Wearable antennas are used for wearing either on humans or animals. They operate near the human body, which affects various parameters, like impedance mismatching and detuning effects. Currently, various studies have been conducted to design various types of wearable antennas. Examples of antennas include the monopole planar antenna [
261], planar inverted-F antenna (PIFA) [
262,
263], microstrip patch antenna [
264], magneto dipole antenna [
265], substrate-integrated waveguide antenna (SIW) [
266], electromagnetic bandgap (EBG)-type antenna [
267], dipole antenna [
268], fractal patch antenna [
269], and cavity slot monopole antenna [
270]. Similarly, a Yagi-Uda antenna was also used for WBAN in the millimeter-wave (mm-wave) band [
226]. A wearable antenna is always encouraged to yield a small form factor in WBAN systems [
271]. Additionally, they should be multiband, flexible, and stretchable, so that they can be used for various applications. The antenna miniaturization is usually related to physical and electrical properties. These miniaturization techniques can be divided into two groups, such as material and topology method. Each group contains various types and shapes, such as space-filling curves, meander lines, fractal shapes, metamaterial surfaces (MT), and high dielectric materials [
102]. Recently, button-shaped antennas and single and multiband antennas using MT are widely used in the WBAN systems. These techniques have minimized the coupling effect between the body and antenna and reduced the backward radiation. Thus, miniaturization techniques resolved all the existing challenges and maintained the compactness of the design. This section briefly explains button antennas and single and multiband wearable antennas.
4.1. Button Antennas
The button antenna is a new design, which can be mounted on textile materials at various locations on-body. The button antenna is usually a rigid surface, which promises a better radiation characteristic as compared to textile antennas [
272]. In [
273], a cuff button antenna was designed for WLAN, which included a conductive G-shaped patch. It was connected with a cylindrical base by a cylindrical metallic tube and mounted on a Velcro substrate. This antenna provided an omnidirectional pattern. In [
274], a button antenna was designed on a pair of jeans for rescue operations at 2.4 GHz and the HiperLAN bands. This design was used to study body propagation, such as the line-of-sight (LOS) and non-line-of-sight (NLOS) channels. In [
275], a new wideband button antenna was proposed at 2.4 GHz using characteristic mode theory, as shown in
Figure 9a. The design demonstrated excellent performance in free space with BW, gain, and radiation efficiency of 658 MHz, 1.8 dBi, and 97%, respectively, whereas for on-body, it achieved the BW, gain, and radiation efficiency of 788 MHz, 5.1 dBi, and 71%, respectively. Additionally, SAR of 0.45 W/kg at 20 dBm (input power) was obtained. In [
276], a novel circularly polarized button shape antenna was designed for a broadside pattern at 5.47–5.725 GHz (UNII Band), as shown in
Figure 9b. The design was based on a button-shaped FR4 substrate with a radiating element on both sides. On the top side of this substrate, a large and a small radiating circle was used with the cylinder being fixed on the small circle, while, on the bottom side, a rectangular radiating strip was used. Moreover, at the other side of the rectangular strip, a feeding probe was connected to the coaxial feed underneath the conductive fabric ground. A BW of 610 MHz was obtained in the range of 5.47 to 6.08 GHz, while an axial ratio (AR) BW of 3 dB was obtained within the frequency range of 5.4 to 5.81 GHz. Moreover, the radiation efficiency and the gain of 79% and 3.5 dBi were obtained, respectively. In [
277], a cuff-shaped button antenna was designed at the ISM and UNII bands, which was a circular patch with a diameter of 22.3 mm. The shape of a cuff button was conceived from a square-shaped polytetrafluoroethylene (PTFE) taconic ceramic material and its dielectric constant was considered 10. The ground was used on the bottom side to avoid the mutual coupling between the body and antenna. The design provided a high efficiency, high gain, and a VSWR of 94%, 1.5, and 2.1 at both operating band of 2.4 GHz and 5.5 GHz, respectively.
In [
278], a new idea was proposed for the design of modular geometry, which acted as a snap-on button and detached the radiation parts easily. All radiators could be used in the same feeding technique. First, a detachable patch achieved a right-hand circular polarization (RHCP), and then left-hand circular polarization (LHCP) by changing the radiator at 5 GHz. Secondly, the design showed a PIFA-antenna by interchanging the resonator at 2.4 and 5.3-GHz. A 3 dB AR BW was obtained from 4.95 to 5.08 GHz for the LHCP and 4.95 to 5.06 GHz for the RHCP, respectively, thus both were centered at 5.01 GHz. Finally, all results showed that the proposed module was less affected by the human body. In [
279], a miniaturized spiral Inverted-F antenna was proposed with the omnidirectional radiation pattern. This structure consists of various components, such as spiral line, stub, metal flange, metal ring, and a ground coplanar waveguide (GCPW)-type feeding. The two vias were used at the ends of the spiral line and the feeding stub. The metal ring was printed at the bottom side of the substrate, as shown in
Figure 9c. Additionally, the gain on-body at the lower and upper band was −0.6 dBi and 4.3 dBi, respectively. However, the limited SAR of 1.6 W/kg was obtained at 26.4 dBm (input power). In [
272], a novel dual-band button shape was proposed for the WLAN band. It showed a quasi-monopole pattern for on-body at 2.4 GHz, while a quasi-broadside pattern band for off-body at 5 GHz. The design showed an efficiency of 91.9% and 87.3% with a measured gain of 0.27 and 4.73 on both bands, respectively. The SAR was checked on various parts of the human body, which was below the limited value (0.20–0.22 W/KG). Currently, a new miniaturized array button antenna was designed, as shown in
Figure 9d. It covered 4.5–4.6 GHz for the IoT applications, whereas the 5.1–5.5 GHz for the WLAN applications. Moreover, the design showed the lowest SAR in all these bands along with the S-parameters and total efficiencies were almost similar to the free space communication [
280]. Thus, it shows the robustness of the proposed button antenna, as shown in
Figure 9d.
In this section, various types of wearable button antennas have been studied, such as semi-flexible and fully-flexible, with unique shape and size of the button antennas. Further, button shapes can be mounted on any parts of the human body. Additionally, these designs provide miniaturization, stable performance, less coupling effects, flexibility, and a high degree of physical robustness using both rigid and flexible materials.
4.2. Miniaturized Single and Multiband Wearable Antennas
A wearable device is an important component in tracking and monitoring systems, especially ones using miniaturized multi-band antennas with circular polarization, as they show stable radiation characteristics during bending, stretching, and crumpling conditions [
281]. Researchers are working to determine new ways of miniaturization, which could have a low profile and provide stable radiation characteristics under bending conditions. These wearable antennas are also using various communication systems, such as the Global Navigation Satellite System (GNSS). They require circular polarization (CP) and broadband patterns, like GNSS with GPS. Similarly, Galileo, GLONASS, and Compass also provide similar global coverage [
282]. These systems are used for various applications, such as rescue operations, military, and health systems. Moreover, they can be used to integrate with cellphones, cameras, wristwatches, computers, and wearable outfits, etc. [
283]. Usually, satellite systems need CP and multi-bands at the receiving side to obtain constant polarization and to avoid the time-varying orientation between transmitting and receiving signals for off-body communication. In [
284], the antenna was designed for outdoor applications at the GNSS L1/E1 band, which provided CP. Recently, in [
285], a CP sleeve badge antenna was designed for the 2.4 GHz ISM band, as shown in
Figure 10a. The design was small enough so that it could fit within the jacket sleeve badge. It provided a wide beam-width with a 5.6% BW, RHCP, and a gain of 4.71 dBi. The SAR was limited to 0.34 W/KG. In [
286], a dual-band wearable textile antenna was proposed for the ISM (2.22–2.48 GHz) and HiperLAN (4.95–5.80 GHz) applications. The antenna was based on a suspended plate. The rectangular radiator was used as cylindrical vias, slits, and slots to obtain the dual-band resonance and broad BWs. The design provided a unidirectional radiation pattern because of the ground and provided the isolation between the body and antenna. The total efficiency was obtained between 67% and 89%, whereas the gain and SAR obtained were 8.33 dBi and 0.14 W/kg on the chest, and a further 0.24 W/kg on the back. In [
287], a miniaturized dual-band patch antenna was designed at 2.4 and 5.2 GHz, as shown in
Figure 10b. The design was miniaturized to 74% using the combination of a DGS and a shorting via. However, dual-band was obtained using a U-slot in the ground. In [
288], a metal rim antenna was proposed for dual-band applications. A T-type feeding was used to connect ground with the metal rim, whereas a shorting patch was used to excite the second antenna. The antenna was fed with a T-type feeding. Additionally, the use of a metal rim and the ground reduced the antenna up to 1500–2300 MHz with 1.6 dB gain in the entire band. The SAR on the phantom was about 0.78 and 0.75 W/kg, respectively, in [
16], a compact miniaturized textile antenna was designed at 2.4 GHz, which used a rectangular slot along with an inserted strip line for the design of an inverted E-shaped antenna to minimize the size up to 75% as compared to a conventional antenna. The antenna showed a stable radiation performance, with 15% BW, and 79% radiation efficiency. In [
289], a dual-band array type antenna was designed for the WiMAX, LTE, and WLAN applications. The MTM structure was used as a ground plane to reduce the size. The radiation efficiency and the gain in the first band were obtained up to 95% and 2.8 dBi, respectively, while, in the second band, the radiation efficiency and gain were achieved up to 85% and 3.48 dBi, respectively. In [
290], a circularly-polarized antenna was proposed at 2.32–2.63 GHz, as shown in
Figure 10d. In that context, a square type ring was asymmetrically truncated at the surrounding edges to get the circular polarization and to reduce the patch size (50.5%). Moreover, the slots and stubs were used in the ground as a DGS to further reduce the size. The antenna showed a BW of 12.53% and a gain of 5.65 dBi along with the axial ratio (AR) of up to 3.27%.
Recently, wearable designs have received special considerations due to their demands in various fields, like health and military applications. However, the textile materials have shown flexibility and conformity on-body as well as easily integrated into the clothing [
291]. In [
292], a soft textile surface was used to decrease the back radiation of the antenna during moving. The backward radiation was significantly reduced on a cylindrical surface. The backward radiations of 6.2% were found in a flat position without a soft surface, while that of 2.2% were obtained when the antenna was surrounded by two-unit cells. The backward radiations of 8.2% and 2.6% were obtained during a curved surface. However, the soft surface improved the radiations in the broadside direction. In [
293], a novel miniature feeding structure was used in the aperture-coupled antenna at 2.4–2.5 GHz. The design reduced the size of the printed circuit board (PCB), which was carrying an electronics circuit and feeding system, as shown in
Figure 11a. The feeding point was found so small that it did not affect the users. However, the main characteristics such as the cross-polarization, the FBR, and the gain were obtained up to −20 dB, 15 dB, and 5.6 dBi, respectively. Additionally, a 47% efficiency and 0.145 W/kg SAR was obtained on the body. In [
294], another compact textile-based antenna was proposed for short-range communications at 5.9 GHz. The design was fully textile-based with a 9.3% BW and a gain of −2.75dBi.
In [
210], a PIFA type antenna was designed from 2.40 GHz to 2.48 GHz using a reverberation chamber as a shorting wall instead of shorting pins for increased BW. Moreover, a slot was used in the patch to widen the BW. The design showed a BW of more than 46% and a gain of 1.5 dBi. In [
295], a textile antenna was designed for the GSM/PCS/WLAN applications, which provided the comfortability with a gain of 2 dBi obtained in all bands. In [
296], a wearable antenna was proposed for military applications at 915 MHz (ISM) and 1.575 GHz (GPS L1) bands. This design used two patches, such as truncated and circular ring patches, with four conductive threads. The circular shape was used for the ISM band, whereas the truncated patch was used for the GPS band. In [
297], the authors proposed an AMC based coplanar waveguide antenna for the ISM bands, as illustrated in
Figure 11b. The AMC structure improved the performance and provided the isolation between the antenna and the human body. The gains of 8.2 dBi and 9.95 dBi at 2.45 GHz and 5.8 GHz were achieved, respectively. The SAR was reduced to 96.5% for 1 g of tissues and 97.3% for 10 g of tissues. In [
298], the EBG based circular ring slot antenna was proposed for the ISM band. The gain improvement of 7.3 dBi, and the efficiency of 70% was achieved. The SAR value of 13.5 W/kg for 1 g tissue and 7.98 W/kg for 10 g was obtained. Moreover, the EBG structure has reduced the SAR from 95.9% to 97.1%, and a new value of 0.554 W/KG was obtained. In [
299], an M-shaped antenna was designed on a Kapton substrate at 2.4 GHz using an AMC structure as a ground plane. This antenna used an AMC structure as a ground, which was fed through a CPW. Consequently, FBR was increased up to 8 dB with a gain of 3.7 dBi as compared to the antenna without the structure. Moreover, the SAR value was reduced up to 64% and showed 0.683 W/Kg. However, a similar antenna without AMC showed up to 1.88 W/Kg. In [
300], two different types of meander line antennas were designed at 915 MHz. Their shapes were like Vivaldi antennas and the edges of both antennas were connected through the meander lines. The gain, efficiency, and BW of one of them was −3.75 dBi, 31%, and 28 MHz (3%), and for the second one, it was −0.3 dBi, 76%, and 73 MHz (8%), respectively. In [
301], a textile antenna was proposed at 5.8 GHz, which used a SIW cavity-feed structure with a circular ring slot. This antenna provided the gain and radiation efficiency of 2.8 dBi and 32.5%, respectively, on the voxel.
A SIW antenna has been considered as a new technology in the WBAN systems. It is highly recommended due to its low cost and the ease of design for upcoming technology, like 5G [
302]. This structure used the vias on its surrounding walls and was supported by a ground. Additionally, it improves the quality factor (Q) of the design as well as provides shielding against thermal radiation between the human body and antenna. In [
303], the SIW technique was implemented for designing two antennas, which were located on the same substrate. In [
304], a multiband wearable antenna was designed on a leather substrate. The copper was used as a radiator, which was pasted on the leather using sticky material to operate for WiMAX, WiFi and military applications. In [
305], a copper taffeta fabric was etched on a felt substrate to obtain a low profile wide-band antenna, which provided the BW and radiation efficiency of 46% and 84%, respectively. In [
306], the MTM-based SIW textile antenna was designed on the same felt substrate using conductive fabric, which reduced the size up to 80% with an efficiency of 74.5% for on-body applications. The research in [
307] proposed a SIW-type fully-textile antenna at 2.45 GHz. This design presented a stable performance and gave a low SAR in bending as well as integration. Therefore, the SIW type antennas are available in various shapes with suitable performance. Recently, a new miniaturized wearable mosaic antenna is designed for cross-body applications with higher performance as compared to other types of wearable antennas, as shown in
Figure 11c [
308]. The design optimally uses the surface waves to support cross-body communication. The mosaic antenna can be used to detect human activity with an accuracy of 91.1% using RF-type recognition methods in the WBAN systems. Thus, it can work as a wearable sensor for motion tracking and human activity recognition (HAR), simultaneously.
Currently, micro and millimeter-wave have received great attention in communication. They can be widely used in various applications including sensing, or medical systems, etc. Thus, the terahertz (THz) technology would be able to fulfill the high demands, such as higher data rates, multiple features in one, miniaturized devices, and 5G networks within the range of 0.1–10 THz. However, the current advanced applications are not being fully used by the concerned society due to the immaturity of THz in light of antennas and some basic components [
309]. Researchers in [
310,
311] investigated various types of THz antennas to fulfil the demands of promising applications. In [
312], a graphene antenna was designed at 0.647 THz, as illustrated in
Figure 11d. The antenna was tested for on-body applications. It showed a BW of 20 GHz with a radiation efficiency of 96% (50%) in free space (on-body), and a gain of 7.8 dB (7 dB), respectively.
In this section, two categories of miniaturized wearable antennas have been discussed in detail with different examples.
Table 10 has been provided to show the performance comparison of these miniaturized wearable antennas. In practice, it is important to use wearable antennas with textile accessories like a button antenna because it can be easily integrated on various locations of the body. Additionally, button antennas use a combination of flexible and hard materials and cannot be easily deformed, and are less sensitive to body humidity. They provide good efficiency and gain in free space and on-body applications, including low SAR. Recently, various unique button antennas have been designed in the literature based on the principle of the chassis or terminal antenna using characteristic mode theory [
275], which can easily improve the performance and reduce their size. Thus, all these advanced techniques are linked with the fast development of smart mobiles, which have reshaped the wireless communication devices [
313].