Calibration methods of a 2GHz evanescent microwave magnetic probe for noncontact and nondestructive metal characterization for corrosion, defects, conductivity, and thickness nonuniformities

Abstract

A near-field magnetic-dipole probe suitable for noncontact and nondestructive imaging of metals is described and the effects of resonator coupling strength, operation frequency, and the probe wire tip geometry on the conductivity resolution of the probe are experimentally determined. Using a simplified circuit model of the resonator, we were able to interpret the system’s output and predict the magnitude of reflected wave and relate it to the properties of the samples under investigation. Thus, the probe was calibrated to perform quantitative conductivity measurements with the ability to detect metal nonuniformities with 1% accuracy and 5×10−3σ and 2×10−2σ conductivity resolutions at 2GHz operation frequency for both the critical and over-coupling probes, respectively. We also discussed the calibration results of probes with different coupling strengths over a 0.91Ω∕square resistive sample. The calibration results of a critical-coupled resonator probe were also compared with a microstrip transmission line probe. It was observed that the resonator probe has 100 times higher conductivity resolution than that of the transmission line probe. Furthermore, we characterized and compared the calibration results of probes with tip wires of different diameters. Images obtained by an evanescent microwave probe are presented.