Tutor: Major Research topic
:Material Characterization Techniques for Microwave Imaging Applications
Advisor: GENTILI GIAN GUIDOAbstract:
Design and Implementation of Microwave and Terahertz Material Characterization Methods
My PhD research work is focused on characterization of dielectric materials for microwave and terahertz imaging systems. Novel source antenna designs are proposed and a couple of time domain data inversion techniques are implemented in order to study dielectric material characterization of solid objects. Initially, stacked patch microstrip and custom shaped horn antennas are designed to improve the antenna bandwidth and spot focusing characteristics at microwave and terahertz frequencies. A waveguide-horn-waveguide structure scheme is used to model a horn antenna in order to avoid lens correction at horn apertures, which is usually needed to enhance the spot focusing. The presented horn antennas are designed and simulated with an accurate proprietary Body-of-Revolution Finite-Element code. Interaction of electromagnetic waves with materials is studied, where the variation in magnitude and phase of the transmitted and reflected wave has been observed with and without presence of dielectric material for calibration purpose. Different data inversion techniques are developed and tested e.g, Fourier inversion and Bayesian inversion. Time and computation efficiency of the post processing techniques has also been enhanced significantly, by using proprietary code in MATLAB. The analysis and estimation of dielectric properties is mostly based on measured data and simulated results obtained using commercial software (Ansys HFSS and CAD FEKO) for the purpose of comparison.
The dielectric measurement are carried out at PoliMI and UPC Barcelona to validate the results in multiple environments and test benches. Different Vector Network Analyzer (VNA) calibration techniques e.g, TRL, waveguide etc. are realized to minimise the measurement errors. Furthermore, the material thickness is also included in the inversion algorithm to reduce possible errors between ground (reference) and material’s scattering parameters. The materials tested have dielectric constant in the range of 2 to 12 thickness ranging from 0.5mm to 10mm. Both single and multi-layered materials at different frequency bands i-e; 26-40GHz, 75-110GHz and 915-925GHz are measured during the experiments. We have successfully verified the accuracy of dielectric estimation as compared to the available data, while the computational power and time is significantly reduced when compared to commercial software like Ansys HFSS and CAD FEKO. The Bayesian inversion method provides a measure of reliability on material properties since, the probability density is calculated and corresponding eigenvectors are plotted to find the confidence on observed parameters over model parameters. The novel horn antenna designs and improved time domain data inversion techniques made in this research, are very useful contribution in the development of latest dielectric measurement and imaging systems. The possible applications of this research are biomedical imaging for detection and diagnoses of cancer, non-destructive testing of structural defects in objects and communication systems with ultra-high data rates.
Advisor: Prof. Gian Guido Gentili
Compact Antenna Test Range for High-Frequency Satellite Antenna Measurements
A compact antenna test range has been designed to be installed in the microwave/millimeter wave anechoic chamber (MAC) present in the Department of physics at the Università degli di Studi Milano. The design is based on a dual reflector Cassegrain system, a compact range with this design will allow testing the far-field characteristics of telecommunication antennas, small telescopes, and payloads for scientific satellites. The performance parameters such as reflector rim dimensions, a shape of serrations, source feed taper, and distance of antenna under test in quiet zone axis have been analyzed with a parametric studied carried out using TICRA Grasp v10.2.0. This design is going to be manufactured with the typical techniques used for ground station antennas.
Advisor: Prof. Gian Guido Gentili and Prof. Enrico Pagana