Effects of Soil Characteristics on Passive Wireless Sensor Interrogation

Applications, such as the long term in-situ monitoring of geotechnical structures and of agricultural soils, motivate the use of wireless sensors that are of a passive design. However, the effectiveness of such passive devices will be dependent on the power received via a forward interrogation link and the power returned to a data collector via a reverse link. For these applications, link losses will be determined by the transmission medium, i.e., soil. This paper presents a pathloss study of microwave attenuation versus soil type and moisture. Then, we present results for a passive frequency doubling reflectenna (FDR), operating at 1.3 GHz for the forward link and 2.6 GHz for the reverse, buried at various depths in agricultural soil of different moistures. Results indicate that a FDR-based wireless sensors operating in this frequency range could be effectively embedded to a depth of 0.5 m or more, significantly deeper than reported for other passive technologies.