Characterisation of cylindrical eddy-current probes in terms of their spatial frequency spectra

The change in impedance of a cylindrical eddy-current coil, when placed on a layered conducting structure, is determined by the material properties and geometry of the structure and the spatial frequency spectrum of the coil. It is shown how the spatial frequency spectrum can be derived from the impedance spectrum of a coil for a characterised specimen. Once this is known, the performance of the coil can be predicted for any specimen. This process is illustrated using impedance measurements of both a large air-cored coil and a small ferrite-cored coil