## 3 Results
### Multi-Coil Arrangement  
The issue of variable lift-off could be eliminated from the object's composition identification challenge by the implementation of a multi-primary rf field coil arrangement. Previously, we have shown that primary rf field coils located on opposite sides of the object with either the same or opposite phases produce an inductive image that could also be used for material identification [23]. In particular, we demonstrated that the inductive image of the tilted object located in the centre between two coils has a homogeneous character in contrast to the image recorded with a single coil. Departure from this central location will introduce inhomogeneity in the inductive image and enables estimation of the object lift-off. Here, the focus is on local measurements and lift-off estimation would be based on a comparison of the outputs of local measurements performed with excitation by a single coil on either side of the object and with the same two coils in phase acting simultaneously with each other. Figure 8: Gradient of the angular signal dependence as a function of the operating frequency recorded for stainless steel (dark blue triangles), brass (green dots), aluminium (blue squares), and copper (red diamonds) plates. The dotted line represents a linear fit to the dependence and indicates the frequency regime where the signal growth is dominated by the induction frequency dependence. Figure 9: (**a**) Gradient and (**b**) offset of the frequency dependence of the angular gradient at high frequencies, Figure 8, as a function of lift-off for a copper (red diamonds), brass (green dots), and aluminium (blue squares) plate. The red dotted curve is an exponential fit to the copper data. Figure 10a shows the amplitude of the rf signal as a function of the angle between the normal to the object surface and the sensor axis recorded for copper plates with excitation produced by a single coil on the same (navy diamonds)/opposite side (red triangles) of the plate with respect to the sensor and the set of two coils ((green points). The distance between the primary rf field coils and the object is different by 5 mm, which is reflected in difference in the rf signal amplitudes. We have recorded angular dependencies, as shown in Figure 10a, for different lift-offs values. For all measurements, the distance between the two coils is the same. This could mimic a scenario, where the coils are located on both sides of a conveyor belt (fixed distance), while the individual lift-offs could vary depending on the position of the object on the conveyor belt. As in the previous subsection, we characterise data sets by the gradient of the angular dependence of the rf signal, as an orientation independent measure of the rf signal. Figure 10b shows the gradients of the angular dependence recorded with the single primary rf field coil as a function of the difference in lift-offs measured from the point where the lift-off between each coil is equal. The values of gradients have been normalised to the value recorded in the two coils arrangement. In the particular case of equal lift-offs gradients are equal to 0.5, i.e., signal recorded with a single coil is equal to half of the signal observed with two coils. The dependence of this normalised gradient value recorded with a single coil could be used to evaluate the lift-off, i.e., position of the object with respect to the coils. As mentioned before the symmetry (or lack of symmetry) in the amplitude might also provide information about the relative lift-off difference. Although we are focusing on local measurement, analysis of the cross-section through the centre of the amplitude image provides insight into the limitation of the method discussed above. Figure 11a shows the amplitude of the rf signal recorded over a 0.5 mm thick square copper plate with an area of \(40\times 40\) mm\({}^{2}\) for the two coils configuration (navy triangles) and single coil arrangements (green diamonds and red points). The data were recorded with plates angled at 30\({}^{\circ}\).