MICROWAVE TECHNIQUES AND METHODS FOR THE CHARACTERIZATION OF DIELECTRIC MATERIALS

Abstract

The characterization of dielectric materials has been crucial for various implementations that vary from communication devices to military satellite services. In fact, the study of material properties at microwave frequencies has always been among the most active areas in multiple fields. In recent years, the increasing requirements for the development of dielectric sensors for a vast variety of applications, encouraged designers and researchers to seek for new measurement methods and techniques, to adapt to the diverse requirements of such applications. This dissertation work presents fully functional microwave methods and techniques for the measurement of the dielectric properties of low-loss materials. It focuses on the analysis and study of the different stages in the material characterization process. The first stage defines the measurement setup and topology used to transmit the electromagnetic waves through the material under test and measure its response. The second stage provides the means for the transition between the measured electromagnetic response, and the dielectric material properties. The dissertation continues, to propose, verify, and discuss novel conversion methods to be used in the second stage of the characterization process, as a bridge between the measured electromagnetic response and the required dielectric properties. Moreover, multiple measurement setups and topologies are proposed and implemented along with the proposed conversion methods. The performance of each setup/conversion combination is tested and analyzed with respect to selected metrics. The dissertation work starts by investigating the different measurement setups and conversion methods available in literature. This includes the study of the limitations and constraints of each of the available setups/methods. Such study helps identify the suitable setup/method combination for each application. It also distinguishes the common constraints between different methods, which provides the entry point for a possible contribution to the field of microwave characterization of dielectric materials. As a result of this investigation, it is found that the need for a properly calibrated setup, is a common constraint between different conversion methods. Such need limits the applicability of the overall characterization method in multiple applications. To overcome such a limitation, a novel conversion method is proposed. The proposed method, denoted by PM, aims to achieve accurate and stable transition between the non-calibrated signal properties and the dielectric constant value of low-loss Material Under Test (MUT). The proposed method relies solely on the frequency values at which the reflection coefficient (S11) resonates and does not require pre-calibration or shifting of the reference measurement plane from the setup’s input port to the material under test air interface. But as measurement setups get more and more complex, random, and systematic errors are more likely to occur. To account for such errors, this work ventures into providing more robust and more reliable solutions. The solution, denoted by PMv2, is proposed to adapt to more complex measurement setups, where errors such as shifts in the measured |S11| resonant frequencies may occur. The proposed method (PMv2) is tested on different waveguide measurement topologies. In the first topology, the material under test is tightly sandwiched between a matched load terminated waveguide section, and an open-ended waveguide sensor. The reflected signal at the waveguide’s input port (|S11|) is then measured and fed to PMv2 to estimate the performance of the proposed conversion method/measurement setup combination. To provide further hassle-free solutions, a reflectometer design is proposed and implemented. The proposed reflectometer aims to reduce the measurements’ cost, by measuring the minimal required data for PMv2 to perform properly. The proposed reflectometer is tested along with the proposed waveguide topology, to estimate the performance of the PMv2/waveguide setup/reflectometer combination. As for the second proposed waveguide measurement topology, the MUT is placed inside an open terminated waveguide section, and the waveguide sensor is placed in a tight configuration against one of the MUT’s faces. The measured reflection coefficient at the waveguide’s input port is then used by PMv2 to estimate the dielectric constant value of the material under test and study the performance of the proposed setup/conversion combination. Taking this analysis to the next level and proving that the proposed method can be combined with even more complex setups, a monostatic horn antenna sensor is proposed and tested for free space characterization of planar low-loss materials. The horn antenna is designed, fabricated, and tested for selected performance metrics. MUT constraints are determined and set based on the antenna’s performance along with the setup’s configuration. The setup is composed of the horn antenna sensor, along with a foam MUT holder for proper positioning. Reflection measurements are done at the input of the horn sensor. A modified version of the conversion method, denoted by PMv2-TEM is then proposed to adapt to TEM propagating mode environments. PMv2-TEM is then used to provide the transition between the measured reflected signals, and the MUT’s dielectric constant value. Finally, a study of the proposed setup’s accuracy in the estimation of the dielectric constant value of sample materials under non-calibrated setup conditions is conducted. As a conclusion, this dissertation proposes new fully functional microwave methods for the measurement of the dielectric properties of low-loss materials. This is done by proposing novel conversion methods, along with the study of the performance of such methods when combined with multiple measurement topologies. In addition, recommendations are presented for future extensions of this work into application specific designs.