# Flexible Antennas for a Sub-6 GHz 5G Band:
## 4 Flexible Antennas for a Sub-6 GHz 5G Band
### Single Element Flexible Antennas for the Sub-6 GHz 5G Band  
#### 4.1.1 Slot/Slit Based Flexible Antenna  
Slot elements are utilized to increase the impedance bandwidth of the antenna using the coupling mechanism in the radiating patch, as well as the ground plane. High gain, efficiency, a larger bandwidth, and a high mutual coupling value can be obtained with the slot antenna [46]. The following section provides slot/slit-based flexible antennas suitable for sub-6 GHz 5G applications. Figure 6: Overview of flexible antennas for the sub-6 GHz 5G band. In [47], a wideband and tri-band antenna that can be utilized for 5G sub-6 GHz communication with independently controllable notch bands, is proposed. The wideband monopole antenna consists of an octagonal geometry with truncated edges that improves the impedance bandwidth of the antenna. The bandwidth is increased by a rectangular stub covering the whole sub-6 GHz band. The slots are introduced in the wideband antenna structure to achieve the band notches to mitigate the C-band and Wi-Max band. The wideband antenna shows a frequency range of 2.8 to 5.35 GHz, and the tri-band antenna is resonating at frequencies viz. 2.45 GHz, 3.5 GHz, and 4.7 GHz. Figure 6(a),b shows the S-parameter of the wideband and tri-band antennas, respectively. A gain of 3.75 dBi is achieved all along the covered spectrum, as shown in Figure 6(c),d.  
Figure 7: _Cont._  
A directional radiation pattern is achieved for the proposed antenna over the entire band, as shown in Figure 7e. The bending analysis is performed to check the conformability of the antenna. For this, the wideband and triband antenna is bent on a Styrofoam cylinder over various radii of 10, 25, and 40 mm. It is seen that there is not much effect on the return of the loss on the various bending radii. Another wideband antenna using conductive graphene film (GAF), is proposed in [48], implementing the slot in the radiating patch to achieve a bandwidth of 3.12-4.42 GHz, that covers the 5G band for the wireless wearable sensor application. The resonant frequency and impedance of the antenna sensor are achieved by using a short rectangular tuning stub. Figure 8a,b shows the antenna prototype and S-parameters, respectively. The bending analysis is carried out with the help of poly(lactic) acid or PLA rings of different testing radii, as shown in Figure 8c. The antenna is tested on an arm under 100 cycles of testing, involving flat and bent conditions. It is noted that the resonant frequency was shifted abruptly but is reverted to the original state when the arm is in a flat position. The antenna shows a directional radiation pattern, as shown in Figure 8d. Figure 7: Multiband slotted antenna: (**a**) Return of the loss of the wideband antenna; (**b**) Return of the loss of the triband antenna; (**c**) Gain analysis of the wideband antenna; (**d**) Gain analysis of the triband antenna; (**e**) Comparison of the radiation patterns of the antenna at 3.5 GHz. Reprinted from ref. [47]. Another approach of the bowtie slot antenna for achieving a wideband with an improved gain, is proposed in [49], where the antenna is designed using a flexible PET substrate, as shown in Figure 9a. To obtain a high gain and a larger bandwidth, the design includes an asymmetric bowtie flare angle with a metal strip incorporated beside a bowtie  
Figure 8: Conductive GAF slotted antenna sensor: (**a**) Fabricated prototype; (**b**) S-parameter of the antenna sensor; (**c**) Bending analysis set-up; (**d**) Radiation Pattern. Reprinted from ref. [48]. slot. The antenna covers a bandwidth of 2.1-4.35 GHz with a peak gain of 6.3 dBi at 4.35 GHz as shown in Figure 8(b). Figure 9: _Cont._  
The antenna exhibits an omnidirectional radiation pattern, as shown in Figure 9c. When the bending analysis of the antenna is carried out, it is observed that there is a variation in the frequency shift during the antenna testing phase, due to the increase in the dielectric constant value of the cylinder used for the bending analysis.