Evaluation of simple transmission line oscillators for soil moisture measurement

Time domain reflectometry (TDR) measures water content in soil by measuring the travel time of an electrical pulse in a transmission line which is buried in the soil. Travel time is determined. in part, by the dielectric constant of the soil. The large dielectric constant of water, relative to soil minerals and air, assures that TDR measurements can register small changes in water content (+/- 0.006 m(3) m(-3)). Conventional TDR methods, however, are expensive and the analysis to determine travel time is complex. We have developed simple oscillators which use travel time in a transmission line to set their frequency. Typical frequencies of oscillation in soil are 30-60 MHz. These oscillators are both inexpensive and easy to use. Oscillators with both transmission and reflection wave guides were designed, and reflection oscillators were designed for both open and shorted wave guides. Best results were obtained with open, reflection oscillators. The relationship between 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 than are conventional TDR calibrations. Oscillators operated satisfactorily in the field for several months. (C) 1998 Elsevier Science B.V. All rights reserved.