Evaluation of Near-Field Electromagnetic Shielding Effectiveness at Low Frequencies

Magnetic induction tomography (MIT) is a novel technology for flow measurement offering significant promise in the measurement of multiphase flows containing low-conductivity fluids such as saline water. Such measurements rely on optimal effective shielding to avoid external field interference and extraneous capacitive coupling that can lead to false readings and overestimations of the eddy current-induced fields. The performance of various attenuation materials in the low megahertz frequency spectrum is presented and compared with outcomes from a numerical computational method. The results demonstrate that the shielding mechanism that prevails at low frequencies is that of reflection. Consequently, hard shields such as metals show superior wave attenuation performance for MIT systems operating below 13 MHz. For higher frequencies, the absorption effect on the incident wave path within soft electromagnetic shields presents enhanced shielding properties. This paper also explores the limitations of traditional testing geometry for shielding effectiveness and proposes an alternative approach to near-field, free-space measurement for MIT sensors. The proposed semi-enclosed approach shows enhanced shielding effectiveness measurements compared with the traditional transversal barrier method. The proposed method was used to assess the electromagnetic shielding effectiveness of ferromagnetic and various metallic materials.