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We developed the design of a waveguide on the exterior of an access tube for use in time-domain reflectometry (TDR) for in-situ soil water content sensing. In order to optimize the design with respect to sampling volume and losses, we derived the electromagnetic (EM) fields produced by a TDR sensor with this geometry. Using this analytical derivation, the effects on sampling area and waveform shape of varying geometry and soil water content were examined. The theoretical results were compared to laboratory measurements of different design variations in air, triethylene glycol, deionized water, sand, and clay in order to evaluate sensor performance and model accuracy. Both theoretical results and lab measurements indicated a positive, though not strong, relationship between electrode separation distance and (EM) field penetration into the soil or other medium with which sensor prototypes were surrounded. Results indicated good correspondence between the hybrid mode EM model predictions and measurements, indicating the value of the hybrid mode analysis. The relationship between measured pulse travel time and soil volumetric water content was quadratic rather than linear as in conventional TDR. Different quadratic calibration equations were obtained for sand and clay soils, indicating that soil-specific calibrations will be required for this design.