## 3 Results and Discussion  
To validate the proposed antenna design, the 4 \(\times\) 4 CP array with a power divider is fabricated based on PCB technology and experimentally tested. Figure 11 displays the photograph of the fabricated antenna array, substrate1 and substrate2 are tightly stacked by a few nylon screws. A 2.92-mm end launch connector is used to feed signals to the microstrip power divider. To ensure the accuracy of the simulation results, the simulation model includes screws and an end launch connector. Meanwhile, the assembling error is also considered in the design process and analyzed by the simulation software Ansoft HFSS.  
Figure 12a,b show the S11 and AR of the antenna array. The practical test results have slight differences from the simulated ones, which is mainly attributed to the errors in fabrication and measurement. Particularly, there is some air gap between the two substrate pieces when using screws. The gap can be removed by using a bonding layer. The simulated bandwidth with S11 \(<-\)10-dB is 40%, between 22 and 33 GHz, and the measured one is more than 35.7% ranging from 23 to 33 GHz. Additionally, the simulated and measured bandwidth with AR \(<\) 3-dB is 28% (24 to 31.8 GHz) and 30.1% (22.5 to 30.5 GHz), respectively. Figure 12c depicts the broadside gains. It is seen that the experimental maximum gain is 19.2 dBic at 30 GHz, and the 3-dB gain variation spans  
Figure 10: Geometry and simulated S-parameters results of the T-junction: (**a**) geometry; (**b**) simulated S parameters results.  
Figure 9: Configuration of the 4 \(\times\) 4 array prototype: (**a**) top-down view; (**b**) bottom-up view.  
22 to 31 GHz, excepting the drop around 24 GHz, which may be caused by the errors in measurement.  
The radiation patterns of the array at three representative frequency points are measured, as shown in Figure 13a-c. Due to the measurement limitation in our laboratory, only the results within \(\pm 40^{\circ}\) angles are measured. The simulated and experimental data show reasonable agreement. There are some ripples in the main beam, which may be attributed to low signal-to-noise ratio in our in-house built platform [37]. The cross-polarization level is always under \(-10\) dB at different frequencies. A high front-to-back ratio is also observed at all the frequency points.  
Figure 11: Photograph of the fabricated array: (**a**) top-down view; (**b**) bottom-up view.  
Figure 12: Simulated and measured results of the antenna array: (**a**) S11; (**b**) AR; (**c**) gain.  
Finally, the performances of the proposed array are compared with other Ka-band CP antenna arrays. As listed in Table 2, the overlapping impedance and AR bandwidth of the proposed design exceeds most of the referenced arrays. Although [10; 18] have wider bandwidth, they need four substrate layers. Although the SIW design in [15] and the microstrip line in [26] also need two substrate layers, the bandwidth of the two arrays is much narrower. Therefore, our proposed antenna array has the advantages of broadband and fewer substrate layers.  
Figure 13: Simulated and measured radiation normalized patterns of the proposed antenna array: (a) 24 GHz; (b) 28 GHz; (c) 31 GHz.