###### Abstract
Further in the work, they analyzed the inadequacy of the algorithm in the field of frequency dependence of material permeability. In 2017, the author [PERSON] published an article Ref. [5] on the use of the eddy current method by frequency sweep and impedance normalization to measure the thickness of the metal plate. In his work, he focused on measuring the thickness of metal plates without knowledge of their conductivity and permeability. Analytical and experimental studies were carried out on non-magnetic and ferromagnetic metallic materials. The results showed that the thickness of non-magnetic metals could be determined even without knowing the conductivity of the material. For ferromagnetic metals, he established a conductivity-insensitive function using an analytical study, with the help of which it is possible to determine the thickness of a ferromagnetic metal. Furthermore, he found experimentally that low frequencies are more stable and more suitable for metal investigations in the field. In 2019, [PERSON] followed up the study with a publication Ref. [6] in which he measured the thickness of a non-magnetic metal with unknown conductivity and the thickness of a non-conductive coating on the metal. In it, he proved analytically and experimentally that the maximum phase of the normalized impedance is related to the thickness of the material and the lift of the probe, therefore it is possible to use the frequency sweep method to measure the thickness of non-magnetic metal and the non-conductive coating on it. He also proved that the conductivity of a non-magnetic material could be determined by using a high-frequency SFECT signal. In Ref. [7], the authors [PERSON] and [PERSON] presented a theoretical study of the measurement of multilayer structures using the SFECT method. In the study, they analyzed the solution based on the transmission and reflection of waves between the layers. Through simulations, they proved that the contributions of the layers depend on the reflection coefficient. In a subsequent study Ref. [8], they confirmed that the thickness could then be characterized by spectral analysis of the reflection coefficients, which represent the individual contributions of the layers. Furthermore, in the study, they found that the individual responses of individual layers reach the maximum value of the normalized reactance difference at different frequencies. They confirmed this assumption in the publication Ref. [9]. They were based on the spectral analysis of the impedance of the two-layer structure. They verified their claims experimentally. [PERSON] et al. also worked on the use of the SFECT method for measuring the thickness of aluminum alloy plating, which they described in the paper Ref. [10]. In the study, after a theoretical analysis, they focus on the design of a detection probe using simulations. Simulations have verified the theoretical basis, which was subsequently confirmed experimentally. The results of the study confirmed that the SFECT method is suitable for measuring the thickness of aluminum alloy coating. In 2020, [PERSON] et al. published an article Ref. [11] on measuring the properties of an ultrathin metal coating using the SFECT method. In their work, they focused on a new technique based on SFECT, with the help of which they simultaneously measured the thickness of the coating with dimensions of several \(\upmu\)m and the conductivity of the metal coating. In the work, they also describe a method of eliminating the lift-off effect (the dependence of the coil lift). The result of the work was a new iterative model from which the thickness and conductivity of the coating can be determined using a suitable numerical method. In 2020, the same author team continued the research in Ref. [12]. In the study, they found that we can consider an electromagnetic wave as a plane wave if the diameter of the coil is smaller than the penetration depth of the wave and the impedance is invariant to a small change in the lift of the coil.