Using Short Soil Moisture Probes with High‐Bandwidth Time Domain Reflectometry Instruments

Time domain reflectometry (TDR) is used to measure moisture content and salinity of soils. Most TDR systems have a bandwidth of 2.5 GHz or less, limiting the precision of measurements using short probes. The primary objective of this research was to develop and test short probes for use with a high‐bandwidth (20‐GHz) TDR instrument. The secondary objective was to determine moisture content in highly conductive media by insulating the probes with Teflon heat‐shrinkable tubing. Laboratory‐packed test cells with probe lengths of 0.025, 0.05, and 0.075 m were used for the primary research objective, and 0.075‐m probes for the secondary objective. Linear relationships between the apparent refractive index and volumetric moisture content were developed for each probe length and type. The accuracy of the probes was ±0.0125, ±0.05, ±0.025, and ±0.035 m 3 /m 3 for the probes of lengths 0.075, 0.075 (insulated), 0.05, and 0.025 m, respectively. Moisture measurements in highly conductive media were possible using insulated probes.