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Soil water measurement is important in water management for irrigation and hydrologic sciences. The purpose of this paper is to describe the design of a cylindrical access-tube mounted waveguide 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 fields produced by a TDR sensor with this geometry. Using this analytical derivation, the effects on sampling area, waveform shape, and losses while varying design and soil water content were examined. It was found that 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 increase delay and dispersion.