A Nondestructive Method for Measuring the Complex Permittivity of Thick Slab Specimen Using the Improved Open Waveguide Method Based on the NRW Method

This article proposes a nondestructive method for measuring the complex permittivity of thick slab specimens using the improved open waveguide (IOW) method based on the Nicolson–Ross–Weir (NRW) method. Conventional methods, generally effective only for thin materials, face significant challenges in accurately measuring the complex permittivity of thick slab specimens. This study addresses the limitations of prior methods by applying a disambiguation technique under nonmagnetic conditions. The accuracy of the proposed methods was validated through numerical analysis and experimental results. Specifically, measurements were conducted on paper specimens of varying thicknesses and a concrete block. For the paper specimens, the results closely matched reference values obtained using the split cylinder resonator (SCR) method, a widely used standard for thin specimens. For the concrete block, numerical analysis was used to validate the derived complex permittivity, confirming the reliability of the proposed methods. While the IOW method relies on numerical analysis, which increases processing time, it provides reliable and accurate measurements of complex permittivity, particularly for specimens where sample preparation is challenging.