Preface
The absorption, reflection, and transmission phenomena of electromagnetic (EM) waves are the most fundamental subjects for those involved in EM-wave engineering.
Although this book is entitled "Electromagnetic Wave Absorber," it has the nature of an EM-wave theory textbook. And, thus, the following two points have been essentially kept in mind. First, the basic physical phenomena are explained as far as possible before describing the detailed theory in each chapter. Secondly, the derivation processes of each important equation are presented in detail along with the appendix explanations.
The first half of this book contains the foundations of EM-wave engineering, viz, the transmission line theories necessary for EM-wave absorber analysis, the basic knowledge of reflection, transmission, and absorption of EM waves, computer analysis, etc.
Based on this, the second half describes specific mediums, the measurement methods of material constants, absorber application examples, methods of absorber design, autonomously controllable EM-wave absorbers, etc.
Now, what is an EM-wave absorber? First, in Chapter 1, in order to understand the overall picture of EM-wave absorbers, they are classified and arranged in a multifaceted manner, including the history of their development process.
After describing the method of determining the amount of EM-wave absorption, their classifications by constituent material, composition form, and frequency are described.
In order to deliver a comprehensive image, these classifications are presented in a table. In addition, the table also contains some application fields of the EM-wave absorber together with the materials to be used, and a new EM-wave absorber, which is described afterwards. The EM-wave problems can be often treated by replacing them as transmission line problems. By doing so, it becomes easy to understand the phenomena and characteristics in the EM-wave problems. Therefore, in Chapter 2, time is devoted to also deepen the understanding of the transmission line theory, including the derivation process of its relevant equations.
After clarifying the relationship between the reflection coefficient and impedances of the transmission line, the principle of the Smith chart constitution is presented in detail along with an admittance chart. Furthermore, this chapter includes the derivation methods of Maxwell's equations, besides presenting the reflection and transmission phenomena of a plane incident wave for perpendicular and oblique incidence.
Chapter 3 studies the reflection coefficients for the cases of perpendicular and oblique incidence on a flat plate-type single-layer wave absorber and a multilayer wave absorber, respectively. In addition, the theoretical analysis of multiple reflections is introduced in the case of an EM-wave absorber placed in a room.
In recent years, a lot of simulation methods or analyses have been introduced for the analysis of EM-wave absorber characteristics. However, it is important to understand their basic theories first.
Chapter 4 describes two powerful simulation methods, viz, the finite difference time domain (FDTD) and finite element (FE) methods. Here, the theories behind the methods are explained in such a way that the reader can understand them well. Regarding the FDTD method, the evaluation of boundary conditions and the cell size division in an analytical region are shown on the basis of an actual wave absorber analysis data.
Next, since the analysis of the FE method is, in principle, based on the variational method, the concept of the variational method is clearly explained with concrete examples. For the FE method, two approaches have been introduced: (i) variational method using a functional and (ii) weighted residual method, being defined as a direct method, without using the functional. For the latter, the three-dimensional current vector potential method is introduced in detail, and an eddy current absorber is demonstrated as an example.
The characteristic of an EM-wave absorber largely depends on its material. Therefore, two typical EM-wave-absorbing materials, carbon and ferrite, are investigated in Chapter 5 from the viewpoints of their crystal structures.
Recently, material technologies have made remarkable progress and many new materials have been produced. Introduction of these materials based on their implementation concepts is also important for new EM-wave absorber designs. Chapter 6 explains three such media, viz, chiral media, ferrite anisotropic media, and metamaterials as an example of special mediums.
Concerning these subjects, the derivation processes of the theoretical equations along with their physical interpretations are explained, and examples of an EM-wave absorber and attenuator are shown.
To know the measuring method of material characteristics of the EM-wave absorber and absorber characteristics themselves is important in order to grasp in advance the required characteristics being determined. In Chapter 7, which is entitled as Measurement Methods of EM-Wave Absorbers, the measurement methods of the EM-wave absorber material constant and the EM-wave absorber characteristics are introduced. Here, in order to understand the fundamental principles of the measurement method and to enhance applicability, the theories including the fundamental principles of measuring material constants together with conventional methods and the measurement methods of EM-wave absorbers are described in detail.
For future EM-wave absorber development, it is also important to understand what kind of absorber is used in which electromagnetic environment. Chapter 8 describes the materials being used and their composition, focusing on absorbers that have been put into practical use; and detailed data are given, from a general flat plate structure to EM-wave absorbers for use in a building wall.
By the way, the commonly used materials for the EM-wave absorber are a low-conductive material, a carbon material, and a magnetic material (typically, a ferrite), and the like.
When designing a new EM-wave absorber having the desired characteristics using these conventional materials, we face difficulties because the absorber materials have to be made through the process of controlling the mixing ratio of raw materials, firing temperature, pressure, etc. Therefore, in Chapter 9, wave absorbers introducing the new concept of "equivalent transformation method of material constant (ETMMC)" are introduced. Here, one method of constructing a new EM-wave absorber is described, which does not require the complicated steps involved in conventional material design. This chapter describes the method of (i) combining two or three conventional materials divided into macro-sizes using a conventional material, (ii) providing small holes in an EM-wave absorber material, (iii) mounting periodical conductive elements on an absorbing material surface, and (iv) introducing integrated circuit concepts.
Currently, artificial intelligence (AI) technologies have advanced rapidly, and it seems that material technologies should assimilate this trend as one aspect. Therefore, it is necessary to design EM-wave absorbers that can be autonomously controlled electrically. Chapter 10 introduces a new EM-wave absorber that can be controlled electrically, called "autonomous controllable metamaterial (ACMM)" absorber. This is an EM-wave absorber based on a new implementation concept satisfying all conditions to be imposed on an EM-wave absorber, and independent of the oblique incidence and polarization characteristics, etc.
As can be speculated from the descriptions, although this is written as a book on EM-wave absorbers, the detailed explanations provided impart to it the properties of a textbook on EM-wave theories.
In publishing this book, I would like to thank Professor Arye Rosen for his devoted cooperation, valuable advice, and encouragement. I owed Mrs. Daniella Rosen for her heartwarming support during my research activities. I express my sincere thanks for her. I acknowledge my heartfelt gratitude to Professor Andre Vander Vorst for his valuable comments and thoughtful suggestions in this regard.
I am deeply grateful to Professor Kunihiro Suetake and Professor Yasutaka Shimizu of the Tokyo Institute of Technology, who provided the valuable opportunity to study the EM-wave absorber and their kind guidance throughout the duration of the research.
This book contains the research contributions of Dr. Mitsuhiro Amano, and I express my appreciation for his sincere efforts in EM-wave absorber studies. I also express my thanks to Professor Emeritus of Ryuji Koga in Okayama University for his cooperation in this book publication.
The basic structural concept in chapter 10 of this book was underpinned by the cancer treatment research on EM-waves guided by the Founder, President Shigeyoshi Matsumae of Tokai University. I would like to express my sincere gratitude for this valuable guidance.
It will be an unexpected delight for the author if this book would be widely helpful from all students at the university level to researchers who are undertaking EM-wave study fields.
23 January 2019
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