# 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
#### 4.1.2 DGS-Based Flexible Antenna  
DGS is a structure where defects and slots are created in the ground plane of the antenna. These structures provide a wide bandwidth, a higher efficiency, and a low mutual coupling [46]. The following section provides the DGS-based flexible antennas suitable for sub-6 GHz 5G applications. A highly efficient and enhanced bandwidth antenna is proposed in [51], where a monopole antenna operating in 2.45 GHz and 3.5 GHz is designed for WBAN/WLAN and 5G applications. The antenna that is fabricated on a flexible Kapton polyimide substrate is fed using a CPW feed. The antenna has two rhombic elements and a partial ground plane, as shown in Figure 10(a). The S-parameter of the antenna is given in Figure 10(b). Figure 11: _Cont._To validate the performance of the antenna and to ensure its safe usage for wearable applications, the SAR analysis of the antenna is carried out with the proposed antenna. The antenna is placed on the chest and forearm at a 5 mm distance from the human body, as shown in Figure 11c. The maximum SAR value for 1 g of tissue is 0.908 and 0.782 in the forearm and chest, respectively, and for 10 g of tissue, it is 0.468 and 0.342 at 2.6 GHz, which is well below the international limits for safe SAR values, as shown in Figure 11d. The DGS is also incorporated for gain enhancement. Such a flexible wideband antenna with an improved gain is proposed in [52]. The antenna volume consists of a CPW feedline, two L-shaped elements, a matching stub, and a DGS fabricated on a flexible Kapton polyimide substrate. A defected ground structure along with the L-shaped elements improved the bandwidth. Further adding of the matching stub helps in achieving a wideband from 1.77 to 6.95 GHz. A gain of 5.9 dBi and an efficiency of 90%, respectively, are achieved. The bending analysis of the antenna is performed for five bending radii 50, 20, \(\infty\), \(-\)50, and \(-\)20 mm using simulation. It is seen that the bending across these two radii has not much difference in the performance of the antenna. The antenna shows a directional radiation pattern in the E-plane and an omnidirectional radiation pattern in the H plane. The radiation pattern is slightly affected across the bending around the radii of 20 and \(-\)20 mm. However, no null is seen in the broadside direction. Ground planes can act as a shield to safeguard the users from harmful radiation. In [53], a wearable dual band ring-shaped folded antenna with a minimum SAR and a high gain is proposed. The antenna comprises two substrates, one which is rigid and acts as the main conductor, and the other which is a textile material that is a ground plane to protect the user from the backward radiation which reduces the SAR. The antenna could achieve a bandwidth of 5.7% (2.40-2.54 GHz) and 4.0% (3.38-3.52 GHz). The antenna has a gain ranging from 6.6 to 6.8 dB and 8.9 to 9.0 dB with an efficiency of 94% and 93% for the lower and higher frequency bands, respectively. The antenna is tested for the bending analysis along a cylinder of the radius R at different radii in the x and y directions. It is seen that there is no shift in the frequency compared to the flat condition. To check the suitability of the antenna as a wearable application, the antenna is tested in free space and on a human body phantom. To ensure the safety for the user, the SAR is estimated for 1 g and 10 g of tissues for both frequencies. The SAR values are 0.2 and 0.1 W/kg for 1 g of tissue and 10 g of tissue, respectively, at 2.45  
Figure 11: Monopole antenna with rhombic structures for the 5G: (**a**) Fabricated prototype; (**b**) Reflection coefficient of the antenna on the chest and forearm; (**c**) Proposed antenna on the chest (**left**) and forearm (**right**); (**d**) SAR analysis of the proposed antenna at a distance of d = 5 mm on the chest and forearm. Reprinted with permission from ref. [51].