## 2 Clothing Components in Wearable Textile Antennas
### Buttons
#### 2.1.2 Snap-On Buttons  
Snap-on buttons are widely used CCs. Still, few researchers have studied their potential use in terms of wearable antenna design and implementation. In [40], the use of commercial snap-on buttons for achieving a modifiable textile antenna design has been presented. The aim is to accomplish resonance frequency reconfigurability. Specifically, a foldable patch antenna is designed that resonates at 8 or 9, or 10 GHz. By simply folding the textile radiator at specific lengths, the resonance frequencies can be varied. The applied design technique allows a practical and agile solution to modify the system response. Similar work was presented in [41] where a planar antenna configuration with detachable radiating parts was developed to offer polarization and resonance frequency reconfigurability. By using snap-on buttons, as radiofrequency (RF) connectors and mechanical holders, multiple microstrip patch antenna designs can be manually interchanged. For instance, a patch antenna design that enables both right-hand circular polarization (RHCP) and left-hand circular polarization (LHCP) is obtained at 5 GHz. Further, a pla  
Figure 1: (a) Fabricated triangular patch button antenna: “Reprinted/adapted with permission from Ref. [14]. © 2014, © [PERSON]”; (b) UWB dielectric button antenna (Top-, Middle-, Bottom-layer): “Reprinted/adapted with permission from Ref. [30]. © 2007, © [PERSON]”. nar inverted-F antenna (PIFA) that resonates at 2.4 and 5.3 GHz WLAN in an alternating manner is presented. It should be highlighted that all antenna configurations are designed on a common feeding structure which consists of a two-layered substrate, five snap-on buttons, a ground plane, and a proximity-coupled feedline. Measurements of the antenna prototypes in free space, on a body tissue phantom, and in bending conditions indicate that modular interchangeable designs can provide easy, manual, and versatile reconfigurability at low cost for wearable applications. A similar research study was described in [42] where a circular patch is presented by introducing a rectangular flap in the middle of the antenna structure. This modular antenna provides interchangeable RHCP, LHCP, and linear polarization (LP). Further, resonance frequency reconfiguration in LP mode is allowed by rotating the antenna and changing the flap states (open or closed). Additionally, the concept of employing commercial snap-on buttons as shorting pins in a textile patch antenna is presented and experimentally validated in [43]. Specifically, the patch antenna can apply different shorting pins arrangements through four sets of two back-to-back male snap-on buttons soldered into a two-layer substrate. By connecting specific male buttons in the female parts onto the ground plane, different antenna radiation patterns can be generated. Furthermore, in [44] a dual-band textile patch antenna with frequency reconfiguration has been presented. The antenna presents tuning ranges of 32.8% and 8.8% for its two frequency bands of operation at 2.45 and 5.8 GHz ISM bands, respectively. The principal idea was to embed the active components (two in-parallel varactors) and the biasing circuit into a compact and mechanically robust module of back-to-back male snap-on buttons. In [45], a textile PIFA is presented by applying a coplanar frequency reconfiguration mechanism. The reconfiguration module includes a compact printed circuit board that embeds the active tuning circuit with snap-on buttons offering the electronic-to-textile connection. The antenna prototype achieves a wide operating frequency range of about 70%. Finally, in [46] a coupled-mode substrate-integrated cavity (CMSIC) antenna is presented. The antenna achieves radiation pattern reconfiguration at 2.45 GHz by employing two snap-on buttons as passive switches. When the buttons are in the OFF state, the antenna generates a broadside radiation pattern with a maximum gain of 9.2 dBi, while, in the ON buttons state, the radiation pattern is omnidirectional with a maximum gain of 3.1 dBi.