Passive radio requestable SAW water content sensor

Using a SAW reflective delay line, a new passive sensor for a remote measurement of water content in sandy soil was built. The system operates in the European 434 MHz ISM-band and consists of a request unit and a passive sensor connected by a radio link to the request unit. The SAW device is fabricated on LiNbO₃, YZ-cut, and mounted and sealed in a standard DIL 16 package. The device includes four electroacoustic transducers (IDTs), three of them being used as reflectors. One transducer is connected to an external antenna to pick up the RF request signal of the request unit and to radiate back the RF response. Two reflectors are used for reference purposes. The bus bars of the median reflector are connected to an electrical transmission line with two rods added to their end. The rods are put into sandy soil. Together, the transmission line and the two rods with the sand-water-mixture between them have a characteristic impedance Z_load, which terminates the second transducer and affects the amplitude and phase of the acoustic reflection. A change of the water content of the soil changes Z_load due to the high permittivity of free water. The amplitude and phase of the acoustic reflection at the second transducer changes due to a variation of the terminating Z_load, which appears in a different attenuation and phase of the corresponding peak in the time domain. Thus the RF response of the sensor carries the information of the water content between the rods, which therefore can be detected and evaluated in the request unit.