A Waveguide-On-Access-Tube (WOAT) TDR Sensor for Deep Soil Water Content and Bulk EC.

A waveguide-on-access-tube (WOAT) TDR sensor was invented and the design optimized through a combination of electromagnetic modeling and several rounds of prototyping and testing in air, water, mixtures of water and ethylene glycol, and sand and silty clay loam soils over a range of water contents and bulk electrical conductivities. Limited field testing showed the feasibility of the sensor system using a conventional TDR instrument cabled through a multiplexer to each waveguide triode. WOAT sensor segments were designed as 20-cm long tubes with waveguide triodes on opposite sides of the tube, a proprietary TDR circuit embedded in the tube wall, and electrical bus connectors joining the bus between these WOAT segments. WOAT segments can be used individually or joined to make an access tube that can be inserted into the soil to indeterminant, user-chosen depth (=> 3 m). The electrical bus carries power and digital signals to the WOAT system head for storage and/or wireless transmission elsewhere. The presentation will discuss field tests from summer 2012 during which WOAT segments will be installed in an irrigated field for testing in a closed-loop feedback sensing system for irrigation managment.