A Frequency Shift Dielectric Soil Moisture Sensor

Abstract
Field implantable and laboratory sensors for the measurement of moisture in soil based on the increase of soil dielectric permittivity with volume fraction of water were developed. A typical sensor consists of a case containing a high-frequency (31-MHz) oscillator whose frequency determining resonance LC network is coupled to the built-in electrode via a capacitor T network. Increases in moisture cause a shift in the resonance frequency, which is detected by an external differential frequency counter or readout. The T network and electrode electronic switches reduce the undesirable influence of soil conductivity on the frequency shift. Although the sensor contains active elements, it acts as a passive transducer since power is supplied to the sensor from the readout unit through the two-conductor cable which transmits the RF.