the increasingly complex electromagnetic environment.Therefore,the electromagnetic shielding,an important approach to realize the EMC,has become the research hotspot in this field inevitably.For the purpose of rapid and accurate calculation of the electromagnetic shielding performances of cavity structures,the calculation or prediction methods for the shielding effectiveness(SE)of apertured cavity,the electromagnetic interference(EMI)of the loads of the transmission line within apertured cavity,and the electromagnetic resonance of apertured cavity have been studied systematically in this dissertation to provide a theoretical basis for the design of the structure of shielding cavities.Specifically,the research includes the following four parts:(1)Study on the modification and extension of the Robinson analytical method.In order to cancel the large deviation in the Robinson analytical method when dealing with electrical small aperture or larger aperture,the factor introduced to account for the coupling between aperture and cavity in the original Robinson method has been corrected based on the transmission line matrix(TLM)method,and a modified Robinson method with high precision for electrical small aperture or larger aperture has been proposed.Furthermore,since the Robinson method can only deal with the cases of centered aperture,centerline observation point,and TE10 mode propagation,an extended Robinson method has been proposed based on the guided wave theory,by introducing the components of voltage response and the coefficient of the aperture position containing higher modes influence.This extended Robinson method can be used to accurately predict the higher resonant modes and the SE at any point in the cavity with arbitrarily positioned aperture.Ultimately,the validation of the two proposed analytical methods are examined by experimental data and TLM method.(2)Study on the analytical method for the SE of apertured shielding cavity with discontinuous structures.In order to analyze the SE of apertured shielding cavity with discontinuous structures,such as ladder,internal metal baffle,equipment with metal housing,etc.,three steps have been carried out.Firstly,the equivalent circuit of the shielding cavity has been established by using the waveguide theory and equivalent circuit principle,and then the scattering matrices corresponding to equivalent circuits of the aperture and internal discontinuous structures have been deduced by employing the microwave circuit theory.Finally,based on the transmission line network BLT(Baum-Liu-Tesche)matrix equation,a fast analytical method for predicting the SE of apertured shielding cavity with discontinuous structures has been derived.The proposed method and TLM method have been used to analyze the influence of discontinuous structures on SE and electromagnetic resonances of the cavity.(3)Study on the analytical method for the EMI of the loads of the transmission line in shielding cavity with apertures.The problem of the external electromagnetic field coupling onto the internal wires through aperture is complicated,which is always difficult to analyze.To solve it,this problem has been decomposed into two independent sub-problems based on the electromagnetic topology(EMT)theory: the problem of the coupling between the aperture and cavity,and the problem of the coupling between the field and line.The coupled electric field in the first problem has been calculated by equivalent circuit method,and the EMI of the loads of the transmission line in the second problem has been solved by the obtained coupled electric field and the Agrawal's BLT equation.Consequently,an analytical method for rapid calculation of the EMI of the loads of the transmission line in apertured cavity can be deduced.(4)Study on the prediction method for the electromagnetic resonant modes of apertured shielding cavity.The electromagnetic resonance of the cavity with apertures is affected by the aperture characteristics and the coupling between the aperture and cavity.Firstly,the influence of the aperture size on the electromagnetic resonance of the cavity has been studied by using TLM method and equivalent circuit method.Secondly,the corresponding relationship between the position of the electrical small aperture and the excited resonant modes in the cavity has been deduced by employing the waveguide excitation principle,and has been further extended from single aperture to aperture array.Finally,a fast prediction method for resonant modes of shielding cavities according to the aperture position and distribution has been suggested based on previous study.Being validated by TLM simulation and experimental data,some conclusions have been drawn to guide the design of apertured shielding cavities.The innovations of this dissertation are mainly embodied in the following three aspects:(1)Proposed analytical methods for the SE of apertured shielding cavity.The modified Robinson method corrects the factor introduced to account for the coupling between the aperture and cavity in original Robinson method,and greatly improves the calculation accuracy of the original Robinson method for electrical small apertures and larger apertures.The extended Robinson method can accurately predict the higher order resonance including TMmn0 and the SE at any point in shielding cavity with arbitrarily positioned aperture,and thus greatly extends the application scope of the original Robinson method.The proposed analytical method for rapid prediction of the SE of apertured cavity with discontinuous structures,such as ladder,internal metal baffle,or equipment with metal housing,expands the application scope of analytical methods,and is more efficient for computer modeling and solving compared with traditional equivalent circuit models.(2)Proposed an analytical method for the EMI of the loads of the transmission line in shielding cavity with apertures.The proposed analytical method utilizes the equivalent circuit method to calculate the internal electric field distribution generated by external plane wave,which exhibits a good accuracy,and shows higher efficiency than other numerical methods and semi-analytical methods.Furthermore,the proposed analytical method can be used to calculate the SE at any point in the cavity with arbitrarily positioned aperture,irradiated by arbitrary incident and polarized plane wave.(3)Proposed a fast prediction method for the resonant modes of shielding cavities based on the positions and distributions of aperture.The prediction method gives the corresponding relationship between the position or distribution of the electrical small aperture and the excited resonant modes in the cavity.The excited resonant modes in the cavity can be obtained quickly according to the relationship.Therefore,by designing the position or distribution of the electrical small aperture,one can easily avoid some resonant modes and improve the SE,which has important application value for the design of shielding cavities with apertures.