# Flexible Antennas for a Sub-6 GHz 5G Band:
## 4 Flexible Antennas for a Sub-6 GHz 5G Band
### Flexible MIMO Antennas for Sub-6 GHz 5G Band
#### 4.3.1 Slot/Slit-Based Flexible Antenna  
A tri-band MIMO antenna is fabricated on a flexible LCP substrate [73]. The MIMO structure consists of an inverted-F branch that provided the two lower frequency bands, as shown in Figure 17a. To achieve the third band, and to have a better isolation between the elements, an I shaped structure and a slot are introduced in the ground plane that gives better isolation of \(-10\) dB. The measured impedance bandwidth is 2.38-2.55 GHz, 3.37-3.60 GHz, and 4.92-5.37 GHz and the isolation achieved is greater than 19 dB for all of the working bands, as shown in Figure 17b. The bending analysis of the MIMO antenna is performed, and it is found that for both the E-plane and the H-plane, there is negligible impact on the isolation of the antenna. The SAR analysis is performed by keeping the antenna at various distances from the human tissue model consisting of different layers of skin, fat, and muscle. The minimum SAR values of 0.64 and 0.99 for 2.45 GHz and 3.5 GHz for 10 g of tissue are achieved at 1 mm from the human body. Figure 17c shows the SAR distribution in 10 g of tissue at 3.5 GHz.  
Figure 17: Triband MIMO antenna: (**a**) Fabricated prototype; (**b**) S-parameter; (**c**) SAR distribution in 10 g of tissue at 3.5 GHz. Reprinted from ref. [73].  
Fabric substrates are widely used in the design of several MIMO systems due to their low dielectric constant values. In [74], a wideband dual-polarized textile antenna is designed on a jeans substrate, as shown in Figure 18a. The antenna has a bandwidth ranging from 1.5 to 3.8 GHz and 4.1 to 6.1 GHz. Figure 18b shows the measured S-parameter of the antenna. The antenna provides an efficiency of 70% with a peak gain of 4 dBi at the desired frequency range, as shown in Figure 18c.  
Figure 18: Dual-polarized slotted textile MIMO antenna: (**a**) Antenna prototype; (**b**) S-parameter of the antenna; (**c**) Gain and efficiency. Reprinted with permission from ref. [74]. Copyright 2020 John Wiley & Sons, Inc.  
The mutual coupling is decreased by implementing a meander line in the ground plane. Introducing another meander line on the top layer enhanced the diversity performance of the antenna in the specific frequency ranges. The antenna exhibits a dipole-type pattern in the E-plane and an omnidirectional type of pattern in the H-plane. The SAR value is below 1.6 W/kg which is below the international standards.  
Another MIMO antenna design on a jeans substrate with a different isolation technique is proposed in [75], in which an E-stub is inserted between the radiators to improve isolation, and the antenna is made up of two square radiators with slots running vertically and horizontally on the front end. A partial ground plane has been etched to increase the antenna's bandwidth. The introduction of stubs contributes to a minimal mutual coupling between the radiators. A SAR analysis of the antenna is carried out to check the suitability of the antenna in wearable applications. The analysis is carried out using a human body model consisting of different layers of tissues. The antenna is placed at different distances from the model. The minimum average SAR over the 1 g volume of tissue at 3.5 GHz is found to be 0.62 for the human body model, placing the antenna at a distance of 5 mm from the model. The antenna diversity parameters are estimated and showed good results suitable for the MIMO.