Material characterization using a two-wire transmission line

Abstract

Summary form only given. Numerous methods exist for determining the permittivity and permeability of a sample at RF and microwave frequencies, including free-space, coaxial probe, and waveguide measurements. This paper investigates the use of a two-wire transmission line for the characterization of homogeneous, isotropic materials. A two-wire transmission line is advantageous because it is a TEM transmission structure. The TEM nature and simple geometry allow for analytical expressions for the S-parameters of the line under various loading conditions. Traditional transmission line theory may be used in the analysis allowing for simple and rapid calculations. Another advantage of a two-wire transmission line is that the fields diminish rapidly with distance from the wires. Tightly bound fields prevent significant interaction with the outside environment and allow for smaller sample sizes. Possible applications for the two-wire transmission line technique include the in-situ characterization of liquids, gases, and solids during manufacturing, transportation, or storage. An example of such an application is the characterization of a homogenously mixed gas or liquid while in a holding vat during the manufacturing process. Various materials may be used to construct the two-wire transmission line allowing for the incorporation of such a system into various equipment and infrastructure. Existing material characterization techniques typically require a sample to be taken from a system and measured in a laboratory setup. An open-ended two-wire transmission line may be used as a wand-like probe for determining the parameters of samples such as soil or water. This paper examines the theory and possible uses of a two-wire transmission line for material characterization. A method for calibration using three shorts is presented. Basic transmission line theory is used to determine the S-parameters for a transmission line in free space with one material sample positioned along the line. Material parameters are extracted from these parameters using standard search algorithms. Computational simulations are carried out and compared to analytical solutions. A similar analysis is presented using a layered medium where the unknown sample is positioned between layers of materials with known permittivity and permeability. Additionally samples of the same material with varying thicknesses are studied so that results using a two-wire transmission line may be compared to existing two thickness free-space measurement techniques.