TUNABLE MICROWAVE AND MILLIMETRE-WAVE METAMATERIAL STRUCTURES AND APPLICATIONS by MARINA MAVRIDOU A thesis submitted to the University of Birmingham for the degree of DOCTOR OF PHILOSOPHY School of Electronic, Electrical and Systems Engineering College of Engineering and Physical Sciences University of Birmingham August 2015 ABSTRACT TUNABLE MICROWAVE AND MILLIMETRE-WAVE METAMATERIAL STRUCTURES AND APPLICATIONS MARINA MAVRIDOU Doctorate of Philosophy School of Electronic, Electrical and Systems Engineering University of Birmingham Novel designs of metamaterial structures as well as novel techniques and configurations for tuning metamaterials are presented in this PhD thesis. The proposed tuning techniques overcome the challenges that exist in other tuning techniques available thus far. Moreover, possible applications of tunable metamaterials in communication systems are proposed. Initially, tunable Electromagnetic Band-Gap (EBG) structures are proposed for low frequencies operation (3GHz to 6GHz) employing a novel biasing technique for varactor diodes. Subsequently, the proposed tunable EBG structures are applied to closely spaced antennas, achieving isolation enhancement for Multiple Input Multiple Output (MIMO) systems. Moreover, a new technique of tuning High Impedance Surface (HIS) structures is presented, with low-loss performance and no parasitic effects, based on employing two types of piezoelectric actuators, each type being suitable to a different frequency band. Particularly, bender piezoelectric actuators are used for configurations operating at low mm-wave frequencies (~15GHz) and stack multilayer actuators for operation at higher mm-wave frequencies (60GHz) where achieving a low loss performance is even more challenging. Two tunable antenna designs are also proposed incorporating both tunable HIS structures (at 15GHz and 60GHz). Finally, novel configurations of tunable Frequency Selective Surfaces (FSS) are proposed based on the concept of piezoelectric actuators to obtain a tunable ii response. Again, this is carried out for two operating bands, 15GHz and 60GHz. The particular designs of tunable HIS and FSS, are directly scalable to even higher frequencies (THz), offering a promising solution at this band. iii To my husband Kostas and my family, iv Acknowledgements First and foremost, I would like to thank my thesis supervisor, Dr. Alexandros Feresidis, for his support and guidance throughout my PhD without which I would not have been able to complete this work. Furthermore, I would like to thank my second supervisor, Dr. Peter Gardner as well as Dr. Costas Constantinou for their useful insight, advices and overall support during my PhD studies. I would also like to thank the technician of the Department of Electronic, Electrical and Systems Engineering, Alan Yates for his valuable assistance with practical issues and for the fabrication of my structures’ prototypes. In addition, I would like to acknowledge the help of Dr. James Bowen, previously working in the Department of Chemical Engineering of the University of Birmingham and currently Lecturer in Materials Engineering at The Open University, for measuring using the optical interferometer. Special thanks go to the PhD and postdoctoral researchers of my group Konstantinos Konstantinidis, Elena Abdo Sanchez, Tade Oluwabunmi and Yuriy Nechayev. Finally, I would like to acknowledge the financial support from the school and from EPRSC. v List of Publications Journal papers: 1. M. Mavridou, A. P. Feresidis, P. Gardner and P. S. Hall, “Tunable millimetre-wave phase shifting surfaces using piezoelectric actuators,” IET Microw. Antennas Propag., vol.8, no. 11, pp. 829-834, Aug. 2014. 2. Q. Li, A. P. Feresidis, M. Mavridou, and P. S. Hall, “Miniaturized Double-Layer EBG Structures for Broadband Mutual Coupling Reduction Between UWB Monopoles,” IEEE Trans. Antennas and Propag., vol. 63, no. 3, pp. 1168–1171, March 2015. 3. M. Mavridou, A. P. Feresidis, P. Gardner, “Tunable Double-Layer EBG Structures and Application to Antenna Isolation”, IEEE Trans. Antennas and Propag., accepted, publication pending. 4. M. Mavridou, K. Konstantinidis, A. P. Feresidis, “Continuously Tunable mm-Wave High Impedance Surface,” IEEE Antennas and Wireless Propag. Lett., submitted. 5. M. Mavridou, A. P. Feresidis, “Dynamically Reconfigurable High Impedance and Frequency Selective Meta-Surfaces”, IEEE Trans. Antennas and Propag., submitted. International conferences: 1. M. Mavridou, A. P. Feresidis, P. Gardner, P. S. Hall, “Tunable defected ground slits for mutual coupling reduction applications,” Antennas and Propagation Conference (LAPC), 2012 Loughborough, UK. 2. P. Gardner, A. P. Feresidis, P. S. Hall, T. J. Jackson, O. Tade, M. Mavridou, Y. Kabiri, X. Gao, “Frequency reconfiguration in single and dual antenna modules,” 7th European Conference on Antennas and Propagation, Gothenburg, Sweden, Apr. 8-12, 2013. 3. M. Mavridou, A. P. Feresidis, P. Gardner, P. S. Hall, “Tunable Electromagnetic Band Gap Slits for Mutual Coupling Reduction”, European Microwave Week 2013, Nuremberg, Germany, October 6-11, 2013. vi 4. M. Mavridou, A. P. Feresidis, P. Gardner, “Tuning Periodic Surfaces with Piezoelectric Actuators”, 8th European Conference on Antennas and Propagation, The Hague, Netherlands, Apr. 6-11, 2014. 5. M. Mavridou, A. P. Feresidis, “A new class of tunable multi-layer meta-surfaces”, 8th International Congress on Advanced Electromagnetic Materials in Microwaves and Optics – Metamaterials 2014 Copenhagen, Denmark, 25-30 August 2014. 6. M. Mavridou, A. P. Feresidis, P. Gardner, “A New Technique for Tuning Millimetre- Wave Artificial Impedance Surfaces”, Antennas and Propagation Conference (LAPC), 2014 Loughborough, UK. 7. M. Mavridou, A. P. Feresidis, P. Gardner, “A Report on Tuning mm-Wave Periodic Structures with Piezoelectric Actuators”, IET 2nd Annual Active and Passive RF Devices Seminar, Birmingham, UK, 29 October 2014. 8. M. Mavridou, K. Konstantinidis, A. P. Feresidis, P. Gardner, “Reconfigurable Beams from Millimetre-Wave Leaky-Wave Antennas”, 9th European Conference on Antennas and Propagation, Lisbon, Portugal, Apr. 12-17, 2015. 9. M. Mavridou, A. P. Feresidis, P. Gardner, “Tunable mm-Wave Artificial Impedance Surfaces Using Piezoelectric Bender Actuators”, 9th European Conference on Antennas and Propagation, Lisbon, Portugal, Apr. 12-17, 2015. vii Contents CHAPTER 1. INTRODUCTION .................................................................................. 1 1.1 BACKGROUND ............................................................................................................ 1 1.1.1 Metamaterials and Meta-surfaces .................................................................................. 1 1.1.2 Overview of Tuning Techniques for Metamaterial Structures ...................................... 7 1.2 MOTIVATION AND OBJECTIVES ........................................................................ 10 1.3 OUTLINE OF CHAPTERS ....................................................................................... 12 References ................................................................................................................................ 15 CHAPTER 2. THEORETICAL BACKGROUND ............................................... 23 2.1 PERIODIC STRUCTURES ....................................................................................... 23 2.1.1 Electromagnetic Band-Gap (EBG) Structures ............................................................. 24 2.1.2 Frequency Selective Surfaces (FSS) ............................................................................ 27 2.1.3 High Impedance Surfaces (HIS) .................................................................................. 30 2.1.4 Leaky Wave Antennas (LWA) .................................................................................... 32 2.2 PERIODIC ANALYSIS AND ELECTROMAGNETIC SOLVERS ...................... 33 2.2.1 Commercial 3D Electromagnetic Solvers .................................................................... 33 2.3 TUNING COMPONENTS ......................................................................................... 35 2.3.1 Varactor Diodes ........................................................................................................... 35 2.3.2 Piezoelectric Actuators ................................................................................................ 37 2.3.2.1 Bender Actuators ................................................................................................... 37 2.3.2.2 Stack Multilayer Piezo-actuators .......................................................................... 39 References ................................................................................................................................ 41 CHAPTER 3. TUNABLE EBG STRUCTURES USING VARACTOR DIODES AND APPLICATIONS ................................................................................. 43 3.1 TUNABLE EBG STRUCTURES .............................................................................. 44 3.1.1 Dispersion Characterization of Slots in a Ground Plane ............................................. 45 3.1.2 Design of Single Tunable Slot-Patch Structure ........................................................... 48 3.1.3 Design of Multiple Tunable Slot-Patch EBG .............................................................. 50 3.1.4 Design of Slit-patch Structures ...................................................................................