## 3 Experimental Results and Discussion  
To verify the antenna performance, the proposed low-SLL differential-fed 6 \(\times\) 16 SIW-based SAA is fabricated and tested. A photograph of the fabricated antenna is displayed in Figure 7 with an overall dimension of 420 mm \(\times\) 145 mm \(\times\) 4 mm. The 6 \(\times\) 16 slot array occupies an area of 247 mm \(\times\) 88 mm, which corresponds to 8.23\(\lambda_{0}\) \(\times\) 2.93\(\lambda_{0}\) (\(\lambda_{0}\) is the  
Figure 6: (a) Implementation of the decoupling walls; (b) effect of the decoupling wall on \(|S_{11}|\) and realized gain of the proposed low-SLL, differential-fed, SIW-based SAA. free-spaced wavelength at 10 GHz). The decoupling walls and the antenna fixture are made of alumina and are fixed to the substrate by small metallic screws. A commercial WG-to-coax adapter (product number: 16094-SF40) is used to feed the fabricated antenna. An Agilent E8362C network analyzer is used to measure the reflection coefficient. The radiation properties are measured in an anechoic chamber with a dimension of 16 m \(\times\) 11 m \(\times\) 9 m, as shown in Figure 8.  
Figure 9 illustrates the measured and simulated reflection coefficients versus frequency. The measurement agrees well with the simulation. The measured \(-\)10 dB reflection bandwidth ranges from 9.57 to 10.28 GHz, i.e., a relative bandwidth of 7.15%. The measured and simulated realized gains in the broadside direction against the frequency are illustrated in Figure 10. The realized gain ranging from 20.30 to 21.92 dB is obtained from the measurement result, while the simulated one is ranging from 20.72 to 22.18 dBi. A slight deviation is found between the simulation and measurement, which is mainly attributed to the fabrication tolerances and the measurement errors. Figure 10 also plots  
Figure 8: Far-field measurement setup of the proposed low-SLL, differential-fed, SIW-based SAA. Figure 7: Photograph of the proposed low-SLL, differential-fed, SIW-based SAA: (**a**) top view and bottom view and (**b**) overall view. angles in the \(E\)- and \(H\)-planes versus frequency of the proposed low-SLL, differential-fed, SIW-based SAA. In reference to the figure, the zero beam squinting is observed over the entire measured frequency band for both the \(E\)- and \(H\)-planes. Figure 11 shows the simulated and measured radiation patterns at 9.6, 10, and 10.25 GHz in both the \(\gamma oZ\)-plane (\(E\)-plane) and the \(XoZ\)-plane (\(H\)-plane). The radiation patterns are measured at elevation angles ranging from \(-90^{\circ}\) to \(+90^{\circ}\), for co- and cross-polarizations. The main beam direction in the \(YoZ\)- and \(XoZ\)- planes stays in the boresight direction at all three frequencies. At 10 GHz, the peak measured SLLs are \(-29.1\) dB in the \(E\)-plane and \(-29.4\) dB in the \(H\)-plane, which are close to the ideal value of \(-30\) dB. The peak measured SLLs in the \(E\)-plane are increased to \(-23.4\) dB at 9.6 GHz and to \(-24.4\) dB at 10.25 GHz. This is because the six-way unequal power divider is designed and optimized at 10 GHz. Meanwhile, the peak measured SLLs in the \(H\)-plane are \(-31.5\) dB and \(-29.8\) dB at 9.6 GHz and 10.25 GHz, respectively. Furthermore, as observed in Figure 11, the measured cross-polarization level of less than \(-30\) dB is obtained in the boresight direction. A comparison between the proposed antenna and the other low-SLL, SIW-based SAAs is conducted and listed in Table 2. It can be seen from the comparison that the proposed antenna has a much wider impedance BW than previous designs while maintaining low SLL features in  
Figure 10: Simulated and measured realized gains and main beam angles. Figure 9: Measured and simulated reflection coefficients. both the \(E\)-plane and \(H\)-plane. Despite having a lower SLL than the proposed antenna, the antenna in [13] was designed with a goal of \(-35\) dB SLL.