ACKNOWLEDGMENTSI would like to begin by extending my gratitude for having the opportunity and privilege to study under the guidance of my advisor, Prof. Marinos Vouvakis, over the course of five years for my Master's and PhD research. His energy, drive, and motivation to not only teach but also to stimulate critical thinking, to invoke a practical mindset, and to create with meaning have left me with invaluable principles. I am indebted to him for challenging me and his endless patience and support throughout this research.I'd like to also thank Prof. Do-Hoon Kwon and Prof. Gopal Narayanan for serving on my dissertation committee and providing many insightful comments and support.I would like to additionally extend my gratitude to Prof. Robert Jackson, Prof.Paul Siqueira, and Prof. Joseph Bardin for serving on my research qualifying exam committee. Also, although I never had the pleasure of having him as a professor at UMass, I'd like to thank Prof. Dan Schaubert for taking the time to inadvertently introduce Vivaldi arrays to a curious kid at a phased array conference nearly six years ago; likewise thanks to Prof. Jorge Salazar for seeing some potential in that kid to encourage him to apply for graduate study at UMass. I appreciate it greatly. Ultra-wideband (UWB) electronically scanned arrays (ESA) with high efficiency, excellent polarization agility, and wide-scan matching remain essential for servicing multifunctional RF front-ends and other communications, sensing, and jamming or countermeasure systems. To this day, the most popular antenna array element in modern UWB-ESA systems is the Vivaldi, or flared notch, due to its superior widescan wide impedance bandwidth, well-known design guidelines, and practical embodiment versatility. Despite their popularity, these arrays tend to radiate unacceptably high cross-polarization levels, thus encouraging a great research opportunity.This dissertation presents the theory and design of a new class of UWB-ESAs, termed Sliced Notch Antenna (SNA) arrays, that remedy the high cross-polarization problems in Vivaldi arrays while maintaining their desirable impedance performance.The critical enabling insight of this work lays in revealing the nature of polarization viii purity or cross-polarization ratio (CPR) degradation in Vivaldi arrays to arise from a highly imbalanced ratio of longitudinal and transverse currents within the element.This work introduces a novel design strategy that intrinsically balances these currents over a UWB operating band, achieving decade-order (10:1) bandwidths and low cross-polarization. Moreover, the design approach is simple, intuitive, and can be implemented in a manner that does not inflate cost expenditures. In fact, the proposed topology can facilitate significantly reduced costs and manufacturing times
