## 2 Background of Electromagnetic Absorption  
Various electromagnetic waves are widely used in many areas, making our daily lives convenient [6]. For example, a wide variety of electromagnetic wave-based electronic devices such as mobile phones, WiFi, Bluetooth, Near Field Communication (NFC), and wireless charging have been developed and play important roles [7; 8]. So, the widespread practical application of various electronic devices and dense systems will cause electromagnetic interference (EMI), which might cause more and more serious problems, such as the malfunction of electronic devices and effects on the human body [9]. Moreover, the wireless devices we use generate unwanted electromagnetic waves that affect other wireless devices [10]. In recent years, as represented by fifth-generation mobile communication systems (5G) and autonomous driving, the frequency band of electromagnetic waves used has been expanded to high frequency bands including millimeter waves. Therefore, there is an urgent need to develop electromagnetic shielding materials and establish evaluation methods [11]. Accordingly, the development of the latest radio wave absorbers and electromagnetic shielding materials using artificial materials that do not exist in the natural world, and evaluation methods, are being researched [12; 13]. The artificial material referred to here is a two-dimensional or three-dimensional periodic arrangement of metal wires, metal patterns, and so on. Their radiometric characteristics are significantly different from those of ordinary conductive materials, dielectrics, and magnetic materials existing in nature. Figure 1: Illustration of SWCNTs and MWCNTs and a photo of dispersed SWCNTs (SEM 5000\(\times\)). In the field of electromagnetic wave-shielding research, electromagnetic wave-shielding measures for suppressing noise caused by electronic devices and electrical automobile components, electromagnetic wave radiation reduction measures for semiconductor packages, and electromagnetic wave-shielding measures for medical facilities and studios have been implemented. Research on shielding measures in necessary facilities and evaluation methods for electromagnetic wave shielding performance are also being conducted [14]. In addition, compared to the fourth-generation mobile communication system (4G) and wireless LAN radio, the fifth-generation mobile communication system (5G), which shifts to a higher frequency band, uses millimeter-wave band frequencies, meaning radio waves can be used. Due to the strength of straightness, it seems that the shield has a large effect on the radio wave propagation characteristics. Since this will have a great impact on indoor and outdoor coverage areas, research on the analysis and evaluation of the radio wave environment with consideration to the shielding-loss characteristics of vehicles and human bodies is also underway [15]. On the other hand, in the field of radio wave absorber research, it is ideal to have a wide band and high absorption performance. However, recently, a radio wave absorber with a narrow band absorption performance specialized for a specific purpose and having various added values such as thinning and weight reduction has been proposed [14; 15; 16; 17]. Furthermore, in the research field on the evaluation of material constants of absorbent materials, various measuring methods for a high frequency region which have been difficult to measure in the past have been proposed. This has become possible since the performance of measuring instruments such as vector network analyzers have been dramatically improved. In this way, research and development of electromagnetic wave absorbers and electromagnetic wave-shielding materials are expected to become more and more important as their use in various fields expands in the near future. Currently, a sheet type of commercialized electromagnetic wave-shielding material, a mixture of rubber and carbon-based material, is the mainstream [14; 18; 19; 20; 21; 22; 23].