Ring‐capacitor sensors are used widely for real‐time estimation of volumetric soil water content θ from measured resonant frequency fr, which is directly affected by the bulk soil permittivity ε. However, the relationship fr(ε) requires improved quantification. We conducted laboratory experiments to characterize the response of the Sentek EnviroSMART sensor system for a full range of ε values from air to water and a range of temperatures. Water–dioxane mixtures were placed into a solvent‐resistant container equipped with custom tools for heating and mixing the fluid, removing air bubbles from sensitive surfaces, measuring permittivity in situ, and creating an axisymmetric metal disturbance to the electric field. Total capacitance C was measured using a vector network analyzer (VNA) connected to one sensor, while four other sensors provided replicated fr readings. The measured temperature response of free water permittivity was linear with a negative slope, which is qualitatively consistent with theory. A precise nonlinear relationship between ε and normalized fr was derived. The instrumental error in ε was RMSEε = 0.226 (for 3 < ε < 43), which corresponds to a measurement precision in θ(ε) derived from Topp's equation of RMSEθ= 0.0034 m3m−3 Axisymmetric numerical simulations of the electric field supplemented the experimental results. The characteristic length scale for the distance measured radially from the access tube is 12.5 mm, meaning that 80 and 95% of the signal are sensed within approximately 20 and 37 mm of the access tube, respectively. The results are crucial for scientific applications of the investigated sensor type to environmental media.

[1] We present a method for extracting spatially resolved water content profiles q(x) from a two-wire time domain reflectometry (TDR) probe. The profile q(x) is represented in terms of the dielectric e r (x) and ohmic s(x) properties in the longitudinal direction of the TDR probe. We solve the inverse problem iteratively by combining a one-dimensional time domain solution of the transmission line equations and a genetic optimization method. The method is capable of finding the global optimum in a complicated error landscape without initial assumptions, except physically reasonable limits. The method utilizes both the position and the magnitude of the TDR signal. We analyze water content profiles from laboratory measurements and demonstrate that the achievable spatial resolution can be made as low as 2 cm and even smaller. The present implementation of the numerical code demonstrates the practical feasibility of spatially resolved water content profiles.

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A scanning probe microscope has been integrated into a microwave resonator tunable from 2.2 to 3.4 GHz with a quality factor Q larger than 1000. Nonlinear phenomena caused generation of higher harmonics when rf fields in the range of 109 V/m were applied between tip and sample. Higher harmonic signals were detected at a bandwidth of 10 kHz on conductor surfaces as well as on thin insulating films and were used as feedback to the control loop for imaging graphite surface features and oxidized silicon surfaces with subnanometer resolution.