# A Design for a Wide-Band Antenna Pair Applied for Mobile Terminals at the Sub-6 GHz Band
## 3 Evolution and Analysis of Antenna Pair  
### Evolution  
To better understand the operation principle of the proposed antennas, the evolution process of the proposed antenna pair is depicted in Figure 2. The proposed antenna pair with a wide band is realized by three model modifications and the relative changes in reflection coefficients are shown in Figure 3. Due to the symmetrical structure of the antenna pair, the results are obtained when port 1 is excited.  
Figure 1: The structure of the proposed MIMO antenna array and antenna pair. (**a**) Geometry and parameters of the proposed eight-port MIMO. (**b**) Geometry and parameters of the proposed antenna pair.  
Inspired by the frequency selective surface antenna presented by the authors of [28], this paper attempts to construct different lengths of resonant current paths by controlling the radiating patch to achieve a wideband design. Firstly, a conventional antenna with two L-shaped patches is constructed (Model 1). The simulation results of the reflection coefficients in Figure 3 indicate that Model 1 can resonate at 3.7 GHz, with a band of 700 MHz. In addition, there is the potential for resonance near 5 GHz. It is evident from Figure 4a that the current distribution at 3.7 GHz is strong on the inner area of the long L-shaped patch and the short L-shaped patch. When working at 5 GHz, the current distribution is only concentrated on the short patch. Since the major resonant element has a significant effect on the antenna resonance, the secondary radiation part on the outside is selected to improve the bandwidth. Above the secondary radiation part, the current-sensitive area, which is the middle of the long patch and the short patch, is changed to create new resonant current paths for exciting further band potential. Thus, the T-shaped patch is introduced on the current-sensitive area, and Model 2 is obtained.  
One can see from the reflection coefficients that compared to Model 1, the resonant points of Model 2 are shifted to the left, and two resonant points of 4.1 and 5.6 GHz emerge. In addition, the impedance bandwidth is improved, which means that at a low frequency, the wide band from 3.52-4.62 GHz is realized, and at a high frequency, it has an insignificant impedance band from 5.48-5.65 GHz. The current distribution of model 2 in Figure 4b indicates that the resonant current at 4.1 GHz is mainly distributed in a T-shaped patch and a short L-shaped patch. The resonant current at 5.6 GHz is strong in the T-shaped  
Figure 3: Reflection coefficients of different antenna models.  
Figure 2: The evolution of the antenna pairs.  
patch and the short L-shaped patch. It can be observed that the addition of a T-shaped patch allows the current path to be shifted on the T-shaped patch and short L-shaped patch and shows great potential for bandwidth expansion between 4.5 and 5.5 GHz. The effect for supporting the resonance of long L-shaped patches is weak. Thus, the long L-shaped patch is the next optimization target. The notch is attempted for the long L-shaped patch to improve impedance-matching for obtaining a wider bandwidth. Along with the above modification, Model 3 is obtained.