Start Date: February 11, 2014
Andreou Evmorfili
The rapid development of wireless communications has prompted the design of low profile, efficient compact antenna with reconfigurable characteristics. Reconfigurable antennas have gained significant interest as they offer the flexibility to adapt on demand their operating characteristics depending on the communication channel conditions. Towards this direction, various reconfiguration mechanisms have been introduced. Among them, the use of magneto-dielectric materials that exhibit anisotropic behavior and tunable permeability by means of an external magnetic field has gained considerable attention. The magnetic bias field acts as a “switching mechanism” that enables the transition of the ferrite material between two magnetization states: the demagnetized state (absence of magnetic field) and the magnetically biased state. The objective of this PhD thesis is to address the major challenges related to the modeling, design and evaluation of reconfigurable antennas exploiting the tuning features of ferrites.
Initially, the modeling of the permeability of ferrites in the demagnetized state and the magnetically biased state is extensively studied. The key factors for the accurate simulation of reconfigurable antennas using ferrites are determined, and a multistage simulation approach is proposed. Two types of reconfigurable ferrite patch antennas are studied: a proximity coupled fed patch antenna with a bulk ferrite sample inserted into its dielectric substrate and a patch antenna printed on a magneto-dielectric composite substrate consisting of ferrite particles dispersed in a polymer matrix. Due to the interaction of EM fields in the antenna cavity with the biased ferrite material, antenna reconfigurability can be achieved. Dynamic control of the antenna operating frequencies and polarization is achieved in an easy manner through the variations of the applied magnetic field, keeping at the same time the design complexity and cost relatively low.
Subsequently, the prototypes of the proposed reconfigurable ferrite antennas were fabricated, and their performance was experimentally evaluated in terms of reflection coefficient, axial ratio, gain and radiation efficiency. Measurements of the antenna parameters were performed in an RF shielded anechoic chamber and commercially available permanent magnets were used for the biasing of the ferrite bulk material or composite polymer-ferrite substrate. For easily applying the variable external magnetic field, a special supporting structure was fabricated. To demonstrate the accuracy of the proposed simulation procedure, a comparative study between simulation and measurement results is presented. For this comparative study, the magnetic field inside the ferrite material was estimated by performing a magnetostatic analysis.
Finally, an approach to control the mutual coupling between the antenna elements of a multiple antenna system using magneto-dielectric materials is presented. The proposed multiple antenna system consists of two separate patch elements printed on the same substrate. They are placed close to each other and two ferrite bulk samples are incorporated in their substrate. The influence of the ferrite materials on the mutual coupling between the antenna elements is investigated, under the application of an external magnetic field. It is proved through simulations and measurements that the proper application of the magnetic field changes the polarization of the antenna elements changes from linear to circular and additionally the mutual coupling between them, changes in a dynamic range of about 10 dB. The direction of the applied magnetic field plays significant role in the reduction of the mutual coupling.