# Wideband Eight-Antenna Array Designs for 5G Smartphone Applications
## 2 Antenna Geometry and Design Evolution  
Figure 1 depicts the MLAE antenna design, integrated on the inner side of the smartphone's longer frame. The antenna, measuring 21 mm \(\times\) 6 mm, is fed by a 50-ohm microstrip line at point F, with its loop structure grounded via shorting point S on the system ground plane beneath the substrate. This MLAE features a main structure comprising two P-type radiating elements arranged back-to-back, connected by a bridge and adorned with two protruding stubs, each measuring 1 mm (\(L_{1}\)) \(\times\) 0.8 mm, as shown in Figure 1. Notably, each P-type radiating element is loaded with a U-shaped slot (with parameter \(L_{2}\) = 1.5 mm), which excites the three loop modes (at 3.6, 4.0, and 5.1 GHz) to cover the bands-of-interest (3300-5925 MHz).  
Figure 2 shows the proposed eight-antenna array, with four MLAEs arranged along each long side frame of the smartphone casing, maintaining a gap distance of 16.5 mm (d1 = d2 = d3) between any two MLAEs. The side frame measures 150 mm \(\times\) 7 mm in area, while the system ground covers an area of 148 mm \(\times\) 73 mm on a substrate that is 150 mm \(\times\) 75.6 mm. The smartphone casing, including the frame and ground, is made of FR4 substrate of thickness 0.8 mm.  
Figure 3 shows the design evolution of the proposed MLAE. It began with a simple conventional loop antenna structure (similar to [10]), denoted as Ref. 1, which can excite two distinct modes at 3.54 and 5.34 GHz. To enhance the operating bandwidth, Ref. 2 was constructed by reconfiguring the loop structure into two P-shaped elements (back-to-back) linked by a narrow bridge. In this configuration, besides the previous modes from Ref. 1 (shifted to 3.59 and 5.62 GHz), an additional middle mode at 4.19 GHz is also excited. However, as the lower frequency of Ref. 2 is at 3370 MHz, it is insufficient to cover the bands-of-interest (3300-5925 MHz). To further improve the matching of Ref. 2 and satisfy the bands-of-interest, the connected bridge of Ref. 2 was enlarged from 1.4 mm to 1.8 mm, denoted as Ref. 3. Although Ref. 3 excited three resonant modes at 3.7, 4.1, and 4.86 GHz, the upper frequency could not cover the high band of n46 (5925 MHz). Therefore, a rectangular slot forming the P-shaped element was reconfigured into a U-shaped type  
(with parameter \(L_{2}\)), and two stubs (with parameter \(L_{1}\)) were protruded from the connected bridge. This modification shifted the higher resonant mode from 4.86 to 5.10 GHz, while the lower and middle modes were slightly deviated to 3.6 and 4.0 GHz, respectively. Here, the proposed MLAE exhibits a desirable 6 dB bandwidth of 3.15-6.75 GHz, and the impedance matching is better as compared to the conventional loop antenna (Ref. 1).  
Figure 1: Geometry of the single antenna element (modified loop), \(L_{1}\) = 1 and \(L_{2}\) = 1.5 (all dimensions in mm).  
Figure 2: Layout and geometry of the proposed 8-antenna array, d1 = d2 = d3 = 16.5 mm.  
Figure 4a,b illustrate the effects of fine-tuning the two parameters, \(L_{1}\) and \(L_{2}\). By linearly increasing the length of parameter \(L_{1}\) by 0.5 mm (0.5 to 1.5 mm), the three modes are also shifted linearly towards the higher spectrum, demonstrating that adjusting the length of \(L_{1}\) affects all resonant modes. In comparison, by linearly expanding the parameter \(L_{2}\) by 0.5 mm (1 to 2 mm), the middle resonant mode shifts to the lower spectrum and combines with the lower resonant mode, forming a dual resonant effect. Since fine-tuning \(L_{1}\) and \(L_{2}\) does not affect the overall impedance bandwidths of the proposed MLAE, we selected \(L_{1}\) = 1 mm and \(L_{2}\) = 1.5 mm as the optimum values.  
Figure 4: _Cont._  
Figure 3: Design evolution from Ref. 1 to proposed antenna (MLAE), and their respective simulated reflection coefficients.