6.1 Introduction

Nowadays, wearable antennas have become the important area of research. The wearable antennas requirement is increased due to the miniaturization process and the use of flexible devices. The wearable antenna is defined as the miniature antenna which can be worn by the wearer [1]. The wearable antenna allows the cloth or any other device to be a part of the communication system [2] and they are light in weight, highly flexible, and can be easily integrated with the clothes and they are comfortable to wear. The wearable antenna is used in different applications which include medical applications, defense applications, computing, and wearable electronic and IoT. There is a vast research utilizing the UWB frequency band. UWB band can be used for broadband transmission. The antennas with fractal geometry and conventional antenna structures are applicable for RFID tags [3].

The IoT usually refers to a group of devices which can sense, accumulate and switch information using internet as a medium besides any human intervention. Recently, wearable units are swiftly rising and forming a new segment––“Wearable IoT (WIoT)” using wearable material antennas due to their functionality of sensing, computing and communicating [4]. The wearable IoT technology helps the person to interact and communicate with the network through the clothes or other wearable devices via the application and the network layer. For wearable textile IoT systems, integrating antennas into textile materials helps the clothes end up a smart interface between the consumer and the network.

Internet of Things technology is useful in each and every aspect of the today’s world which includes smart cities, smart homes, and connected health systems. Wearable technology is playing an essential role in enhancing the features of the smart projects by interconnecting the smart devices with the networks. So, IoT wearable technology smart devices will be highly approached devices in the coming years. It aims to enhance the living standards of the IoT users with the help of smart devices.

Wearable IoT devices can be helpful in the medical systems with the use of heart rate measurement, etc. These devices make use of the sensors which can sense and collect the required information and interact with the user and the network.

Wearable technology helps in living the smart and the healthy life. By the collaboration of the wearable and IoT technology, the smart devices can behave as most reliable devices for the IoT technology users.

Another feature of the wearable technology is the smart clothing. Wearable technology-based clothing is applicable in medical, defense, lifestyle, sports, security system, etc. Therefore, nowadays, IoT technology is working with different textile brands for developing the smart textile wearable IoT devices.

The antenna is the main component of the wearable devices. The various researches have been done in improving the antenna designs. It includes the use of metamaterials, EBG, defected ground planes, reconfigurable antennas, electro-textile antennas, etc. The main emphasis is on the textile and the fractal antennas so that the advanced wearable multiband antennas can be designed. The antenna geometries of different types of antennas highly affect the performance behavior of the antenna.

The flexible antennas’ requirement has paved a way for the antenna designs with the textile materials. The different types of textile materials are used as a substrate in designing the wearable antennas like cotton, foam, jeans, polyester, and nylon. These textile antennas are fabricated using different textile substrates for the wireless systems including Wi-Fi, WiMax, BAN, Bluetooth, and WLAN applications [5]. The human body behaves as a lossy medium which affects the performance behavior of wearable antenna. So as to plan a body wearable antenna, the structural parameters and diverse electromagnetic properties of substrates ought to be wisely considered [5]. It is also required to take care of the SAR values of the wearable antenna so that back radiation cannot harm the human body. In the textile antenna designs, there is use of textile material as a substrate material and electrically conducting materials are used for ground plane and microstrip patch. These conducting materials should have a low electric resistance to minimize the losses [6]. Thus, for achieving the necessary performance of these antennas, electromagnetic properties of the materials should be taken into consideration. There is possibility that many researches may come into existence in the near future which may cause many electronics to be built into the clothing. The idea to be taken into consideration, the embroidered antenna came into existence. The embroidered antennas are also more comfortable and highly durable design for the smart clothing [7].

In 1988, first fractal antenna was worked by Dr. Nathan Cohen. Fractal antennas have self-comparable and self-repetitive qualities. The thought behind fractal antennas originated from designs existing in nature. They have space filling properties that used for structuring antennas for wideband behavior. Fractal antennas are the mix of antennas that are working at various frequencies with a compact size [8]. The miniature antennas can be integrated into smart clothing for various telecommunication-based applications. The hybrid antennas came with the advanced features with highly promising results in the field of fractal antenna technology. In hybrid fractal antennas, the various fractal geometries are fused together to design an antenna.

This chapter provides insight information about the wearable antennas. The wearable antenna system is fast growing field in various wireless and health monitoring applications and paves the way for upcoming technology. To achieve the wider bandwidth and multiband characteristics, the different fractal antennas with the textile substrate can be designed as multiband wearable antennas. A multiband antenna is an antenna which is manufactured to operate in different range of frequency bands. The design of multiband antenna is such that the different parts of the antenna are working for different frequency bands, respectively. The multiband antennas are useful for different applications like RFID (0.924 GHz), GSM (1,710–1,785 MHz and 1,805–1,880 MHz), GPS (1.575 GHz), Wi-Fi (2,400 MHz), ISM (2.4–2.4875 GHz), WLAN (2.45 and 5.8 GHz), Wi-MAX (3–3.63GHz), UWB (3.1–10.6 GHz), two-band MBOFDM (3.1–4.8 GHz), SWB (4.3–29.6 GHz), and DS-UWB (6–8.5 GHz) [4, 5, 8]. The multiband antennas gain is low and they are physically large as compared to single-band antennas to accommodate the multiple bands. It is required to study different geometries of fractal antennas, conventional antennas, hybrid fractal antennas, and textile antennas. By understanding in depth the behavior of such antennas, new correlation can be achieved about the geometry and performance of the antenna to design advanced wearable textile antennas.

6.2 Types of Antennas

Wearable miniaturized antennas are a fundamental component of each and every wearable 5G, IoT, and Medical applications. Antenna converts RF frequency into electrical signals and vice-versa. There are various types of antennas depending upon the transmission and the reception requirements.

6.3 Design of Wearable Antennas

There is an intense work and research has been done in the field of wearable electronics. So, there is a great demand for the adjustable electronic designs. These designs require special focus and they have several utilizations especially for health monitoring and commercial wireless communication systems. Several frequency band allocations have been allocated for different applications. Table 6.1 describes the chronology for the wearable antennas which considered the advancements in the antenna designs and the intense research work being done in the area of wearable technology.

There are different techniques being used for designing the antennas based on the requirements. The structure of the antenna adversely affects the functioning of the antenna. So, while designing the antenna, the geometry of the design plays a vital role. There are various methods are described as follows.

6.3.1 Effect of Substrate and Ground Geometries on Antenna Design

6.3.1.1 Conducting Coating on Substrate

The antenna performance is affected by the conductive parts of the substrate. Using the immensely conductive coating on textile substrate causes the reduction of losses. In [34], the dielectric substrate consists of Polyamide lossy (Nylon-6) while the of nickel-copper-nickel coating was used as coating on the textile substrate for the conductive parts, i.e., patch and ground plane as shown in Figure 6.1. The design worked on the description of fives layer of human body model which consists of textile, air, skin tissue, bone, and fat layer as shown in Figure 6.2. The antenna provided good bandwidth by working descriptively on the five-layer model.

Year	Antennas used
2004	A Novel Circularly Polarized Textile Antenna [19]
2006	Textile Patch Antenna [20]
2007	Aperture- Coupled Patch Antenna [21]
2008	Electro-Textiles Wearable Antenna [22]
2009	Body-worn E-textile antennas [23]
2011	Sierpinski Carpet Fractal Antenna [24]
2012	Terahertz Microstrip Antenna on Photonic Bandgap Material [25]
2013	Dual resonant shorted patch antenna [26]
2014	Metamaterial-based wearable microstrip patch antennas [27]
2015	Wearable Rectangular Patch Antennas with Partial Ground [28]
2016	Wearable Antennas applicable for Tele-Medicine [29]
2017	EBG-based Textile Antenna [30]
2018	Textile Antenna Arrays for smart clothing applications [31]
2019	Wearable Antenna with Circular Polarization using NinjaFlex-Embedded Conductive Fabric [32]
2020	Wearable EBG-Backed Belt Antenna [33]

A multiband antenna designed with magnetic properties applicable in medical field, RFID, WLAN, and wireless monitoring. In [35], the conducting material was applied on the patch and the ground plane, while the antenna substrate was non-conducting textile. The human body has lower conductivity and permittivity values, so when the antenna is brought closer to the human body, the electric field’s electrical impedance is reduced while there is increase in magnetic field. Therefore, magnetic type antennas are very suitable for close to the body applications. The antenna performance parameters like gain, bandwidth, resonant frequency, and radiation pattern are not changed in the closeness of the human body due to the magnetic properties of the antenna design. Table 6.2 describes the SAR values that are measured at different distances from the phantom in order to investigate the change in the SAR value by positioning the antenna at near and far positions from the body [35].

Distance from human body	SAR value (W/kg)
(mm)	Freq. 0.923 GHz	Freq. 2.44 GHz	Freq. 5.81 GHz
0	0.11	0.41	0.52
5.12	0.03	0.40	0.10
10.2	0.01	0.33	0.09
20.5	0.00	0.22	0.03

6.3.1.2 Ground Plane With Spiral Metamaterial Meandered Structure

In various researches, different implementations of the MTM design structures have been demonstrated. In [36], the design was introduced with the use of spiral metamaterial meandered structure in the ground plane, to decrease the SAR of the antenna. The antenna is fabricated by using photolithography technique. The use of MTMs greatly helped in decreasing the SAR value and getting the results according to international safety standards (FCC and ICNIPR). Figure 6.3 describes the antenna design with MTM cells.

Table 6.3 explains the SAR values with the change in distance from the human body. With the increase in the distance from the human body, SAR value is decreasing. The specific absorption rate (SAR) value describes the absorption rate of radiation by the human body on their exposure to radio waves.

Resonance frequency (GHz)	SAR(W/kg)
	10 mm	20 mm	30 mm	40 mm	50 mm
1.57	0.452	0.321	0.241	0.1914	0.155
2.7	1.1	1.02	0.898	0.786	0.662
3.4	0.67	0.532	0.398	0.294	0.218
5.3	0.75	0.5318	0.407	0.303	0.278

6.3.1.3 Partial Ground Plane

There are various design techniques for ground plane which can be adopted. The antenna can be designed with the full ground plane or the defected ground plane and the antenna behaves differently for different ground geometries. In [37], the antenna was designed on denim material with partial ground plane and rectangular patch with triangular cut as presented in Figure 6.4. The partial ground plane can alter the antenna performance parameters into the desirable measurements. The antenna was designed to work on the frequency of 2.4 GHz. The measured impedance bandwidth ranges from 1.948 to 4 GHz. The antenna design was tested at different bent angles as shown in Figure 6.4.

Parameters	Antenna’s bending (cylindrical diameter)
	Flat dimension ∞	10 mm	20 mm	30 Mm	40 Mm
Resonant Frequency in GHz	2.4	2.11	2.16	2.18	2.17
Impedance Bandwidth in %	68.9	72.7	72.9	72.4	72.3

The comparison of impedance bandwidth at different bending positions is described in Table 6.4. It is proved that the impedance bandwidth changes with the change in the bent conditions. If the cylindrical diameter was increased, then the resonance frequency also increased accordingly [37].

6.3.3 Embroidered Antenna

Another form of antenna geometry is embroidered conductive yarn antenna with wearable electronics. In the embroidered antennas, there are two types of loss which includes the loss with the vicinity of the human being and the loss due to conductive yarn like Silverpam. In [40, 41], the antenna design included the different sewing patterns. The measured radiation efficiency is same as that of referred copper antenna. Using these sewing patterns, radiation efficiency can be increased and the conductive yarn usage can be reduced which further help in reducing the losses.

While constructing the embroidered antenna, firstly, the embroidery is done on the textile material and then the embroidered part is cut and pasted on the dielectric material with the help of adhesive sheet. The embroidery is done using the SWF MA-6 machine. The stitches pattern and the density of the stitches make a big difference in the behavior of the antenna characteristics.

6.3.4 Wearable Antenna Based on Electromagnetic Band Gap

The electromagnetic band gap (EBG)–based wearable antenna design describes the compact and unremarkable method to fulfill the need of the wearable technology. To avoid losses due to presence of human body, the EBG structure is used as it reduces back radiations. The EBG material acts as a resonant cavity. The quality factor Q increases the gain of the antenna with EBG patches. There are various methods by which the EBG patches can be designed. Some designs include EBG with FSS which consists of cuboids, slotted cylinders, cones, etc. The antenna design using EBG patch is shown in Figure 6.6. This antenna resonates at frequency of 2.4 GHz and used for medical applications. There is 95% improvement in SAR value due to the effect of EBG [41]. The antenna simulation results in 7.8 dBi gain, 2.17–2.83 GHz impedance bandwidth. In [42], wearable coplanar antenna was designed which was combined with an EBG surface and it operated at 2.45 and 5.5 GHz frequency bands. Even the small EBG-based structure is helpful in the reduction of back radiations and SAR values.

6.3.5 Wearable Reconfigurable Antenna

An antenna with reconfigurable feature is able to change its performance dynamically, in a controlled method. Reconfigurable antennas can be designed by integrating PIN diodes or MEMS devices into the geometry of the antenna. In [43], the O-shaped antenna design used a small slot structure with dimensions of 11 mm × 11 mm, so it was helpful for on-body application due to its small structural design. The copper was used as a conducting plane which is integrated with foam. Steps 1 and step 2 are, respectively, considered as the ON and OFF mode which works at different frequency bands as shown in Figure 6.7. The ON mode resonated at 2.38 to 2.52 GHz, and for OFF mode, antenna resonated at 5 to 5.5 GHz.

6.4 Textile Antennas

The textile antennas are the antenna designs which uses textile material as a substrate and the conducting and the ground plane are designed with any conducting materials. The antenna should be of small size and low weight. The antenna should be capable of providing wide bandwidth and the proper radiation pattern meeting the requirements of the application. Textile materials that are used as an antenna’s substrates are of two types: natural and man-made fibers.

In [44], the three patch antennas with rectangular patch were designed using different textile substrates. One antenna design which used cotton substrate measured a reflection coefficient of −15 dB, antenna using jeans substrate showed reflection coefficient of −17 dB, and antenna with silk substrate measured reflection coefficient of −12 dB at 2.44 GHz. The proposed antennas are useful in medical applications for patient monitoring and the interconnection between PAN devices.

The electromagnetic properties of the textile substrate should be analyzed before the antenna design. The change in the properties of the substrate also cause change in the dimensional geometry of the antenna. The antenna with different substrate materials resonates at different frequencies and thus changes its band of application.

The complex dielectric permittivity of the fabric is given by ε = εo (εr1 − jεr2) where εo = 8.854 × 10–12 F/m is the permittivity of the vacuum. The dielectric constant of the textile material is the real part of the permittivity, εr1. The permittivity of the textile materials is close to 1 because they are porous materials and they have low dielectric constant. The antenna bandwidth of the textile antenna can be changed by variation in its permittivity but by lowering the substrate permittivity, the resonance frequency can be increased.

The different textile substrates with their electromagnetic properties are listed in Table 6.5.

In textile wearable antennas, there are different textile materials which can be used as the substrate. Textile materials that are used as an antenna’s substrates are of two types: natural and man-made fibers. The antenna should not be affected by the near field effects of the human body. If the antennas are insensitive to the body effects, then it reduces detuning and improves the battery life time.

The impacts of addition of layers of cloth on the behavior of wearable electro-material UHF RFIDs was studied [45]. Figure 6.8 presented the wearable dipole antenna design dimensions. The electro-textile label antennas were pressed on the cotton T-shirts with the help of hot-melt adhesive and covered with a stretchable defensive encapsulant. The behavior of the labels was assessed on-body in different conditions. Two kinds of winter covers on the T-shirts were studied to analyze the impacts of wearing over the RFID tags. It has been proved that including a thick layer of coat on the shirt also does not prevent the tag from working yet diminishes the top read extend from 7 to 5 meters. The manufactured electro-material tags can be read from distance of 2 to 5 meters, all over the range of UHF-RFID frequency band. Figure 6.9 describes that the thick winter coat had the same impact on the readable distance as the thin coat.

Textiles are able to communicate with the outer world without any requirement of costly equipment. These textile antennas are small in size and flexible antennas, so they can be integrated in any type of sensors and accessories. Therefore, the textile wearable antennas can be applicable for the IoT systems. Textile antennas embedded into clothes provides wireless interface for the IoT applications.

The wearable antennas are the antennas which are easy and comfortable to wear. The antenna designs use different materials as a substrate in their designs. If the substrate material used is the textile antenna, then it proves to be the comfortable wearable antennas. Based on the various textile antenna designs, the description of effect of the substrate materials on the antenna performance is described in Table 6.6.

The textile materials have the permittivity and loss tangent values which are not readily available. The textile materials are also more inhomogeneous materials than the other high-frequency counterparts. The antennas which are made up of textile/foam substrates offer high flexibility but introduce additional losses. To reduce losses, the electric resistance of the conducting materials is required to be low and stable. The increase in impedance bandwidth is due to increase in spatial waves. Textile fabrics’ thickness and density are variable with the low pressure as the textile materials are porous and compressible materials. Textile fabrics’ thickness is also an important parameter, so the thickness of the fabric material should also be precisely chosen while designing the textile antennas.

6.5 Comparison of Wearable Antenna Designs

A comparative description of the various wearable antenna designs is presented in Table 6.7. The wearable antenna design required to be adhering with FCC standards regarding the safety of the human body by the wearable electronics. Hence, while designing the antennas for wearable applications, the interfering effects should be taken into consideration. As seen, energy antenna performance parameters like bandwidth, gain, and return loss distinguish the various antenna designs from each other. Some antenna designs used textile material as a substrate; others used non-textile material like copper and bronze. The textile materials are popular among them.

6.6 Fractal Antennas

The numerous researches have been done in the field of wearable antennas which are applicable in different areas. The fractal antenna designs came into being in which single antenna is useful in different frequency bands. The fractal antenna refers to the self-similar structures. Fractals are usually utilized to review natural objects. Fractals are the space fillers; hence, this property helps in accurately packing into small size. The powerful packing technique is helpful in the miniaturization if the antenna. The scaled geometry of fractal antennas help in designing antenna to resonate at different frequency bands. Fractal antennas have many applications in wireless communication system. Fractal antennas can be designed using various shapes. For example, a quarter wavelength monopole can be transformed into a similarly miniaturized antenna by using the Koch fractal shape.

Wearable antenna can be designed using a microstrip antenna which is used in Body Area Network. The different antennas are designed using fractal geometry but using different textile substrate and keeping same dimensions for antennas. The antenna designs used different textile materials as a substrate. These antennas provide the multiband operations at different resonant frequencies. There are various researches have been done in the field of fractal wearable antennas. The different papers published on wearable antenna designs using fractal geometries.

Low profile antennas play important role in the development of 5G communication systems and IoT. There is a need of the development of miniaturized antennas with high efficiency and large gain values for 5G and IoT applications. The antenna designs with the metamaterials and fractal geometries are helpful in gaining high efficiency than the other antenna geometries.

6.6.1 Minkowski Fractal Geometries Using Wearable Electro-Textile Antennas

In [46], the Zelt and Flectron two electro-textile materials, and the polyester fabric material as a dielectric medium were used. For 132 MHz, the gain provided by Flectron-based antenna was 6.54 dB and the Zelt antenna provided 7.4-dB gain with 104 MHz of impedance bandwidth. The gain efficiency for Zelt antenna is described in Figure 6.10. The miniaturization was achieved by using Minkowski structure for the antennas. The first iteration antennas were applicable for WiBro frequency band and the next iteration antennas were more miniaturized and tuned to GSM 1900 applications. Zelt-based antenna provided far better results than the Flectronbased antenna.

6.6.4 Development of Embroidered Sierpinski Carpet Antenna

In fractal geometry also, various embroidered antenna designs have been developed. In [49], the embroidered antenna design included two different textile materials as substrate materials. The geometry of the antenna is based on the Sierpinski carpet with successive iterations as shown in Figure 6.12. The silver-nylon yarn conductive threads were used in radiating and the ground planes as described in Figure 6.13. Both antennas showed miniaturization effects, flexibility, and omnidirectonal pattern. The Felt antenna showed gain of 7.9 dB and 90.47% efficiency, whereas the jeans material showed 7.1 dB gain and 89% efficiency.

This exploration work will open a way for planning fractal antennas by using other fractal geometries. According to the requirements of the wearable systems, fractal antennas are useful in multiband frequencies for different areas of wireless applications and IoT market [51]. The wearable fractal antenna technology helps us to design miniaturized, comfortable to wear, and multiband antennas with large bandwidth into a single device.

6.7 Future Challenges of Wearable Antenna Designs

The process of designing wearable antennas has to face various challenges as these antennas have a direct touch with the human body. The lower the value of SAR of Multiband Magnetic Type Wearable Antenna, the lesser the quantity of radiation is absorbed by body tissues. There are some threshold values above which the SAR values are not safe for the human exposure [50]. The standard values for SAR are 2 W/kg for 10 g and 1.6 W/kg for 1 g of body tissues. As the antennas come nearer to the human body, antenna performance parameters’ responses get changed. In wearable antennas, omnidirectional radiation patterns are desirable so that the antennas can be placed at any position on the human body. The antenna should be flexible and of small size so that the sensors can be integrated in the antenna. Although the antenna is required to be efficient and immune to be detuned by the human body effects, the antenna can be incorporated with re-configurable feature also so that the antenna behavior can be further improved. The angle of bending plays a crucial role in the antenna performance. Being a wearable antenna, the location of the antenna affects the antenna behavior due to various factors like bending, twisting, and wrinkling. So, while designing the wearable antennas, it is important to test the antenna according to the location of the human body where the antenna is required to be used according to the application.

The efficiency of the antenna ought to be improved to increase the battery life span of the wearable antenna. By decreasing re-transmissions, the battery life can be increased. The higher gain and pattern diversity is linked with the link budget. The rise in the link budget improves the battery life of the antenna. Antenna performance parameters aside from electrical resistance, like radiation efficiency, gain, and radiation patterns, are also adversely influenced by human body existence in the nearby vicinity [10]. It is needed to compare the free space which is zero absorbing material with that of the antenna efficiency within the presence of an absorbing material like human body.

6.8 Conclusion

Based on the survey, it has been predicted that there are numerous performance parameters for concern in designing the antennas which includes bandwidth, return loss, bending performance, radiation characteristics, and gain. It has been predicted that the geometry of the antenna design also affects the performance behavior of the antenna. The substrate height, material properties, and the design dimensions are the main parameters to be taken into consideration while designing the wearable antennas. The geometry of the antenna design specifies the resonance frequency and the application bands for the specific antenna design. From different antenna designs, it has also been observed that the SAR values reduced as the distance of the antenna increases from the human body.

Textile antennas act as a vital part in today’s wireless communication in the areas of tracking, navigation, medical, IoT, and computing application. Due to the low dielectric constant value of textile material, there is an increase in the spatial wave which causes increase in the impedance bandwidth of antenna. The textile antenna can be embedded into the clothing, easy to wear, comfortable, and light weight antennas which proves them to be a quite helpful in the wearable antennas market. There are large number of fractal antennas which are helpful in multiband frequency applications like medical, UWB, SWB, WiFi, WiMax, Bluetooth, 5G, IoT, WiBro, and GSM. The fractal antenna has an advantage that the single antenna can be used for multiple frequency band applications. The different hybrid fractal antenna design geometries and other conventional antenna designs studied so far can be redesigned as textile wearable antennas by using the textile materials as substrates. A comparative description of the different textile substrates and wearable antenna designs is presented in a categorized way with the expectation of providing guidance for concerned researchers.