In standard measurement methods such as NSA 94-106, the low-frequency magnetic shielding effectiveness of a shielding enclosure is tested using the near field of loop antenna. Under this near-field configuration, there is no analytical or closed-form solution for volumetric shielding like box/cavity except for planar shielding like a sheet of infinite extension. Exploring the correlation between volumetric shielding and planar shielding can provide simple prediction methods for volumetric shielding based on planar shielding. As a taste to this end, this article explores the difference between the shielding effectiveness of a double-sheet cavity and a single sheet under the NSA 94-106 standard. We derived the exact solution in integral form for electromagnetic fields inside the cavity and calculated the curves of shielding effectiveness on the frequency with different sheet material, thickness, and sheet-to-sheet distance. The results show that when the distance from the receiving antenna to the back sheet is greater than the diameter of the loop antenna, the results of a double-sheet cavity tend to be consistent with a single-sheet configuration. When the distance is less than the diameter, the difference between the two depends on material type and sheet thickness.
"Low-Frequency Magnetic Shielding of a Cavity Formed by Two Imperfectly Conducting Sheets: Effect of Sheet-to-Sheet Distance and Comparison with the Single-Sheet Configuration," Progress In Electromagnetics Research M,
Vol. 121, 13-26, 2023. doi:10.2528/PIERM23082601
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