## 2 Clothing Components in Wearable Textile Antennas
### Buttons
#### 2.1.1 Buttons
At the lower frequency band, the antenna yields linear polarization and radiates as a top-loaded monopole, being appropriate for on-body communication. At the higher band, it yields right-handed circular polarization (RHCP) and radiates as a crossed dipole, being appropriate for off-body communication. The respective gains are similar with those of a dipole (on-body) and a patch (off-body) antenna. Additionally, an innovative dual-band dual-polarized button antenna is proposed in [28]. The button is miniaturized (diameter = 19.5 mm). The antenna operates at 4.5-4.61 GHz and at 5.04-5.5 GHz Unlicensed National Information Infrastructure (U-NII) 5G Internet of Things (IoT) and WLAN. At both operating bands, vertical and horizontal linear polarizations are propagated. The final antenna is assembled to a 10 \(\times\) 1 array. The antenna is fed by two ports. Both ports are of similar scattering parameters (reflection coefficient S11, S22). This makes the antenna appropriate for MIMO and for off-body communications. Additionally, ultra-wideband (UWB) button-shaped antenna designs are proposed in [29; 30; 31]. The UWB technology promises high-speed data transmission at low-cost for short-range communications making it a good candidate for wireless on-body networks. In [29], a compact metallic button design has been proposed to operate in the 3.1 GHz to 10.6 GHz frequency band. The radiation patterns are omnidirectional and the achieved gain ranges from 3.45 dBi to 5.6 dBi across the operating UWB band. In [30], a button antenna design is developed by using the circular metal nail found in some buttons for size reduction. Simulated and measured results indicate that the antenna is able to operate in the UWB band. The radiation patterns show good omnidirectionality with measured gain values from 1.9 dBi to 3.4 dBi within the frequency band. The proposed button antenna can be used on a textile jacket. Additionally, an UWB state-of-the-art pattern diversity dual-polarization button antenna is proposed in [32]. Two ports are applied to this antenna. The first port yields UWB operation (4.12-9.12 GHz) with a wide 3 dB axial ratio AR (5.71-8.69 GHz) and a maximum gain of 8 dBi. The second port yields multi-resonant and multi-mode operation at 2.45, 3.7, and 5.85 GHz with modes TM01 (Omnidirectional), TM11 (Broadside), and TM21 (Side-Omnidirectional) of linear polarization. The gain ranges from 1 to 6.6 dBi which is appropriate for on- and off-body wireless links. Button antenna designs have also been proposed for the mm-wave frequencies where multi-gigabit data transmission rates for very short-range communication scenarios are enabled [33; 34; 35]. In [33], a compact patch antenna in the shape of a shirt button is designed to operate at 62 GHz with a wide impedance bandwidth. The radiation pattern is directional with a maximum gain of almost 5 dBi. The antenna performance is quite stable under tissue phantom conditions. In [35], a disc-like antenna has been designed and experimentally validated for body-centric applications. The antenna operates at a 61 GHz frequency band with an omnidirectional radiation pattern of vertical polarization. The measured maximum gain is 4.7 dBi. The presence of a tissue phantom impacts the antenna radiation performance. A magneto-dielectric button-shaped broadband patch antenna operating at 868 MHz has been reported in [36]. To decrease the antenna size for compatibility with a jacket button while maintaining its impedance matching and radiation properties, a barium-strontium hexaferrite has been employed. A low radiation efficiency of 1.5% is observed which is mainly attributed to magnetic losses. A state-of-the-art circularly polarized button antenna is proposed to operate between 5.47-5.725 GHz U-NII band in [37]. The gain of this antenna lies between 2 and 5 dBi. The antenna presents a gap-loaded loop design of small dimensions (diameter = 2 cm). The antenna is unidirectional.