126,99 €
Mehr erfahren.
- Herausgeber: John Wiley & Sons
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
Addresses the importance of EM wave absorbers and details pertinent theory, design, and applications Demands for various EM-wave absorbers are rapidly increasing along with recent trends toward complicated electromagnetic environments and development of higher-frequency communication equipment, including AI technology. This book provides a broad perspective on electromagnetic wave absorbers, as well as discussion of specific types of absorbers, their advantages and disadvantages, their applications, and performance verification. Electromagnetic Wave Absorbers: Detailed Theories and Applications presents the theory behind wave absorbers and their practical usage in design of EM-wave absorber necessary particularly for EMC environments, and similar applications. The first half of the book contains the foundations of electromagnetic wave engineering, specifically the transmission line theories necessary for EM-wave absorber analysis, the basic knowledge of reflection, transmission, and absorption of electromagnetic waves, derivation of Maxwell's equations and computer analysis. The second half describes special mediums, absorber application examples, simplified methods of absorber design, autonomously controllable EM-wave absorber, and more. This valuable text: * Provides detailed explanations of basic theory and applied theory for understanding EM-wave absorbers * Discusses the material constant measurement methods of EM-wave absorption characteristics that are necessary for designing EM-wave absorbers * Includes examples of novel EM-wave absorber configurations Electromagnetic Wave Absorbers: Detailed Theories and Applications is an ideal read for researchers and students concerned with electromagnetic wave engineering. It will also appeal to computer software engineers and electromagnetic field theory researchers.
Sie lesen das E-Book in den Legimi-Apps auf:
Seitenzahl: 427
Veröffentlichungsjahr: 2019
Ähnliche
Table of Contents
Cover
Preface
1 Fundamentals of Electromagnetic Wave Absorbers
1.1 Introduction to Electromagnetic‐Wave Absorbers
1.2 Fundamentals of Absorber Characteristics
1.3 Classifications of Absorbers
1.4 Application Examples of Wave Absorbers
References
2 Fundamental Theory of EM‐Wave Absorbers
2.1 Transmission Line Theory
2.2 Smith Chart
2.3 Fundamentals of Electromagnetic Wave Analysis
Appendix
References
3 Methods of Absorber Analysis
3.1 Normal Incidence to Single‐layer Flat Absorber
3.2 Oblique Incidence to Single‐layer Flat Absorber
3.3 Characteristics of the Multilayered Absorber
3.4 Case of Multiple Reflected and Scattered Waves
Appendix
References
4 Basic Theory of Computer Analysis
4.1 FDTD Analysis Method
4.2 Finite Element Method
4.3 Three‐Dimensional Electric Current Potential Method
Appendix
References
5 Fundamental EM‐Wave Absorber Materials
5.1 Carbon Graphite
5.2 Ferrite
5.3 Hexagonal Ferrite
References
6 Theory of Special Mediums
6.1 Chiral Medium
6.2 Theory of Magnetized Ferrite
6.3 MW‐Propagation of Circular Waveguide with Ferrite
6.4 Metamaterial
Appendix
References
7 Measurement Methods on EM‐Wave Absorbers
7.1 Material Constant Measurement Methods
7.2 Measurement of EM‐Wave Absorption Characteristics
Appendix
References
8 Configuration Examples of the EM‐wave Absorber
8.1 Quarter‐wave‐Type Absorber
8.2 Single‐Layer‐Type Absorber
8.3 Two‐Layered Absorber
8.4 Applications as Building Material
8.5 Low‐Reflective Shield Building Materials
References
9 Absorber Characteristic Control by Equivalent Transformation Method of Material Constants
9.1 Basic Concepts and Means
9.2 Examples of ETMMC Absorbers
References
10 Autonomous Controllable‐Type Absorber
10.1 Autonomous Control‐type Metamaterial
10.2 Configurations of the ACMM Absorber
10.3 The Main Point as the Technical Breakthrough
10.4 Characteristics as the EM‐Wave Absorber
10.5 Input Impedance Characteristic
10.6 Examples of Application Fields
References
Index
End User License Agreement
List of Tables
Chapter 1
Table 1.1 Representations of reflection coefficient in wave absorbers.
Table 1.2 Classifications of wave absorbers.
Table 1.3 Examples of main wave absorber use.
Chapter 4
Table 4.1 Flowchart of deriving the electric current vector potential and the ma...
Table 4.2 Discretization processes of the basic equation.
Table 4.3 Electrical constants of each part of human body model.
Chapter 5
Table 5.1 Main compositions ofM
2+
.
Chapter 6
Table 6.1 Dielectric constant of chiral media.
Chapter 8
Table 8.1 Characteristics of sintered ferrite absorber.
Table 8.2 Characteristic example of rubber ferrite absorber (Ni–Zn system, from ...
Table 8.3 Characteristics and dimensions of BMDM and the new BMDM.
Table 8.4 Reflection loss (dB) of BMDM walls.
Chapter 9
Table 9.1 Characteristics and functions of the EM‐wave absorbers with periodic c...
Table 9.2 Example of each parameter normalized by the wavelength of the center a...
Chapter 10
Table 10.1 Dimension example of a two‐dimensional unit cell.
Table 10.2 Dimensions of a three‐dimensional unit cell.
Table 10.3 Configuration dimensions in a broadband absorber of ACMM type.
Table 10.4 Specifications of ACMM‐type absorber characteristics.
List of Illustrations
Chapter 1
Figure 1.1 Main classifications of the wave absorber. (a) Plane type, (b) λ/4...
Figure 1.2 Classification by frequency band. (a) Narrowband type, Δ
f
/
f
0
× 100 ...
Chapter 2
Figure 2.1 Equivalent circuit representation of the electromagnetic‐wave trans...
Figure 2.2 Equivalent relationship of transmission lines. (a) Infinite length ...
Figure 2.3 Explanation of the reflection coefficient along the transmission li...
Figure 2.4 The formation of the Smith chart. (a) Reflection coefficient. (b) P...
Figure 2.5 Construction process of the Smith chart. (a) Circle group by Eq. (2...
Figure 2.6 Smith chart.
Figure 2.7 Admittance chart.
Figure 2.8 Explanation of the Smith chart.
Figure 2.9 Transmission model terminated by a load with
Z
R
.
Figure 2.10 Explanation of a single‐stub matching method.
Figure 2.11 Explanation on how to treat the Smith chart.
Figure 2.12 The case of transmission line with multiple stubs.
Figure 2.13 Matching method explanation in the case of double stubs.
Figure 2.14 The positional relationship of the magnetic field in a small recta...
Figure 2.15 Current interlinked to a small rectangle
ABCD
.
Figure 2.16 The positional relationship of the electric field in a small recta...
Figure 2.17 Magnetic flux interlinking perpendicularly to a small plane
ABCD
.
Figure 2.18 Explanation of the oblique incidence into general mediums 1 and 2.
Figure 2.19 Explanation of the normal incidence.
Figure 2.20 Standing wave distributions.
Figure 2.21 Normal incidence case at different medium interfaces.
Figure 2.22 Electromagnetic field at the medium interface.
Figure 2.23 The case where a plane wave is obliquely incident on the boundary ...
Figure 2.24 Rotation of coordinate axes.
Figure 2.25 Oblique incidence of the TM wave.
Figure 2.26 Reflection and transmission of the normal incidence in the presenc...
Figure 2.27 Illustration of multiple reflected waves and transmitted waves. (F...
Figure 2.A.1 Explanation of magnetic current. (a) The law of Ampere's circuita...
Chapter 3
Figure 3.1 Single‐layer EM‐wave absorber.
Figure 3.2 Configuration of the EM wave obliquely incident on the single‐layer...
Figure 3.3 Multilayer EM‐wave absorber. This simple and excellent analysis met...
Figure 3.4 Example of a measurement model diagram in the case where there exis...
Figure 3.5 An example of the case of a beat.
Chapter 4
Figure 4.1 Central difference.
Figure 4.2 Closed analysis area.
Figure 4.3 Electromagnetic field arrangements on the time axis.
Figure 4.4 Spatial arrangement of the electromagnetic fields when focusing on ...
Figure 4.5 Arrangement of the magnetic field around the electric field at latt...
Figure 4.6 Analysis model.
Figure 4.7 Examination of the effectiveness of the boundary condition by analy...
Figure 4.8 Analytical model for investigation of PML absorption boundary condi...
Figure 4.9 Characteristic evaluation in the case of 16 layers of PML absorptio...
Figure 4.10 Characteristic evaluation in the case of eight layers of the PML a...
Figure 4.11 Error evaluation of variable cell size part.
Figure 4.12 Evaluation of variable cell size. (The case where reflection coeff...
Figure 4.13 Relations between cell size and convergence in a small hole.
Figure 4.14 Optical path of a medium whose refractive index is continuously ch...
Figure 4.15 The image for the concept of finite element method (an example of ...
Figure 4.16 Triangular element example.
Figure 4.17 (a) Primary tetrahedron element. (b) Elements of the primary tetra...
Figure 4.18 Configuration of magnetic field irradiator.
Figure 4.19 Three‐dimensional analysis results of normalized average loss.
Figure 4.20 Surface temperature measurement by thermography. (a) Thermographic...
Figure 4.21 Configuration of the eddy current absorber.
Figure 4.22 An analytical model when an eddy current absorber is loaded on a p...
Figure 4.23 Relationship between each dimension of the absorption bolus and th...
Figure 4.24 Heat distribution by absorption bolus using conductivity obtained ...
Figure 4.25 Measurement result of heat generation distribution using unnecessa...
4.A.1 (a) Primary tetrahedron element. (b) Each tetrahedral element constructe...
Chapter 5
Figure 5.1 Crystal structure of carbon graphite. (a) Planar structure of the c...
Figure 5.2 EM‐wave‐absorbing characteristics of the charcoal in nature.
Figure 5.3 Magnetic coercive force. (a) Hard magnetic material (
H
c
: large valu...
Figure 5.4 Atomic layout of the crystal's closest packing structure. (a) Oxyge...
Figure 5.5 Electromagnetic‐wave absorption characteristics in the case when th...
Chapter 6
Figure 6.1 Examples of chiral medium structures.
Figure 6.2 Behaviors of the incident wave in a helical conductor coil.
Figure 6.3 Reflected and transmitted waves at the chiral medium interface.
Figure 6.4 Construction of the chiral medium EM‐wave absorber.
Figure 6.5 EM‐wave absorption characteristics in microwave band in a single‐la...
Figure 6.6 Geometry of the EM‐wave absorber composed of multi‐chiral mediums.
Figure 6.7 Absorbing characteristics of the EM‐wave absorber composed of 3‐ an...
Figure 6.8 Absorbing characteristics of EM‐wave absorber composed of 5‐ and 10...
Figure 6.9 Electron rotation and magnetic dipole.
Figure 6.10 Image of precession motion. (a) Explanation of angular momentum. (...
Figure 6.11 Behaviors of the real and imaginary parts of relative permeability...
Figure 6.12 Ferrite‐loaded circular waveguide.
Figure 6.13 The case of loading a ferrite in a part of a circular waveguide.
Figure 6.14 The case of loading a ferrite fully in a coaxial waveguide.
Figure 6.15 (a) Propagation constant characteristics when taking frequency (do...
Figure 6.16 Artificial material in the early days.
Figure 6.17 Metal model introduced by Pendry in the medium composition. (a) Do...
Figure 6.18 Interdigital circuit.
Figure 6.19 (a) Relation of (
E
,
H
,
k
) in right‐handed system. (b) Relation of ...
Figure 6.20 Electric field distribution.
Figure 6.21 Dielectric values, permeability values, and propagation forms. (a)...
Figure 6.22 Conversion of the material medium concept to an equivalent transmi...
Figure 6.23 Right‐/left‐handed composite equivalent transmission line. (Subscr...
Figure 6.24 Conducting thin wires to explain the negative dielectric constant....
Figure 6.25 Analysis model of double‐split rings [24].
Figure 6.26 Negative refraction characteristic.
Chapter 7
Figure 7.1 Standing‐wave measurement using a waveguide. (a) Waveguide loaded w...
Figure 7.2 Material constant measurement with a coaxial tube. (a) When one mea...
Figure 7.3 Measurements of dielectric constant and magnetic permeability with ...
Figure 7.4 General figure of a micro‐sample being put into a closed space.
Figure 7.5 Cylindrical cavity resonator for the TM
011
mode with a small measur...
Figure 7.6 Dielectric constant measurement by the TM
010
mode using a small rod...
Figure 7.7 Permeability measurement by the TE
011
mode using a small rod. (a) C...
Figure 7.8 Examples of TEM mode transmission line and waveguide. (a) Parallel ...
Figure 7.9 Reflection coefficient measurement by a network analyzer.
Figure 7.10 Explanation of standing‐wave measurement by a coaxial waveguide me...
Figure 7.11 Measurement of EM‐wave absorption characteristics using a strip li...
Figure 7.12 Example of a configuration form of a TEM cell.
Figure 7.13 Example of configuration when measuring radio wave absorber charac...
Figure 7.14 Measurement of wave absorption characteristics due to the waveguid...
Figure 7.15 Spatial standing‐wave method. (a) Layout drawing of the space meas...
Figure 7.16 (a) The case of moving the absorber to be measured up and down. (b...
Chapter 8
Figure 8.1 Principle of 1/4‐wavelength‐type wave absorber. (a) Quarter‐wavelen...
Figure 8.2 Example of a wave absorber made of 1/4 FRP structural material. (a)...
Figure 8.3 Relationship between center frequency and thickness in a
λ
/4‐t...
Figure 8.4 A
λ
/4‐type absorber using a resistive cloth.
Figure 8.5 Frequency characteristics of the reflection coefficient due to chan...
Figure 8.6 Oblique incidence characteristics of the marine radar wave absorber...
Figure 8.7 Structure of the two‐layer‐type ferrite absorber.
Figure 8.8 Frequency characteristics of the reflection coefficient of a two‐la...
Figure 8.9 Absorber consisting of granite, ferrite layer, and concrete layer [...
Figure 8.10 Characteristics due to the prototype absorber panel.
Figure 8.11 Curtain‐wall‐type absorber configuration. (a) Structural concept o...
Figure 8.12 Reflection characteristics of the EM‐wave transmission‐type curtai...
Figure 8.13 Schematic diagram of the PC board of the mounting ferrite core.
Figure 8.14 Ferrite core arrangement.
Figure 8.15 EM‐wave reflection coefficient of a specimen with ferrite cores.
Figure 8.16 Construction of a low‐reflection‐type shielding material.
Figure 8.17 Wave absorption characteristics of a low‐reflective shield materia...
Chapter 9
Figure 9.1 Structure of a ferrite microchip integrated‐type absorber.
Figure 9.2 Absorption characteristics of the ferrite microchip integrated abso...
Figure 9.3 Absorbing characteristics of ferrite microchip integrated‐type abso...
Figure 9.4 Examples of small hole shape for equivalent transformation of mater...
Figure 9.5 Rubber ferrite with small holes. (Small holes are made by laser per...
Figure 9.6 Complex relative permeability characteristics. (Type I in the figur...
Figure 9.7 Absorption characteristics when the size of a small hole changes.
Figure 9.8 Absorption characteristics when the spacing between small holes cha...
Figure 9.9 Relationship between the thickness of absorber material and the hol...
Figure 9.10 Relationship between the thickness of absorber material and the ad...
Figure 9.11 Examples of EM‐wave absorber with conductive square frame patterns...
Figure 9.12 Comparison of analytical and measured results of absorbing charact...
Figure 9.13 Permeability characteristics of rubber ferrite materials.
Figure 9.14 Analytical results of absorption characteristics when the distance...
Figure 9.15 Analytical results for absorption characteristics when the size of...
Figure 9.16 Relationship between the size of the square conductor frames and t...
Figure 9.17 The input admittance behaviors on the Smith chart when looking int...
Figure 9.18 Input admittance characteristics. (Conductor width
a
is taken as a...
Figure 9.19 Configuration examples of an absorber loaded with conductive line ...
Figure 9.20 Values of relative permeability (A: sintered ferrite, B: rubber fe...
Figure 9.21 (a) Wave absorption characteristics in a lattice type. (As the mat...
Figure 9.22 (a) Absorber characteristic with cross patterns. (b) Actual appear...
Figure 9.23 Absorbing characteristics when the tip of the cross‐shaped element...
Figure 9.24 Absorber characteristics when the conductor line width changes. (a...
Figure 9.25 Absorption characteristic when the size
b
of a square line frame c...
Figure 9.26 Absorber characteristics with adjacent space
c
as a parameter (
ε
...
Figure 9.27 Normalized input admittance (NIA) with space
c
as a parameter.
Figure 9.28 Theoretical analysis of matching characteristics of a double‐layer...
Figure 9.29 NIA characteristics in the case of a double‐layered‐type absorber ...
Figure 9.30 Integrated circuit–type absorber compatible with vertical and hori...
Figure 9.31 Analysis results of absorption characteristics when each parameter...
Figure 9.32 Analysis result of the absorption characteristics when the spacer ...
Figure 9.33 A photograph of an IC type prototype absorber.
Figure 9.34 Comparison between the theoretical analysis and measured values.
Chapter 10
Figure 10.1 Fundamental concept of ACMM construction.
Figure 10.2 Constructions of two‐dimensional and three‐dimensional unit cells.
Figure 10.3 Examples of absorber consisting of (a) two‐dimensional and (b) thr...
Figure 10.4 (a) Photograph of the initial substrate with 144 feeder lines on o...
Figure 10.5 Architecture of the 3D unit cell microwave absorber designed using...
Figure 10.6 Characteristics to be satisfied by the absorbers.
Figure 10.7 Measurement results of the incidence characteristics of the three‐...
Figure 10.8 A new configuration of ACMM absorber composed of 3D unit cells wit...
Figure 10.9 Absorption characteristics of the (a) TE and (b) TM waves at obliq...
Figure 10.11 Absorption characteristics of a new type of double‐layered ACMM a...
Figure 10.10 Configuration of the absorber for controlling the EM‐wave absorpt...
Figure 10.12 Configuration of a broad bandwidth absorber. (The dimension of th...
Figure 10.13 Example of the broadband absorbing characteristics of the ACMM‐ty...
Figure 10.14 Input impedance characteristics of the ACMM‐3D‐type absorber. (a)...
Guide
Cover
Table of Contents
Begin Reading
Pages
ix
x
xi
xii
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
145
146
147
148
149
150
151
152
153
154
155
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
317
318
319
320
321
322
323
324
325
318
Electromagnetic Wave Absorbers
Detailed Theories and Applications
Youji Kotsuka
Copyright
Copyright © 2019 by John Wiley & Sons, Inc. All rights reserved.
Published by John Wiley & Sons, Inc., Hoboken, New Jersey.
Published simultaneously in Canada.
No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, scanning, or otherwise, except as permitted under Section 107 or 108 of the 1976 United States Copyright Act, without either the prior written permission of the Publisher, or authorization through payment of the appropriate per‐copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, (978) 750‐8400, fax (978) 750‐4470, or on the web at www.copyright.com. Requests to the Publisher for permission should be addressed to the Permissions Department, John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, (201) 748‐6011, fax (201) 748‐6008, or online at http://www.wiley.com/go/permission.
Limit of Liability/Disclaimer of Warranty: While the publisher and author have used their best efforts in preparing this book, they make no representations or warranties with respect to the accuracy or completeness of the contents of this book and specifically disclaim any implied warranties of merchantability or fitness for a particular purpose. No warranty may be created or extended by sales representatives or written sales materials. The advice and strategies contained herein may not be suitable for your situation. You should consult with a professional where appropriate. Neither the publisher nor author shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages.
For general information on our other products and services or for technical support, please contact our Customer Care Department within the United States at (800) 762‐2974, outside the United States at (317) 572‐3993 or fax (317) 572‐4002.
Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic formats. For more information about Wiley products, visit our web site at www.wiley.com.
Library of Congress Cataloging‐in‐Publication Data:
Names: Kotsuka, Y. (Youji), 1941‐ author.
Title: Electromagnetic wave absorbers : Detailed theories and applications /
Youji Kotsuka.
Description: Hoboken, New Jersey : John Wiley & Sons, Inc., 2019. |
“Published simultaneously in Canada“–Title page verso. | Includes
bibliographical references and index. |
Identifiers: LCCN 2019003710 (print) | LCCN 2019011758 (ebook) | ISBN
9781119564140 (Adobe PDF) | ISBN 9781119564386 (ePub) | ISBN 9781119564126
| ISBN 9781119564126q(hardback) | ISBN 1119564123q(hardback) | ISBN
9781119564140q(ePDF) | ISBN 111956414Xq(ePDF) | ISBN
9781119564386q(epub) | ISBN 1119564387q(epub)
Subjects: LCSH: Electromagnetic waves–Transmission. | Absorption.
Classification: LCC QC665.T7 (ebook) | LCC QC665.T7 K68 2019 (print) | DDC
539.2–dc23
LC record available at https://lccn.loc.gov/2019003710
Cover Design: Wiley
Cover Image: © KTSDESIGN/Getty Images
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
Youji Kotsuka
Japan
1Fundamentals of Electromagnetic Wave Absorbers
Needless to say, learning the theory and application of wave absorbers entails learning the fundamentals of electromagnetic (EM)‐wave engineering itself. In short, this means learning about a broad range of basic matters such as the following:
(a) Transmission line theory, which will aid in understanding the fundamental phenomena of EM waves;
(b) Analytical methods to learn EM‐wave reflection and transmission phenomena;
(c) Various behaviors of EM waves;
(d) Theory of EM‐wave analysis by computer simulation;
(e) Basic knowledge of EM‐wave materials;
(f) Measurement of EM‐wave material constants;
(g) The EM‐wave environment associated with wave absorbers;
(h) Fundamental concepts of artificial materials;
(i) Knowledge of EM‐wave absorbers that can be assimilated with
artificial intelligence
(
AI
) technology, and other matters.
Even if called just an “EM‐wave absorber,” its application fields are broadly extended. Particularly, in recent years, higher frequency applications in various kinds of communication systems have advanced rapidly. However, as the frequency region becomes higher, measures against EM scattering and diffracted waves are inevitably required.
Also, as is well known, EM waves are widely used in fields ranging from communication technologies to medical applications. Therefore, the existence of a radio wave absorber plays an important role ranging from preservation of such communication environment safety down to human body protection [1].
In this chapter, in order to make it easier to understand the contents of this book, basic matters on EM‐wave absorbers are arranged from various perspectives.
After first defining what an EM‐wave absorber is, Section 1.1 briefly describes the history of EM‐wave absorber development along with the application fields.
In Section 1.2, the quantitative representation method of the EM‐wave absorption characteristic, namely, the reflection coefficient is defined. In Section 1.3, the EM‐wave absorbers are classified and described from the viewpoint of appearance, composition form, material, and frequency characteristics; these are summarized in a table. In Section 1.4, various applications of EM‐wave absorbers are introduced together with the literature. Finally, new wave absorber technologies described in the later chapters are briefly introduced.
1.1 Introduction to Electromagnetic‐Wave Absorbers
As the name of the EM‐wave absorber, “radio wave absorber,” is often interpreted conventionally. However, the expressions “electromagnetic wave absorber” or, more simply, “absorber” are, except for a special case, adopted in this book. The EM‐wave absorber refers to structures that can absorb an incident EM wave based on the principles of transforming the incident EM‐wave energy into Joule heat or canceling mutually the phases between the incident EM wave and the reflected wave.
An object that completely absorbs all light wavelengths is known as a black body, and carbon is considered as nearly a black body. As for sound wave environments, sound‐absorbing materials have been often utilized, and glass fibers, rock wools, etc. have been used as materials that absorb sound waves well. Thus, even before the EM‐wave absorber was developed, objects that can be referred to as “absorbers” have been used in various scenarios in our daily lives.
The study of wave absorbers is said to date back to the study of EM‐wave absorbers for the 2‐GHz band carried out in the mid‐1930s at the Naamlooze Vennootschap Machinerieen in the Netherlands [2].
Ever since the various types of EM‐wave absorbers were developed, mostly for anechoic chamber applications, they basically have been composed of carbon‐based materials.
During World War II, research began to be carried out, associated with the deep interest in wave absorbers for military use. For example, in the German Schornsteinfeger Project, two types of wave absorbers used for radar camouflage by mounting them on the periscope and snorkel of a submarine were developed [3]. One of the wave absorbers was made of a material called “Wesch,” in which a carbonyl iron material is dispersed in a rubber sheet. The other, namely, the Jaumann absorber [3], was one in which a resistance sheet and a dielectric (plastic plate) were alternately superimposed, as shown in Figure 1.1d in Section 1.3. In addition, in the United States, in a project organized by O. Halpern at the MIT Radiation Laboratory, with the aim of realizing a coating‐type wave absorber, theHalpern antiradar paint (HARP) was developed.
This was an EM‐wave absorber using an artificial dielectric with a thickness of approximately 0.6 mm. It had a high‐performance wave absorber with a resonance characteristic at the X‐band. Furthermore, the “Salisbury screen absorber” was also developed at the same time in the Radiation Laboratory [4]. This was a resonant‐type wave absorber, as shown in Figure 1.1b, and its structure was composed of the resistive sheet with a resistance value of 377 Ω, which was placed in a location λ/4 away from the back conductor plate.
In addition, from a practical standpoint, such as for performing measurements related to electronic devices and antenna characteristics, there is a need for an anechoic chamber. For this countermeasure, a pyramidal wave absorber capable of absorbing broadband EM waves was developed by Neher et al. in 1953. Owing to the development of this kind of a wave absorber, high accuracy has been achieved in experiments such as in the measurement of antenna radiation patterns in an anechoic chamber [5].
From a theoretical approach viewpoint, scattering waves from a planar multilayer absorber and a wedge‐type absorber aimed at use for broadband wave absorbers for anechoic chambers were analyzed. This kind of analysis was conducted by G. Franceschetti and colleagues, who introduced an approximate analysis method of Riccati differential equations and optical approximation [6].
Currently, as wave absorbers based on new concepts, autonomously controllable wave absorbers [7,8] have been promoted aggressively. In addition, wave absorbers based on the idea of a left‐handed metamaterial [9] have been proposed.
In the next section, the EM‐wave absorber is explained in detail from various viewpoints.
1.2 Fundamentals of Absorber Characteristics
The ideal wave absorber is able to absorb all incident EM‐waves, regardless of the incident wave direction, polarization, and frequency. In other words, it is an object that does not cause any reflection waves. In practice, however, an ideal EM‐wave absorber does not exist. Therefore, the performance of EM‐wave‐absorbing characteristics has been defined by the method of providing beforehand the allowable value assigned as the reflection coefficients. Usually, the reflection coefficient is defined to be −20 dB or less; when high performance is required, it is assumed to be −30 dB or less, as shown in Table 1.1 [10].
Table 1.1 Representations of reflection coefficient in wave absorbers.
Reflection coefficient (dB)
Electric‐field reflection coefficient (S)
Electric‐field standing‐wave ratio (VSWR)
Power reflection coefficient
a
,
b
−20
0.1
1.2
0.01
−30
0.03
1.06
0.001
a−20 dB means that 99% of the EM‐wave energy incident on the absorber is absorbed if converting to energy.
b At −30 dB, 99.9% of energy is absorbed.
As the quantitative value that indicates the EM absorption performance, the reflection coefficient is represented generally in decibels. This reflection coefficient can be also regarded as return loss. A value of −20 dB corresponds to an electric‐field reflection coefficient of 0.1 or a power reflection coefficient of 0.01.
From an energy viewpoint, a value of −20 dB means also that 99% of the EM‐wave energy that is incident on the wave absorber is absorbed. Also, for a reflection coefficient of −30 dB, 99.9% of the incident EM‐wave energy to the EM‐wave absorber is absorbed. Conventionally, the absorption amount of the EM‐wave absorber was evaluated using a voltage standing‐wave ratio (VSWR). Recently, the reflection coefficient or return loss mentioned earlier has been used. For the VSWR value, −20 dB is equivalent to 1.2. As a special case, in the wireless local area network (LAN) field, which has led to increased demand for wave absorbers, the acceptable reflection coefficient is regarded as −6 dB or less.
1.3 Classifications of Absorbers
As is well known, EM‐wave absorbers are classified according to various factors such as the structure, the material to be used, and the frequency band to be applied, as listed in Table 1.2. In this section, the wave absorbers related to an incident wave radiated from a far oscillator are explained – that is, the absorber against a plane wave case.
Table 1.2 Classifications of wave absorbers.
Classification
Item
Remarks
1. Material classification
Conductive material
Dielectric material
Magnetic material
Metamaterial
Special material
Materials based on equivalent transformation method of material constant
Substrate‐type material mounting an integrated circuit
Substrate‐type material equipped with autonomous‐control‐type circuit
Carbon materials such as carbon black or graphite have become a major material. Also, metal‐based material, or the like, having a resistance is used
Carbon rubber, carbon‐containing foamed urethane, and carbon‐containing foamed polystyrene, which are made by mixing carbon into rubber or urethane
Ferrite or carbonyl iron material is used mainly
Metamaterials called as left‐handed are used
Special material
By means of combinations or modifications of existing materials, the wave absorber materials create new characteristics
Wave‐absorbing material consisting of a microwave integrated circuit substrate
Material composed of active elements, sensors, and a microchip computer, on the same substrate
2. Classification by configuration form
(I) Classification from the number of layers
Single‐layer‐type wave absorber
Two‐layer‐type wave absorber
Multilayered wave absorber
EM‐wave absorber which is made from a single layer
EM‐wave absorber which consists of two layers having different material constants
Wave absorber which is constituted of three or more layers
(II) Classification by shape
Flat plate‐shaped wave
Quarter‐wavelength wave absorber
Multilayered wave absorber
Jaumann absorber
Chevron‐shaped wave
Pyramidal wave absorber
Flat configuration of radio wave incident surface
Wave absorber having the configuration where the film‐shaped resistor is placed in quarter wavelength apart from a conductive plate
Wave absorber having configurations of different layered material constants
Wave absorber superimposing alternating resistive sheet and the dielectric plate
Wave absorber composed of chevron shape at radio wave incident surface
Wave absorber composed of tapered pyramidal structure at incident side
(III) Classification by frequency characteristics
Narrowband‐shaped wave absorber
Broadband‐type wave absorber
Ultra‐wideband‐shaped wave absorber
Wave absorber having the fractional bandwidth
f
/
f
0
= 10–20% approximately.
P
value is more than 20%, and a wave absorber having peak or twin peaks.
In more than a certain lower limit frequency, the wave absorber shows an allowable reflection attenuation characteristic or less
1.3.1 Classifications by Appearance
First, let us classify the EM‐wave absorbers from their appearance [10]. There are various types of EM‐wave absorbers, such as those shown in Figure 1.1.
1.3.1.1 Single‐layer‐type Absorber
As shown in Figure 1.1a, a flat‐plate‐type wave absorber is composed of a structure in which the wave absorber surface against the normal incident EM‐wave direction is flat. A typical example of this type of wave absorber is a ferrite wave absorber.
Figure 1.1 Main classifications of the wave absorber. (a) Plane type, (b) λ/4 type, (c) multilayer type, (d) Jaumann absorber, (e) sawtooth type, (f) pyramidal type, and (g) metamaterial type.
1.3.1.2 Quarter‐wavelength‐type Absorber
A quarter‐wave‐type wave absorber is constructed by placing a conductor plate in a position a quarter wavelength away from the film‐shaped resistor, as shown in Figure 1.1b.
1.3.1.3 Multilayered Absorber
As illustrated in Figure 1.1c, multilayered wave absorbers are constructed by layering the absorbing materials to obtain the matching characteristic by adjusting each input impedance for each material stepwise.
1.3.1.4 Jaumann Absorber
As shown in Figure 1.1d, a Jaumann absorber consists of a configuration in which alternating resistive sheets and dielectric plates are superimposed.
1.3.1.5 Sawtooth‐shape Absorber
This EM‐wave absorber surface is a sawtooth type and has a kind of tapered shape, as shown in Figure 1.1e. Because of this configuration, this absorber shape has been called a chevron‐shaped absorber. Although this is a single‐polarization‐type EM‐wave absorber, it becomes possible to absorb EM waves over a wide frequency band efficiently, as with the pyramid type, which is treated next.
1.3.1.6 Pyramidal Wave Absorber
As shown in Figure 1.1d, because the pyramidal wave absorber adopts a pyramidal shape from the EM‐wave incident side, this absorber exhibits EM‐wave absorption characteristics over a wide frequency band to both polarized EM waves. This wave absorber is made by impregnating urethane foam, Styrofoam, or the like with a carbon material. This absorber has been widely put into practical use.
1.3.1.7 Absorbers by Artificial Materials and Special Materials
Recently, as described later, wave absorbers related to left‐handed metamaterials have been proposed.
1.3.2 Classifications of Material
1.3.2.1 Conductive Absorber Material
Wave‐absorbing materials that have been used since the wave absorber was invented include lossy conductive metal materials, resistive powders, and resistive films. These can be said to be typical EM‐wave absorbers. This is because they are based on the principle of changing the currents generated in the absorber by the incident wave into Joule heat. As conductive wave‐absorbing materials, there exist materials having predetermined resistance values. These are composed mainly of carbon‐based materials such as carbon black or graphite. They are widely used in the form of platelike or filmlike materials for the conductive type of wave absorber material. Furthermore, excellent EM‐wave‐absorbing characteristics are realized if using a specific conductive fabric. A typical example of an EM‐wave absorber using a resistive conductive material is a λ/4‐type wave absorber, which is a basic EM‐wave absorber configuration.
1.3.2.2 Dielectric Absorber Material
Examples of dielectric wave‐absorbing materials are carbon rubber, carbon‐containing urethane foam, and carbon‐containing expanded polystyrene. These materials are made by mixing carbon material with rubber, urethane, etc. This kind of material is used to realize broadband absorption characteristics and is applied to multilayer‐structure, wedge, or pyramid types of wave absorbers, as described earlier.
1.3.2.3 Magnetic Absorber Material
A thin wave absorber configuration can be realized using ferrite, carbonyl iron, and the like, which are magnetic loss materials available at frequencies higher than the very high frequency (VHF) band. In this case, the EM‐wave‐absorbing characteristic is strongly governed by the frequency dispersion characteristic of magnetic material and, thus, by permeability value.
1.3.2.4 Metamaterial
Recently, wave absorbers have been proposed as one of the applications of the metamaterial that is called “left‐handed.” Exploiting the idea of the left‐handed metamaterial has made possible new types of absorbers that do not require a back conductor plate [9], and terahertz band absorbers have been suggested. Further, EM‐wave absorbers based on a novel configuration concept have been proposed, and they are summarized as follows.
(a) To realize new EM‐wave‐absorption characteristics, an absorber based on the idea of equivalently converting material constants by means of loading some kind of metal pattern on an existing material surface or making small holes, and the like has been proposed (see
Chapter 9
). These methods are unified as the “equivalent transformation method of material constants.” By introducing this concept, wave absorbers much thinner than the conventional ones can be realized [
11
,
12
].
(b) A new wave absorber is a type composed of a microwave integrated circuit. This wave absorber has a simple, yet lightweight, structure, and the broadband‐absorbing characteristics can be realized effectively, even beyond the microwave frequencies [
13
,
14
].
(c) An autonomously controllable metamaterial‐type wave absorber is a wave absorber based on a completely new material concept; thus, its structure is composed of a type of artificial material that is equipped with the active element circuit, sensors, and microchip computer on the same substrate [
7
,
8
].
The concept of this material configuration is based on the autonomy of living tissue, as is described in Chapter 10.
1.3.3 Classifications by Configuration Forms
Furthermore, the wave absorber is categorized from the viewpoint of the “number of layers” constituting each absorber layer and the “shape of appearance” in the absorber structure.
1.3.3.1 Classification from Layered Numbers
(a) Single‐layer‐type absorber
An absorber made from a single‐layer material is called a “single‐layer‐type wave absorber.” Normally, a metal plate made of aluminum, copper, or the like is attached to the back of an absorber. This type of EM‐wave absorber can be seen in those using ferrite, carbonyl iron material, and other such materials.
(b) Two‐layer‐type absorber
This is a wave absorber that has two layers composed of different material constants. This configuration is often introduced when aiming at improving a single‐layered absorber's characteristic to realize a more broadband absorber characteristic.
(c) Multilayered absorber
The multilayered wave absorber is usually a wave absorber consisting of three or more layers. In the multilayered wave absorber, the wideband characteristics are obtained by increasing the number of layers, and this kind of absorber can be used, for example, for an anechoic chamber.
1.3.4 Classifications by Frequency Characteristics
Regarding the quality of the EM‐wave absorber characteristic, the “goodness of absorption characteristic” is defined by introducing the idea of a “figure of merit [10].”
For example, when evaluating a reflection coefficient below −20 dB as a good EM‐wave absorber, if the bandwidth cut by the level of −20 dB values is assumed to be Δf, by dividing the bandwidth values Δf with the center frequency f0, the figure of merit can be defined as Δf/f0. This characteristic is mainly classified into the three types shown in Figure 1.2.
Figure 1.2 Classification by frequency band. (a) Narrowband type, Δf/f0 × 100 = 10 – 20%; (b) broadband type Δf/f0 × 100 = 20 – 30%; and (c) ultra‐wideband type Δf/f0 × 100 = 30% and above [10].
1.3.4.1 Narrowband‐type Absorber
This is usually associated with the case of the characteristic that can be found in a single‐layer wave absorber, or the like. In this case, if the figure of merit is expressed as a percentage, it is approximately 10–20%, as illustrated in Figure 1.2a. When a narrow frequency band is needed, as in the case of a radar application, this type of absorber is used.
1.3.4.2 Broadband‐type Absorber
Notice that the distinction between the case of the wideband and the narrowband types is not clearly defined. In the case where the percentage of Δf/f0 is not less than 20%, the EM‐wave absorbers often show a peak or twin‐peak characteristic, as shown in Figure 1.2b. These cases are generally referred to as “broadband wave absorbers.”
1.3.4.3 Ultra‐wideband‐type Absorber
This type of absorber possesses a wideband‐absorbing characteristic in the above‐assigned absorbing frequency, which is set beforehand. This results in an absorption characteristic of a type in which the lower limit frequency of the allowable reflection coefficient to the EM‐wave absorber meets the specified frequency.
In other words, it is an EM‐wave absorber with broadband characteristics that can absorb EM waves above a frequency determined beforehand, as depicted in Figure 1.2c. Of course, because Δf/f0 becomes infinite, the definition of the figure of merit cannot be used in this case. In general, the multilayer absorber, wave absorber of the saw‐tooth shape, and pyramidal shape exhibit this kind of property.
1.4 Application Examples of Wave Absorbers
The examples of the main application of the wave absorber and the related materials used therein are given in Table 1.3. As shown in Table 1.3, the EM‐wave absorber application fields are expanding along with the development of communication technologies.
Table 1.3 Examples of main wave absorber use.
Application examples
EM‐wave absorber, and material used
For anechoic chamber (more than 30 MHz)
Wave absorber of combination multilayer structure with carbon‐based material and ferrite
Pyramid‐type wave absorber material being produced by mixing carbon in urethane foam material, or wave absorber in which sawtooth type unit absorbers made of a carbon material are arranged alternately in vertical and horizontal directions.
Ferrite single‐layer wave absorber (simplified type)
Electromagnetic‐wave absorber material that is composed of a combination of a dielectric comprising metal fiber material and a ferrite
Improvement of radar characteristics
Absorber material using sintered ferrite
Absorber material of rubber ferrite
Absorber material composed of a nonwoven fabric and metal fibers
For high‐rise building wall (TV ghost prevention measures, 100 MHz – an example of the old analog broadcasting)
Absorber material of ferrite tile
Absorber material using ferrite and dielectric combination
Absorber material mixing ferrite grains into concrete
For electromagnetic interference prevention (for prevention of leakage wave of a microwave oven, wireless LAN measures) (2.45 and 5.2 GHz)
Wave absorber using rubber ferrite
Wave absorber using resin ferrite
Wave absorber composed of carbon‐based dielectric material and building materials
Wave absorber composed of resistance film‐based materials and building materials
Wave absorber composed of ferrite and building materials
Countermeasure for electronic circuit noise (10 MHz to 5 GHz)
Sheets composed of special magnetic materials and electrically conductive material
Insulation sheet which has ferrite powder mixed with polymer
Composite material made from metallic flat powder
Small cylindrical‐shaped ferrite
For mobile communication (malfunction prevention measures of electronic automatic billing system, 5.8 GHz)
Wave absorber material consisting mainly of ferrite
Foam material containing a conductive material
Wave absorber material containing metal fiber in nonwoven fabric
Wave absorber material coated with a conductive paint on synthetic fibers
Paved road wave absorption material consisting of carbon fiber and asphalt material
As shown in Table 1.3, with respect to the wave absorbers of the anechoic chamber described previously, many studies have been conducted, and much research that incorporates the latest analysis has been published. For example, with respect to the pyramid‐type or wedge absorbers, research based on theory and experiments [18], the moment method [19,20], and the frequency‐domain finite‐difference method [21] have been reported. Further, careful experimental studies have been conducted to study the anechoic chamber [22–24].
In addition, as one of the main application fields of the EM‐wave absorber, the topic of radar technology improvement has been examined. Particularly, applications of the EM‐wave absorber associated with radar problems have been studied from the early stages of wave absorber development [25–30
