A Fully Passive UHF RFID Soil Moisture Time-Domain Transmissometry Based Sensor

In this work, it is proposed the integration of a time-domain transmissometry soil moisture sensor with a fully passive UHF RFID tag. A coplanar waveguide based sensor is modeled and simulated, whereas the propagation signal delay was analyzed when the sensor is surrounded by several permittivity scenarios. It is also proposed a low power interface circuit that enables a UHF RFID tag to obtain data from the developed sensor, which is accessible by any standard reader. The proof of concept prototype was developed and tested on soil with different moisture and salinities. From the experiments, it was found that the proposed sensor presents similar behavior for the tests performed with distilled and saltwater solutions with conductivities up to 1584 μS/cm.