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A simple approach to estimate the surface impedance of a thin ink deposition on dielectric substrates is described. The measured sample is transversally accommodated in a waveguide, and the scattering parameters are measured. Then, the surface impedance value is recovered by recurring to an efficient transmission-line approach. Closed-form relations are proposed, which allow removing the dispersive effect of the substrate on the estimated surface impedance. The robustness of the retrieving procedure is analyzed, showing that the one based on the transmission coefficient provides more accurate results than the reflection-based one. The positioning of the sample is also investigated, highlighting that the placement inside the waveguide leads to incorrect estimations of the surface impedance with fictitious high imaginary parts because of the presence of air gaps between the sample and the waveguide walls, which transform the uniform layer to a capacitive resistive frequency selective surface. A modified setup with a uniform resistive ink deposition pressed between the waveguide flanges is therefore adopted. Additional experimental results on the effect of the dielectric substrate and on the homogeneity of the ink deposition are reported.