Low-Profile Wideband Antennas Based on Tightly Coupled Dipole and Patch Elements Dissertation Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University By Erdinc Irci, B.S., M.S. Graduate Program in Electrical and Computer Engineering The Ohio State University 2011 Dissertation Committee: John L. Volakis, Advisor Kubilay Sertel, Co-advisor Robert J. Burkholder Fernando L. Teixeira c Copyright by Erdinc Irci 2011 Abstract There is strong interest to combine many antenna functionalities within a single, wideband aperture. However, size restrictions and conformal installation requirements are major obstacles to this goal (in terms of gain and bandwidth). Of particular importance is bandwidth; which, as is well known, decreases when the antenna is placed closer to the ground plane. Hence, recent efforts on EBG and AMC ground planes were aimed at mitigating this deterioration for low-profile antennas. In this dissertation, we propose a new class of tightly coupled arrays (TCAs) which exhibit substantially broader bandwidth than a single patch antenna of the same size. The enhancement is due to the cancellation of the ground plane inductance by the capacitance of the TCA aperture. This concept of reactive impedance cancellation was motivated by the ultrawideband (UWB) current sheet array (CSA) introduced by Munk in 2003. We demonstrate that as broad as 7:1 UWB operation can be achieved for an aperture as thin as λ/17 at the lowest frequency. This is a 40% larger wideband performance and 35% thinner profile as compared to the CSA. Much of the dissertation’s focus is on adapting the conformal TCA concept to small and very low-profile finite arrays. Three particular designs are presented. One is a 6×6 patch array occupying a λ/3 × λ/3 small aperture (mid-frequency is at 2.1 GHz). Remarkably, it is only λ/42 thick yet delivers 5.6% impedance bandwidth (|S11| < −10dB), 4.4dB realized gain (87% efficiency) and 23% gain ii bandwidth (3dB drop). The second finite TCA consists of 4×2 patches and occupies a λ/3.2×λ/3.2 aperture on a λ/26 thick substrate (mid-frequency is at 2 GHz). This antenna delivers 17.3% impedance bandwidth, 4.8dB realized gain (95% efficiency) and 30% gain bandwidth. That is, more than twofold impedance bandwidth is delivered as compared to a single patch antenna of the same size on conventional or EBG substrate. The third array being considered consists of 3×2 patches occupying a λ/3.2×λ/3.2 aperture and situated on a λ/22 thick substrate (mid-frequency is at 1.95 GHz). Although of very low-profile and of small size, this TCA achieves a 26% wideband performance with 4.5dB realized gain (97% efficiency) and 40% gain bandwidth. It therefore covers DCS, PCS and UMTS bands (1.7 GHz – 2.2 GHz) for mobile communications. As compared to a conventional patch antenna that can cover the same frequency bands, the TCA is 85% smaller in size while also being 60% thinner. This dissertation demonstrates that the tightly coupled array concept is quite versatile for miniaturization, bandwidth enhancement and applicability to a diverse range of next generation antennas. iii Dedicated to my parents. iv Acknowledgments I would like to express my sincere gratitude to my advisor Prof. John L. Volakis and my co-advisor Dr. Kubilay Sertel for their continuous guidance, encouragement, patience and understanding. Their great vision and pursuit for excellence had always raised the bar. Therefore, this dissertation was a challenge to myself for reaching their high standards. I feel very honored, grateful and lucky to have worked with them. In these past four years, they had a huge impact on me, both professionally and personally, that I find very invaluable. I would like to thank Prof. Robert J. Burkholder and Prof. Fernando L. Teixeira for participating in my defense committee, reading and commenting on this disserta- tion and also for their helpful suggestions. I would like to thank Jon Doane, Mustafa Kuloglu, Gokhan Mumcu and Ioannis Tzanidis not only for their friendship but also for many fruitful technical discussions. I also thank Ugur Olgun and Dr. Lanlin Zhang for their friendship and extensive help in fabricating my antenna prototypes. Additionally, I would like to thank all my friends at ESL: Elias Alwan, Nil Apaydin, Kenny Browne, Jae-Young Chung, Stylianos Dosopoulos, Faruk Erkmen, Baris Guner, Justin Kasemodel, Gil-Young Lee, Will Moulder, Hayrettin Odabasi, James Park, Tao Peng, Nathan Smith, Brandan Strojny, Orbay Tuncay, Georgios Trichopoulos, Feng Wang, Salih Yarga and Jing Zhao. v Vita February 10, 1982 . .......................Born - Eskisehir, Turkey 2004 ........................................B.S. Electrical and Electronics Eng. Bilkent University, Turkey 2007 ........................................M.S. Electrical and Electronics Eng. Bilkent University, Turkey 2004-2007 ..................................Grad. Research & Teaching Assistant Electrical and Electronics Eng., Bilkent University, Turkey 2007-present . ..............................Graduate Research Associate ElectroScience Laboratory, Electrical and Computer Eng., The Ohio State University, USA Publications Research Publications E. Irci, K. Sertel and J. L. Volakis, “Antenna Miniaturization for Vehicular Platforms Using Printed Coupled Lines Emulating Magnetic Photonic Crystals,” Metamaterials, vol. 4, no. 2-3, pp. 127–138, Aug.-Sep. 2010. N. Apaydin, E. Irci, G. Mumcu, K. Sertel and J. L. Volakis, “Miniature Antennas Based on Printed Coupled Lines Emulating Anisotropy,” Microwaves, Antennas & Propagation, IET, vol.4, no.8, pp.1039–1047, Aug. 2010. E. Irci, K. Sertel and J. L. Volakis, “Miniature Printed Magnetic Photonic Crystal Antennas Embedded into Vehicular Platforms,” Applied Computational Electromag- netics Society Journal, vol. 26, no. 2, pp. 109–114, Feb. 2011. vi E. Irci, K. Sertel and J. L. Volakis, “An Extremely Low-Profile, Compact and Broad- band Tightly Coupled Patch Array,” Radio Science, submitted. Conference Publications E. Irci, K. Sertel and J. L. Volakis, “Unidirectional transmission characteristics of printed magnetic photonic crystals,” IEEE AP-S/URSI International Symposium, San Diego, CA, vol. APS, pp. 1–4, Jul. 5-11, 2008. E. Irci, K. Sertel and J. L. Volakis, “Miniature Antenna Using Coupled Microstrip Lines Emulating Magnetic Photonic Crystals,” 2009 USNC/URSI Meeting, BDS1-7, Boulder, CO, Jan. 2009. E. Irci, K. Sertel and J. L. Volakis, “Antenna Miniaturization Using Coupled Mi- crostrip Lines Emulating Magnetic Photonic Crystals,” 2009 IEEE AP-S/URSI In- ternational Symposium, Charleston, SC, vol. APS, pp. 1–4, Jul. 5-11, 2009. K. Sertel, J. L. Volakis and E. Irci, “Small Wideband Antennas Based on Mag- netic Photonic Crystals,” 3rd International Congress on Advanced EM Materials in Microwaves and Optics, London, UK, 1-4 Sep. 2009. E. Irci, K. Sertel and J. L. Volakis, “Miniature Printed Magnetic Photonic Crys- tal Antennas Embedded into Vehicular Platforms,” in Proceedings of the 26th In- ternational Review of Progress in Applied Computational Electromagnetics (Applied Computational Electromagnetics Society), Tampere, Finland, Apr. 2010. E. Irci, K. Sertel and J. L. Volakis, “Miniature Multiband Microstrip Antenna De- sign via Dispersion Engineering,” 2010 URSI Radio Science Meeting, Toronto, ON, Canada, Jul. 2010. E. Irci, K. Sertel and J. L. Volakis, “Ultrathin Miniature Antenna to Mitigate Plat- form Loading Effects,” in Proc. 2010 IEEE Antennas and Propagation Soc. Int. Symp. Toronto, ON, Canada, pp. 1-4., Jul. 2010. E. Irci, K. Sertel and J. L. Volakis, “An Extremely Low-Profile, Compact and Broad- band Tightly Coupled Patch Array,” 2011 USNC/URSI Meeting, Boulder, CO, Jan. 2011. E. Irci, K. Sertel and J. L. Volakis, “Bandwidth Enhancement of Low-Profile Mi- crostrip Antennas Using Tightly Coupled Patch Arrays,” in Proc. 2011 IEEE An- tennas and Propagation Soc. Int. Symp., Spokane, WA, pp. 1-4., Jul. 2011. vii Fields of Study Major Field: Electrical and Computer Engineering Studies in: Metamaterials and Photonic Crystals Dr. K. Sertel, Prof. J. L. Volakis Frequency Selective Surfaces Dr. K. Sertel, Prof. J. L. Volakis Ultrawideband Phased Arrays Dr. K. Sertel, Prof. J. L. Volakis Antenna Miniaturization Dr. K. Sertel, Prof. J. L. Volakis viii Table of Contents Page Abstract ....................................... ii Dedication ...................................... iv Acknowledgments .................................. v Vita . ....................................... vi List of Tables .................................... xii List of Figures ................................... xiii 1. Introduction .................................. 1 1.1 Ultrawideband Phased Array Antennas: Motivation, Challenges and Objectives ................................ 1 1.2 Small, Low-Profile and Broadband Antennas: Motivation, Chal- lenges and Objectives ......................... 9 1.3 Contributions and Organization of the Dissertation ......... 14 2. A Low-Profile and Ultrawideband Tightly Coupled Double-Legged Dipole Array ..................................... 18 2.1 Dipole Arrays in Free Space and Over a Ground Plane ....... 19 2.1.1 Dipole Arrays in Free Space: Wheeler’s Current Sheet Con- cept and its Realization Using Connected Dipoles ...... 19 2.1.2 Dipole Arrays over a Ground Plane: Munk’s Tightly Coupled Current Sheet Array ...................... 23 2.2 Double-Legged Dipole Array Design . ............... 26 2.2.1 Skewed Dipole Arrangement .................. 26 2.2.2 Regular