Abstract :
Real-time and wireless monitoring of soil properties would give important information for the decision-making process. Autonomous underground Soil Scouts for monitoring soil properties like moisture, temperature, nutrient level and pH can be produced with present radio electronics. A model has been developed to estimate the signal strength in varying conditions. The model includes three different mechanisms of signal power loss: (1) attenuation; (2) reflection; and (3) angular defocusing. Complex dielectric permittivity is the main soil parameter concerning radio wave attenuation.
In field tests, a radio wave was generated at 869.5 MHz and conducted to an underground antenna. Signal strength was 101.4 mW (equals to 14 dB/mW). The signal was received at the soil surface with a dipole antenna and the amplitude was measured. Measurements were carried out in sandy and loamy soils.
The model succeeded in estimating the signal strength and both the moisture and angle dependencies of the attenuation and in estimating heavy moisture alterations.
Signal power requirements for transmitting from 25 cm depth were simulated. The results suggested that a 1 km working distance can be achieved with commercial single-chip radio components and thereby instrumentation issues are solvable.
