Characterization of a spiral-shaped time domain reflectometry probe

Abstract

Time domain reflectometry is one of the most common methods to determine water content in porous media. Transmission lines used to carry the electromagnetic pulse can take many different forms and shapes and can be tailored to specific needs. Here we describe the design and characterization of a miniaturized transmission line, specifically designed for applications in laboratory settings. The sensor consists of a spiral-shaped Cu waveguide, which is constructed by etching techniques used in printed circuit board manufacturing. The waveguides are 0.508 mm wide and have a separation of 0.508 mm. The overall diameter of the spiral is 42 mm, and the overall length of the waveguide is 38 cm. The probe was tested experimentally in media of different dielectric properties using a Tektronix cable tester. The electrostatic field distribution of the probe was simulated using a two-dimensional numerical solution of the Laplace equation. The numerical analysis was used to compute a weighting function of the measurement and to determine the equivalent dielectric permittivity. For the range of permittivities expected in environmental porous media, such as soils and sediments, the sampling area of the probe was not affected by the water content of the medium.