## 5 Feasibility and Measurement  
To validate the new antenna design under realistic hardware conditions, a prototype was fabricated and integrated into a commercial smartphone chassis (Figure 6). The antenna structure was placed along the upper section of the metal frame using a shared-coupling configuration. Four feeding ports (Ports 1-4) were symmetrically distributed. The top view image of Figure 6a shows the detailed layout, including three intermediate ground taps strategically positioned between the feed points to confine the current distribution along the frame and enhance structural stability. Each port is connected via soldered, coaxial feed lines to surface-mounted SubMiniature version A (SMA) connectors routed through the rear of the device to enable direct interfacing with a VNA that made S-parameter measurements. The complete assembled testbed is shown in Figure 6b. All ports are actively connected to facilitate simultaneous evaluation of VSWR, isolation, and the ECC. The measurement configuration closely replicates real-world smartphone integration. The internal components include a battery, shielding structures and a realistic PCB. The setup therefore enables practical assessment of factors that affect performance. These include detuning, coupling artifacts, and mechanical constraints. Such elements are often idealized or overlooked during simulations. The results presented in the figures below show that slight frequency shifts and deviations in performance underscore the need for layout optimization and physical prototyping during the final stages of antenna development. Figure 5: Simulated radiation efficiencies of the sub-6 GHz antenna ports across the 5G NR bands; (a) ANT #1, (b) ANT #2, (c) ANT #3, (d) ANT #4. The prototype was fabricated on an FR4 substrate with a nominal thickness of 0.8 mm (\(\pm\)0.05 mm tolerance), a relative permittivity of 4.3 \(\pm\) 0.2, and a loss tangent of 0.025 \(\pm\) 0.003. For the conductor layers, adhesive copper tape with a thickness of 35 \(\upmu\)m (1 oz equivalent) and an average surface roughness of approximately 2 \(\upmu\)m RMS (root mean square) was applied. The feed network was implemented as a 50 \(\Omega\) microstrip line measuring 1.5 mm in width and 3 mm in length, which transitions to a slot-fed stub with dimensions of 17 mm \(\times\) 3 mm positioned 0.6 mm beneath the edge of the metal frame. For the measurements, standard SMA connectors (Amphenol 135101-R1) were employed. The vector network analyzer (Keysight E5071C) calibration was performed up to the SMA connector reference plane using a two-port SOLT (Short-Open-Load-Thru) procedure without any additional de-embedding of the connector assembly. Figures 4 and 7, respectively, show the simulated and measured VSWR characteristics of the sub-6 GHz multi-port antenna system. The simulations predict good broadband impedance matching (VSWR \(\leq\) 3) across the n78 (3.3-3.8 GHz), n77 (3.3-4.2 GHz), and n79 (4.4-5.0 GHz) bands for all four antenna ports (ANT #1-ANT #4). Dual resonant responses are observed, enabled by the optimized metal frame coupling and slot geometry. The resonance peaks are strategically positioned around 3.4-3.6 and 4.7-4.9 GHz. Figure 7 shows the measured VSWR when the antenna is implemented in a realistic smartphone (Figure 6). Although the overall impedance behavior is consistent with the simulations, notable frequency shifts are observed at all antenna ports. In particular, the resonance frequencies of ANTs #1 and #4 are downshifted by more than 200 MHz. ANTs #2 and #3 exhibit shifts of approximately 100 MHz. Sometimes, these deviations trigger partial misalignment with the target n78 band. The discrepancies reflect challenges associated with  
Figure 6: Prototype implementation of the new sub-6 GHz four-port antenna system: (**a**) top-view close-up of the metal-frame-integrated antenna showing Ports 1 through 4 with their grounding segments; (**b**) the fabricated smartphone testbed with SMA connectors interfaced to each port. These facilitate VSWR and isolation measurements.