UWB RF sensing with a 3D-printed lens for improved accuracy in refractive index determination for non-destructive testing

This study presents the development of a robust RF sensing system to address design as well as measurement challenges such as operating bandwidth, gain, near field coupling, and external factors during measurement in industrial ultra-wideband (UWB) applications. The proposed system integrates UWB radio sensors, specialized dielectric loads, and a 3D-printed refractive index lens to enhance overall sensing accuracy and performance. The system was designed, manufactured, and prototyped specifically for non-destructive testing, with a focus on accurately determining the complex refractive index ( n +j k ) of various industrial materials. The sensors with dielectric loads were experimentally tested to examine the near field response. The lens increases the antenna gain from 3.1 to 4.8 dBi in the azimuth plane and from 2.5 to 4.3 dBi in the elevation plane, resulting in higher received signal strength and an improved signal-to-noise ratio. The system was experimentally validated using solid wood samples with moisture contents ranging from 1% to 11% over the 1.5–2.5 GHz frequency band. The enhanced antenna gain enabled more reliable complex refractive index estimation despite the increased dielectric losses associated with higher moisture content. The lens suppresses near field and coupling effects, such that unbiased values for n and k are obtained. Using the lens gives a moderate decrease in relative error, Δ n / n from 0.09 to 0.08, and a significant decrease in Δ k / k from 0. 18 to 0.13. Polarimetric measurements were also performed to account for the anisotropic properties of wood. The results demonstrate that the proposed sensing platform provides accurate and repeatable refractive index characterization. For parallel polarization, n ranges from 1.45 to 1.82, while k varies from 0.23 to 0.46. For perpendicular polarization, n ranges from 1.28 to 1.55, and k from 0.10 to 0.36, demonstrating the capability of the proposed system to characterize the anisotropic dielectric properties of wood. The results were further compared with previously reported far-field measurements on solid wood (same samples), where the incident EM waves are approximately planar, demonstrating good agreement and validating the proposed near-field UWB sensing approach.