A passive capacitive soil moisture and environment temperature UHF RFID based sensor for low cost agricultural applications

In this paper, we present the design, fabrication and calibration of a passive UHF RFID tag based soil moisture and environment temperature sensor. The inter-digital double-sided capacitive sensor developed modifies its capacitance as the soil moisture changes, due to modification of the soil permittivity. The implemented smart tag includes an external sensor front-end that is capable to process analog sensors and send digitized measurements to the reader. In order to obtain reliable readings, a process of calibration was performed and described. The used RFID chip also includes a built-in temperature sensor. The developed sensor tag presented a 6.95 ADC LSB variation per soil water content percentage and the capability to measure temperatures in a range of 0.3-59.6 °C with a 0.058°C resolution.