## 2 Antenna Design
### Multi-Communication Mode  
In this section, multiple NB antenna modes, namely NB antenna modes #4 to #7, are analyzed. When multiple antennas are excited, the active S-parameters and S-parameters are different because the reflection coefficient at each port is affected by the inter-port coupling [33]. To investigate this phenomenon, the active S-parameters for multiple communication modes are compared with the S-parameters for a single communication mode. Figure 12 shows the S-parameters comparison between single communication modes and multi-communication modes. In single-mode operation, only one NB antenna is activated. On the other hand, two or more antennas are excited simultaneously in the multi-communication mode.  
For communication mode #4 (P2 & P3: ON), the active S-parameters for ports 2 and 3 do not agree well with those of the single mode at the same frequencies in low and mid bands, as shown in Figure 12a. As illustrated in Figure 12b, the agreement between the two modes is excellent in communication mode #5 (P2 & P4: ON) due to the relatively large distance separation in terms of wavelength in the high band. In addition to that, the currents excited on the ground plane by NB Antenna #1 and Antenna #3 are almost orthogonal to each other, as shown in Figures 6 and 9. For communication mode #6, the S-parameters match well with each other both in mid and high bands, since the separation between the two antennas is long enough to minimize the coupling effect. For communication mode #7, (P2, P3 & P4: ON), there is a discrepancy in the low and mid bands, as depicted in Figure 12d. Especially, the reflection coefficient in the mid band is affected considerably by the coupling from both NB Antennas #1 and #3. This coupling effect can be alleviated by using a defected ground, a decoupling network, and (or) pattern diversity [24; 25; 26]. However, since the \(-10\) dB reflection coefficient requirement is satisfied almost over entire the desired frequency band, we did not utilize such technologies.  
To further investigate the S-parameter degradation of NB Antennas #2 and #3 in some frequencies under multi-communication modes, namely modes #4 and #7, the surface current distributions are analyzed and shown in Figure 13. Since the performance degradation around 5.2 GHz is evident, as shown in Figure 12a,d, the surface current distributions at 5.2 GHz are evaluated for both communication modes #4 and #7. As shown in Figure 13a, when NB Antennas #1 and #2 are activated simultaneously, currents excited along the left and upper left sides of the ground plane cause electromagnetic coupling between NB Antennas #1 and #2. In turn, the reflection coefficients deteriorate slightly. For communication mode #7 (P2, P3, & P4: ON), a similar coupling mechanism is generated between NB Antennas #1 and #2 as in the case of communication mode #4. However, since a very weak current is excited by NB Antenna #1 along the lower right side of the ground plane, as shown in Figure 13b, the effect of coupling in the reflection coefficient performance of NB Antenna #3 is minimal. The surface current distributions for communication modes #5 and #6 are not shown, since the reflection coefficient comparisons shown in Figure 12b,c are in good agreement.  
Figure 11: Simulated surface distribution of the NB antenna at the resonant frequency of 9.5 GHz.  
Figure 12: Comparisons of simulated active S-parameters for multi-communication and S-parameters for single communication: (**a**) communication mode #4 (P2 & P3 ON), (**b**) communication mode #5 (P2 & P4 ON), (**c**) communication mode #6 (P3 & P4 ON), and (**d**) communication mode #7 (P2 & P3 & P4 ON).  
Figure 13: Simulated surface current distributions for multi-communication mode at 5.2 GHz. (**a**) Communication mode #4 (P2 & P3 ON); (**b**) Communication mode #7 (P2 & P3 & P4 ON).