**2. Antenna Structure**

The overall view and lateral perspective of the proposed antenna array are shown in Figure 1. Eight antenna elements are printed along the inner side of two sideboards with a size of 145 × 4.2 × 0.8 mm3, which are constructed perpendicularly to the system board. The size of the system board is 145 × <sup>70</sup> × 0.8 mm3. The sideboards and system board substrate are an FR4 substrate with loss tangent = 0.02 and relative permittivity = 4.4. The height of the whole smartphone is only 5 mm, since the sideboards are placed on the system board. A 2 mm-wide microstrip line feeds each element through an SMA connector via the hole from the bottom of the system board. The designed DBs are separately printed on the inner face and upper side of the sideboard and system board, which are welded together. A ground plane (145 × 70 mm2) with two rectangular ground clearances (145 × 3.5 mm2) is fabricated on the bottom of the system board. The dimensions of DB1(2) and the detailed construction of a fundamental antenna element are illustrated in Figure 1c,d. Parameters that affect antenna performance are described as variables rather than a fixed value. Notably, the values of *S1* of DB1 and DB2 are 11 mm and 9 mm, respectively.

**Figure 1.** Perspectives of the proposed antenna. (**a**) Overall view, (**b**) side view, (**c**) dimensions of DB1 and DB2, and (**d**) detailed structure of an antenna element. (All values are in millimeters).
