PhD thesis defense to be held on December 20, 2022, at 15:00 (Multimedia room 2)

Thesis title: Antenna Reconfigurability using Magneto-Dielectric Materials: Modeling, Design and Evaluation

Abstract: The rapid development of wireless communications has significantly increased the need for low profile, efficient compact antennas that can adapt their operating characteristics depending on the communication channel conditions or specific system requirements. This fact has prompted antenna engineers to use novel materials in place of conventional dielectrics in antenna design. This PhD thesis focuses on the modeling, design and evaluation of reconfigurable microstrip patch antennas using magneto-dielectric materials that exhibit anisotropic behavior and tunable permeability by applying variable DC magnetic field. The use of magneto-dielectric materials in antenna design offers dynamic control of the antenna impedance and radiation characteristics in an easy and controllable manner, keeping at the same time the design complexity and implementation cost relatively low. Initially, emphasis is given on the modeling of the permeability of the magneto-dielectric material. The magnetic bias field acts as a “switching mechanism” that enables the transition of the material between two states, the demagnetized state (absence of magnetic field) and the magnetically biased state, and thus the antenna operation in two discrete states. The key factors for the accurate simulation in both operating states are determined, and a proper methodology is proposed for the design and simulation of reconfigurable antennas with magneto-dielectric materials. Subsequently, the design, simulation and fabrication of a proximity-coupled fed microstrip patch antenna with a sample of magneto-dielectric material placed in the upper substrate is presented. It is proved that when a magnetic bias field is applied, two resonance frequencies appear and antenna polarization changes. Also, a comparative study between simulation and measurement results is presented to validate the proposed methodolody. Then, a suppression technique of mutual coupling between antennas of a multiple antenna system using magneto-dielectric materials is proposed. The design of a system consisting of two patch antenna elements with two cylindrincal samples of magneto-dielectric material placed in the substrate is presented, and the reduction of the mutual coupling between the elements is investigated for various combinations related to themagnetization state of each sample. The idea is to exploit the polarization reconfigurable properties of each antenna element under the application of magnetic field. It is proved that the magnetization state of each element along with the direction and magnitude. of the applied static magnetic field achieved play significant role in the reduction of the mutual coupling.
Aiming to overcome fabrication difficulties that arise by replacing part of the dielectric substrate with a bulk sample of magneto-dielectric material, the design and fabrication of microstrip patch antennas printed on a substrate consisting of ferrite particles homogeneously dispersed into a non magnetic polymer matrix are studied. Frequency and polarization reconfigurability is investigated by applying variable static magnetic field with intentionally varying strength. Also, the influence of the dielectric and magnetic losses of the substrate material on the antenna radiation efficiency is investigated and the feasibility study of fabricating a composite substrate with improved characteristics is presented. Finally, using the cavity model, the operation of a microstrip circular patch antenna on a polymer-magnetodielectric material substrate in the magnetically biased state is analyzed.The Thesis is concluded with an overview of the evaluation of the reconfigurable antennas that were designed and fabricated in this research study. The evaluation is performed through measurements of the input matching and operating frequency bandwidth, the axial ratio, the radiation patterns and the radiation efficiency of the proposed antennas in both operating states.

Supervisor: Professor George Fikioris

PhD Student: Evmorfili Andreou