Coupling Characteristics Of Low Frequency Magnetic Fields Into An Apertured Shielding Box

With the increasing complexity of the electromagnetic environment,low-frequency magnetic shielding has application requirements in many fields.For ventilation and heat dissipation and equipment wiring,it is usually necessary to open an aperture in the shielding box,which will reduce the shielding effectiveness(SE)of the shielding box,and at the same time,it will also make analytical research difficult.At present,there have been many results for the research on the shielding box with apertures,which are mainly based on numerical calculation methods.There is not much explanation of the mechanism.Starting from the diffusion effect,aperture effect and edge effect,this article further explains the mechanism,deepens the theoretical study of the coupling characteristics of the low-frequency magnetic fields into an apertured shielding box,and develops analytical or approximate analytical methods.First,the metal shield box with apertures is decomposed into two sub-models:the finite conductivity shield box without the aperture and the perfect conductor shield with the apertures,which correspond to the diffusion effect and aperture effect of magnetic field coupling respectively.The total magnetic field can be obtained by the superposition of the magnetic fields of the two sub-models.In this paper,the analytical formulas for the SE of spherical shields with an aperture,rectangular shield with an aperture and periodic apertures are deduced in detail,and the validity of the formulas is verified by comparison with finite element simulations.The calculation formula of the induced voltage on a coil inside the shielding box is further deduced,the calculation results show that when the external magnetic field is parallel to the aperture,the voltage increases with the increase of the coil area;when it is vertical,the voltage increases with the increase of the coil area at low frequencies,and when the frequency is high,the voltage first increases and then decreases as the radius of the coil increases.Finally,an approximate analytical method is proposed to calculate the edge effect field of a finite disk against the magnetic field generated by toroidal coil,and it is verified by simulation and experiment,it also reveals the influence of the size and position of the disk on the edge effect.