2.4GHz On-Board Parallel Plate Soil Moisture Sensor System

Moisture content measurements are very important to the evaluation and maintenance of highway pavements. Such applications require sensors can provide relative long time, continuous, and accurate measurements in hostile pavement environments. And sensors should be in contact with soils, concrete, asphalt or other pavement material In this paper, a 2.4GHz integrated parallel-plate soil moisture sensor system is presented. The moisture sensor is based on the principle that the phase shift in the transmission line is the function of medium dielectric constant, which is related to the moisture content. Compared with other methods like TDR and microstrip resonator, the phase shift detection method has better performance for these applications. Detailed sensor head design is based on the consideration of sensitivity, linearity, impedance match, dynamic range and power plane requirement. The solution is fully digital with integrated RF synthesizer, micro-controller, phase detection circuitry, baseband signal processing circuits, and digital communication protocol. The system calibration is built in the software program. The power management system reduces power consumption. The sensor network is able to handle as much as 256 sensors with single pair RS485 interface