The penetration of electromagnetic fields through open and loaded apertures

To ensure that an electronic system will work properly under all conditions, its designer must control the flow of electromagnetic (EM) fields to and from the system. The principal method of control is the use of a metallic enclosure or shield. In many cases, the effectiveness of the shield is determined, not by the bulk material properties, but by apertures that exist in the shield. These apertures may be introduced intentionally, as in the case of windows in the skin of an aircraft, or unintentionally, as in the case of an improperly welded seam of poorly gasketed door. To minimize the penetration of EM fields through a large aperture, the aperture is sometimes loaded with a conductive material. Previous workers have shown that the penetration of EM fields through open or loaded apertures can be calculated by determining the equivalent magnetic surface currents, $\vec M\sb{s}$, that exist over the surface of the aperture. For small apertures, the equivalent magnetic and electric dipole moments (and polarizabilities) are related to the irrotational and solenoidal components of $\vec M\sb{\rm s}$ respectively. In this thesis, the relevant integro-differential equations are solved using the Method of Moments to determine $\vec M\sb{s}$ and to calculate the equivalent polarizabilities and shielding effectiveness of a small, square aperture loaded with an impedance sheet. Results of measurements of the penetration of EM fields through an aperture loaded with conductive material made using a dual transverse electromagnetic (TEM) cell over a frequency range of 2 to 200 MHz are presented and compared with the numerical results. The conductive materials examined include carbon, hybrid and non-carbon composite materials, and metallic grids and meshes. The composite samples were fabricated to study the effects of fibre orientation, conductivity, sample lay-up, and degree of hybridization on the electromagnetic shielding properties. Wire grids and meshes were fabricated to study the effect of wire spacing on shielding properties.