## 4 Results and Discussion  
The EM sensor used in eNDE must accomplish two roles:  
Induce eddy current into the conductive material to be examined, and  
Emphasize their flow modifications due to material degradation. The simplest method to create time-variable magnetic fluxes that can induce eddy current into the material to be examined is represented by the coils circulated by alternative currents, by current impulses, or more alternative currents with different frequencies. The emission part of the EM sensor has this role. To emphasize the induced eddy current and effect of material degradation over their propagation, sensors sensitive to variation of magnetic field can be used, in our case a sensor with MM lens. The theoretical and experimental study of eigenmodes [71] that appear in the studied MSGs open new domains of applications in the eNDE of stratified structures. Figure 4: Experimental setup. The method and the developed sensor can serve as the eNDE of conductive strips, in order to eventually detect interruptions as well as non-alignments of silver strips or short-circuits between the traces. According to [59], in the case of MSG with silver strips and sub-wavelength features, excited with TM\({}_{\text{z}}\) polarized incident EM field, causing a single evanescent mode to appear in the space between strips but disappearing when water is inserted (\(\varepsilon_{\text{water}}=81\)). This mode could be detected and visualized using the sensor described above. For MSG with features compatible with the \(\lambda\) value of incident EM field, TM\({}_{\text{z}}\) polarized; the structure presents known selective properties of transmission and reflection [85]. When an EM TM\({}_{\text{z}}\) polarized waves acts, at normal incidence, for MSG, according to Ref. [27], the reflection coefficient for a strip grating with \(x_{0}<<\lambda_{0}\) is practically 1. When MSG represents a layered structure having the features \(x_{c}=0.6\) mm, \(x_{d}=0.4\) mm, and thickness of 10 \(\upmu\)m deposited on polyimide having a relative permittivity of 4.8, the study focuses on the appearance of abnormal and/or evanescent modes for the cases where dielectric fluids fill the gaps between the strip. Using the sensors with MM lenses, it is experimentally confirmed that, in the space between the strips, evanescent and abnormal modes appear in the case of modes excited with a TM\({}_{\text{z}}\) polarized EM wave with \(\lambda=0.6\) m. The images present an increasing of the amplitude of the signal induced in the reception coil of the evanescent and abnormal modes created in slits at the scanning of a \(1\times 1\) mm\({}^{2}\) region with a 10 \(\upmu\)m step in both directions. When the slits are filled with air, only the evanescent mode will be generated; the amplitude of the signal induced in the reception coil has the shape presented in Figure 5a. In the central zone of the slits the amplitude is maximum, followed by an accentuated decreasing towards the flanks of the metallic strips. The existence of a single evanescent mode, foreseen theoretically [7], is experimentally confirmed by the existence of a local maximum in the middle zone of the slits, with maximum amplitude on the middle of the slits, followed by an accentuated decreasing, symmetrically on the flanks of the strip. The two secondary maxima with smaller amplitude are at \(\pm 42\)  
Figure 5: Image of evanescent and abnormal modes generated in slits for TM\({}_{\text{z}}\) polarized excitation at frequency of 476 MHz: **(a)** slits filled with air; **(b)** slits filled with isopropyl alcohol. In slits abnormal modes are generated, in the case of excitation with a TM\({}_{x}\) polarized wave, for large values of the liquid dielectric constants larger than 10, when \(\varepsilon_{d}=17.9\), (isopropyl alcohol) (Figure 5b).