Impact of the environment on a hygrometry sensor response

The measure of hygrometry is a current approach for soil monitoring in geophysical applications. In that case, in-situ subsurface earth moisture control is tackled with the design of wireless and free from battery sensors interrogated from the surface. Since the most significant impact of humidity on ground physical properties is on the relative permittivity ε_r, the measurement is based on the capacitive pulling of an acoustic resonator. The connection of a variable capacitance in parallel with an antenna feeding the resonator with incoming power induces losses and hence, range losses through capacitive short circuit under strong moisture contents. Consequently, an alternative solution is considered. It consists in the design of a coupled resonator in which antenna and sensor (capacitive load) are electrically separated and linked by the acoustic field confined in the cavity between the electrodes forming the two ports. When probed by a Ground Penetrating RADAR, the echo delay, or in the case of a resonator, the frequency difference between the two modes is expected to depend on the humidity level. Thus, this study is dedicated to the understanding of the impact of humidity on the different elements of sensor. This work also designs a surface acoustic wave resonator to convey the signal. In such a way, this work exhibits that under ideal load the set-up above mentioned indeed allows the measure of ε_r due to capacitive pulling, leading to a wired hygrometer. Then, the possibility to carry out wireless sensing using an antenna both as an antenna and as a sensitive area is explained.