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Magnetic Field Penetration through Protective Metal Screens Using an Equivalent Magnetic Dipole Representation of an Array of Elliptical Apertures

Introduction This paper presents the development of a model to predict the coupling of external magnetic fields into a cylindrical object through an aperture shielded with a metal screen. Also presented, are the results from an experiment designed to test the model. The coupling of these exterior fields through a single elliptical aperture is approximated by the radiation from an equivalent dipole into the object. The magnetic moments for this dipole are determined by the physical size of the aperture. The screen is then modeled as a linear superposition of many individual aper­ tures. The total field at an interior point is calculated by summing the components of the fields from each of these apertures. In order to test this model, an aluminum cylinder was constructed to be used as a test object. This cylinder was the meters long with a diameter of one meter. The ends of the cylinder were tapered to provide a constant impedance at the terminations. The entire object was mounted in a multiple wire coaxial return path to provide a nominal 50 Ohm imped­ ance. The center section of the cylinder cylinder contained an aperture that was 1.5 m long and that extended along a circumferential arc of 120 degrees. Covers for this aperture were constructed using different sizes of expanded metal mesh. An impulsive current generator was used to inject currents axially onto the cylinder. The peak currents levels were up to 30 kiloamps (kA). The resulting magnetic fields across the screened aperture were measured and compared with the simul­ taneous fields at interior points.