# Uni-Planar MIMO Antenna for Sub-6 GHz 5G Mobile Phone Applications
## 2 Proposed Uni-Planar MIMO Antenna
### User's Impact on Antenna Characteristics  
In this section, the user's impact on MIMO antenna performance is examined. Performance is evaluated in terms of S-parameters and radiation efficiencies. The MIMO antenna performance is evaluated in the presence of the user's single and double hand, as shown in Figure 11(a),b.  
The developed MIMO antenna has appropriate reflection coefficients in the region of the human hands, as shown in Figure 12(a),b. For the single-hand scenario, \(>\)10 dB of isolation is noted between the antenna elements, as shown in Figure 13(a), while for double-hand mode, the isolation is \(>\)15 dB (see Figure 13(b)). Besides this, from Figure 14(a),b, it can be noted that the efficiencies of antenna elements degraded. In case of single-handed operation, the reduction in radiation efficiency is noted for Ant-1 and Ant-3, whereas for the double-hand scenario, Ant-2 and Ant-3 provide less radiation efficiency. This is due to the features of human body tissue (fingers near the antenna), which can absorb a lot of antenna radiation power [24]. On the whole, the proposed MIMO antenna offers radiation efficiencies in the range of 39-68% for both scenarios for the desired frequency band.  
Figure 11: Radiation characteristics of the proposed sub-6 GHz MIMO antenna when port-2 is excited.  
Figure 14: Transmission coefficients of the proposed MIMO antenna in case of (**a**) single-hand and (**b**) double-hand scenarios.  
Figure 12: MIMO array placement for (**a**) single-hand and (**b**) double-hand scenarios.  
Figure 13: Reflection coefficients of the proposed MIMO antenna in case of (**a**) single-hand and (**b**) double-hand scenarios.  
For antennas used in mobile phone applications, one of the most important considerations is the specific absorption rate (SAR) [13, 25]. The SAR is a parameter that measures the amount of electromagnetic waves that are absorbed by the human body [26]. The SAR characteristics with user-head are explored and displayed in Figure 16. The minimum SAR is noted to be 0.94 W/Kg for Ant-2, while the maximum SAR value is 2 W/Kg for Ant-3. It is noted that the closer the elements are to the head phantom, the higher the SAR value, and vice versa.  
A comparison of previously reported and designed sub-6 GHz MIMO antennas is given in Table 2. One can observe from the table that the electrical dimensions of the single antenna element are small compared with the designs of [7, 8, 12, 13]. The design presented in [11] offer high gain and low ECC, but due to their non-planar configuration, they can rarely be used in handheld devices. On the other hand, the planar antennas designed in [7, 12, 13, 18] offer high isolation within their operational bandwidth, but they offer low antenna efficiency. Therefore, it can be concluded from Table 2 that the proposed MIMO antenna offers high radiation efficiency, acceptable gain, and low ECC even with a lower number of elements.  
Figure 16: SAR analysis when MIMO antenna used in talk-mode.  
Figure 15: Radiation efficiency of the proposed MIMO antenna in case of (**a**) single-hand and (**b**) double-hand scenarios.