EVALUATION OF SIMPLE TRANSMISSION-LINE OSCILLATORS FOR SOIL-MOISTURE MEASUREMENT

Time domain reflectometry (TDR) measures water content in soil by meas uring the travel time of an electrical pulse in a transmission line wh ich is buried in the soil. Travel time is determined. in part, by the dielectric constant of the soil. The large dielectric constant of wate r, relative to soil minerals and air, assures that TDR measurements ca n register small changes in water content (+/- 0.006 m(3) m(-3)). Conv entional TDR methods, however, are expensive and the analysis to deter mine travel time is complex. We have developed simple oscillators whic h use travel time in a transmission line to set their frequency. Typic al frequencies of oscillation in soil are 30-60 MHz. These oscillators are both inexpensive and easy to use. Oscillators with both transmiss ion and reflection wave guides were designed, and reflection oscillato rs were designed for both open and shorted wave guides. Best results w ere obtained with open, reflection oscillators. The relationship betwe en travel time in the wave guide and water content for the oscillators is similar to that for conventional TDR, but oscillator calibrations are more sensitive to bulk electrical conductivity and temperature tha n are conventional TDR calibrations. Oscillators operated satisfactori ly in the field for several months. (C) 1998 Elsevier Science B.V. All rights reserved.