High dielectric constant microwave probes for sensing soil moisture

Implantable soil moisture sensors suitable for long-term monitoring of moisture in highway subgrades and for similar applications are needed. Two candidate designs of microwave sensors (operating range 4 to 6 GHz) have been investigated for such applications. One design uses the fringing field of a low-loss dielectric slab waveguide (relative dielectric constant of 25) to obtain good resolution for finely divided soil such as bentonite clay with moisture ranging from 10 to 50 percent by dry weight for effective sample volumes of 20 to 40 cm². The response of the dielectric waveguide sensor has been calculated in terms of the effective dielectric constant of the soil-water mixture. A model based on index of refraction yields an effective dielectric constant in reasonable agreement with experiment when effects of ionic conduction are accounted for. Another sensor design, better adapted for coarse materials, such as crushed limestone aggregate, uses waves launched from a tapered dielectric slab. By using either frequency or spatial averaging methods, the launched wave sensor accommodated aggregate particles passed by a 0.63-cm mesh sieve, and was found to have satisfactory resolution for the range of 0- to 10-percent moisture by dry weight.