This paper presents a measurement setup, an extraction algorithm, and the results from material characterization measurements in the millimeter-wave (mm-wave) regime using ultrawideband frequency-modulated continuouswave (FMCW) radar transceivers. The complex permittivity of dielectric and nonmagnetic materials is derived from radar echoes using a high-gain dielectric lens antenna setup and a measurement setup comprising elliptic mirrors. The radar transceivers perform fast and accurate measurements from 200 to 250 GHz within milliseconds. The FMCW transceivers are calibrated using a frequency-domain model that describes the systematic errors in the measurement setup. The characterization is done by a holistic model-based approach. Several well-known dielectric materials, such as polytetrafluorethylene, polyvinylchloride (PVC), or acrylic glass, are characterized among others to validate the setup's accuracy. The characterization is also done for different samples of polylactide, which is commonly used in additive manufacturing processes and 3-D printing, making it of high interest for the construction of mm-wave components.
A method for the calibration of an ultra-wideband microwave tomography system is presented. The objective for this measurement system is to calculate the spatial distribution of the dielectric parameters and thereby different materials. Scattering parameters are used to describe the setup and are given to the reconstruction algorithm for evaluation. The measured wave parameters of this system are influenced by the measurement instrument as well as by the setup itself. In addition, it is also necessary for the reconstruction to obtain the scattering parameters at a well defined reference plane. Both requirements can be achieved by the help of a multiport error correction algorithm used for the calibration of a Vector Network Analyzer (VNA). In a first step, an error model for the tomographic system is presented. Secondly, this paper provides a concept for calibration standards which can be used for this application. Since a tomographic system do not offer the possibility for the use of classic reflection standards or a change in the mechanical length of the transmission other calibration standards have to be found. The idea of a homogeneous filling of the tomograph is presented. With the help of a simulation model the requirements for the system as well as the accuracy of the results are analyzed. In addition, measurement results are shown to validate the proposed approach.
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