DEVELOPMENT OF AN ULTRA-WIDEBAND LOW- PROFILE WIDE SCAN ANGLE PHASED ARRAY ANTENNA DISSERTATION Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University By Henry H. Vo, B.S., M.S. Graduate Program in Electrical and Computer Engineering The Ohio State University 2015 Dissertation Committee: Prof. Chi-Chih Chen, Advisor Prof. John Volakis, Co-Advisor Prof. Joel T. Johnson © Copyright by Henry H. Vo 2015 Abstract Coupling in phased arrays is a major issue. Mutual coupling causes both gain and bandwidth reduction. Such coupling arises from the presence of adjacent elements that produce scattering and losses during low-angle beam steering. The scattering effect is comprised of (1) structural scattering and (2) antenna-mode coupling and associated losses. Losses occur when the coupled energy received by adjacent elements is dissipated at the back-end loads, resulting in lower gain at wide scan angles. In addition, the interference from periodic nature of large arrays or feed networks may produce undesired scattering modes and traveling waves that limit the upper bound of the operational frequency and maximum scan angle in ultra-wideband (UWB) arrays. As a result, current ultra wideband (UWB) array designs typically have limited scanning to no more than 45° from normal. In this dissertation, we examine the low angle scanning issues. These issues are verified via full-wave simulation. Our studies show that mutual coupling in the H-plane is stronger than in the E-plane, likely due to the dipole element pattern shape. Another focus of this dissertation is the development of an UWB dual- polarization and low angle beam steering array based on the concept of tightly coupled ii dipole arrays. For this array, we suppress/minimize mutual coupling by redesigning the antenna element, feed geometry, and array structure. Some key design parameters include (1) the simple feed of tightly-coupled dipoles, (2) array height above ground plane, (3) dielectric superstrate, and (4) parasitic coupling ring. The common mode issue is avoided by retaining the ground plane height to less than λmid/4 and the array unit cell size to 0.45λhigh. The final design is also fabricatable on a low-cost PCB. The PCB uses (1) 0.35 mils thick copper corresponding to a standard ¼ oz. copper lamination, (2) 2 mils coupling slot width and plated-thru vias manufacturable using standard PCB process, and (3) standard Roger RT/Ruroid 5880LZ substrate with dielectric constant of 1.96 and Roger RT/Duroid 5880 superstrate with dielectric constant of 2.2. An 18x18 prototype array is fabricated and measured to verify the final design. The total array height of the fabricated prototype is 0.122λ at the lowest operating frequency. It is also demonstrated that the fabricated array is capable of scanning down to more than 60° in the E- and H-planes with impedance bandwidth of 2.62:1 subject to VSWR ≤ 2. Good agreement was also observed between simulations and measurements. iii Dedication Dedicated to my family iv Acknowledgements I dedicate this dissertation to my parents, my sisters, and my brothers, for their hard work and constant support throughout my education. Without their support, I would not have what I have today. I am thankful to my advisor, Dr. Chi-Chih Chen for his support, encouragement, guidance, and endless patience throughout my time and work at the ElectroScience Lab (ESL). I thank him for sharing of his expertise and teaching me the key points of antenna design, antenna miniaturization, and as well as antenna measurement. It is my honor to have such an outstanding advisor and to be his research group member. I would also like to thank my co-advisor, Dr. John Volakis for the advice and motivation I have received from him. Thanks Dr. Joel Johnson for his valuable suggestions on this dissertation and for his participating on all my examination committee. Very special thanks go to Dr. Teh-Hong Lee for his friendship and providing me all the valuable information in antenna measurement. I would like to express my very special thanks to Patricia Toothman for her endless support, advice, and guidance. Thanks to all my friends at ESL for their friendships and insightful discussions in research as well as in course-work. v Vita December 31, 1974………………………….….…. Born – Quang Nam, Vietnam 2002……………………………………………….. B.S. Electrical Engineering, Cal Poly State University 2008……………………………………………..… M.S. Electrical Engineering, Cal Poly State University 2004-2006…………………………………………. Microwave Engineer, L-3 Com/Randtron Antenna Systems 2006-2009…………………………………………. R&D Engineer, Space Systems/Loral 2009-2011…………………………………………. Sr. R&D Engineer, Space Systems/Loral 2011-present………………………………………. Graduate Research Associate, The Ohio State University Publications H. H. Vo and C.C. Chen, “A Very Low-Profile UWB Phased Array Antenna Design for Supporting Wide Angle Beam Steering,” in review. H. H. Vo and C.C. Chen, “Frequency and Scan Angle Limitations in UWB Phased Array,” European Conference on Antenna and Propagation (EUCAP), Hague, Netherlands, Apr. 2014. H. H. Vo and C.C. Chen, “Causes of Low Scanning Angle Issues in Phased Array Antennas,” Antennas Measurement Technique Association (AMTA), Columbus, OH, Oct. 2013. vi N. K. Host, H. H. Vo, and C.C. Chen, “Causes of Low Scanning Angle Issues in Phased Array Antennas,” Antennas and Propagation Society International Symposium, Orlando, FL, Jul. 2013. Fields of Study Major Field: Electrical and Computer Engineering Studies in: Electromagnetics Microwave/Antenna Design and Measurement RF Circuits vii Table of Contents Abstract ............................................................................................................................................ ii Dedication ....................................................................................................................................... iv Acknowledgements .......................................................................................................................... v Vita.................................................................................................................................................. vi Table of Contents .......................................................................................................................... viii List of Tables ................................................................................................................................... x List of Figures ................................................................................................................................. xi Chapter 1: Introduction .................................................................................................................... 1 Chapter 2: Wide-Angle Scanning and Bandwidth Issues/Limitations of Phased Array Antennas .. 6 2.1 Mutual Coupling in Phased Arrays ........................................................................................ 6 2.2 Low-Angle Scanning Issues ................................................................................................... 7 2.2.1 Antenna Structure Mode Scattering Issue ....................................................................... 9 2.2.2 Absorption by Adjacent Elements ................................................................................ 11 2.2.3 Antenna Mode Scattering ............................................................................................. 13 2.3 Bandwidth and Wide Scan Angle Limitations ..................................................................... 15 2.3.1 Lattice Scattering Mode in Scatter Array ..................................................................... 15 2.3.2 Lattice Scattering Mode in Phased Arrays .................................................................... 18 Chapter 3: Development of Wide Scan Angle Phased Array ........................................................ 26 3.1 Revisiting TCDA Operational Principles............................................................................. 27 viii 3.1.1 Ground Plane Effect in Ideal TCD Array ..................................................................... 28 3.2 Wide Scan Angle Array Design Procedure .......................................................................... 33 3.2.1 Tightly-Coupled Array Element Design ....................................................................... 33 3.2.2 Feeding Lines ................................................................................................................ 36 3.2.3 Effect of Array Height on Wide Scan Angle ................................................................ 38 3.2.4 Effect of Element Pattern on Wide Scan Angle ............................................................ 41 3.2.5 Effect of Mutual Coupling on Wide Scan Angle .......................................................... 42 3.2.6 Effect of Parasitic Ring ................................................................................................. 46 3.3 Design Parameter Tolerance Analysis ................................................................................. 48 3.3.1 Element-to-Element Coupling Slot Width Tolerance Study ......................................... 48 3.3.2 Bonding Layer Thickness Tolerance Study .................................................................. 50 3.3.3 Feed-line Diameter Tolerance Study ...........................................................................