The fourth edition of Antenna Theory is designed to meet the needs of electrical engineering and physics students at the senior undergraduate and beginning graduate levels, and those of practicing engineers. The text presumes that the students have knowledge of basic undergraduate electromagnetic theory, including Maxwell's equations and the wave equation, introductory physics, and differential and integral calculus. Mathematical techniques required for understanding some advanced topics in the later chapters are incorporated in the individual chapters or are included as appendices. The book, since its first edition in 1982 and subsequent two editions in 1997 and 2005, has been a pacesetter and trail blazer in updating the contents to keep abreast with advancements in antenna technology. This has been accomplished by: • Introducing new topics • Originating innovative features and multimedia to animate, visualize, illustrate, and display radiation characteristics • Providing design equations, procedures, and associated software This edition is no exception, as many new topics and features have been added. In particular: New sections have been introduced on: 1. Flexible and conformal bowtie 2. Vivaldi antenna 3. Antenna miniaturization 4. Antennas for mobile communications 5. Dielectric resonator antennas 6. Scale modeling • Additional MATLAB and JAVA programs have been developed. • Color and gray-scale figures and illustrations have been developed to clearly display and visualize antenna radiation characteristics. • A companion website has been structured by the publisher which houses the MATLAB programs, JAVA-based applets and animations, PowerPoint notes, and JAVA-based interactive questionnaires. A solutions manual is avail- able only for the instructors that adopt the book as a classroom text. • Over 100 additional end-of-chapter problems have been included. While incorporating the above new topics and features in the current edition, the book maintained all of the attractive features of the first three editions, especially the: • Three-dimensional graphs to display the radiation characteristics of antennas. This feature was hailed, at the time of its introduction, as innovative and first of its kind addition in a textbook on antennas. • Advanced topics, such as a chapter on Smart Antennas and a section on Fractal Antennas. • Multimedia: