Submitted to: IEEE Sensors Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/9/2011
Publication Date: 5/21/2012
Citation: Casanova, J.J., Evett, S.R., Schwartz, R.C. 2012. Design of access-tube TDR sensor for soil water content: Theory. IEEE Sensors Journal. 12(6):1979-1986.
Interpretive Summary: Measuring the amount of water in soil is important in managing crop irrigation. Current methods of soil water estimation are limited by accuracy, precision, ease of installation, and cost. This paper presents a new sensor design and investigates the performance of the sensor using physical theory. This theoretical study will reduce the time spent prototyping the actual device and allow design/cost tradeoffs to be properly evaluated so that a reasonably low cost yet accurate and easily installed sensor system can be delivered to users.
Technical Abstract: The design of a cylindrical access-tube mounted waveguide was developed for in-situ soil water content sensing using time-domain reflectometry (TDR). To optimize the design with respect to sampling volume and losses, we derived the electromagnetic fields produced by a TDR sensor with cylindrical geometry. Using this analytical derivation, the effects on sampling area, waveform shape, and losses were examined while varying the geometrical design and soil water content. When the soil and tube substrate have identical dielectrics, then sampling area has a local extremum. Tube radius has the largest impact of any geometrical parameter on sampling area with increases in radius causing increases in sampling area. Increasing electrode separation angle increases the sampling area slightly. The effects on TDR waveform are greatest for soil water content, tube dielectric, and tube radius; where increasing any of these increases delay and dispersion.