Fig. 2. (a) Photo of a concentric rings microstrip resonator, in which the dimensions of the substrate are 49 × 67 × 3.2 mm3 ; (b) Reflection coefficient versus frequency for different relative humidity concentrations (1601 points; IF band = 100 Hz); and (c) Analysis of the resonant frequencies versus relative humidity concentrations, from [10], used with permission. Fig. 3. (a) Photo of an IDC covered by a thick layer of TiO2 20 × 30 × 1.6 mm3 dependent behavior of the magnitude of S11 , (substrate dimensions: ); (b) Impact of the oxygen concentration on the frequency;(c) Dip magnitude variations at different oxygen concentrations (signal filtered using a low pass filter); (d) Resonant frequency change at three oxygen concentrations. (From [18], used with permission.) 46 IEEE Instrumentation & Measurement Magazine May 2022