## 3 Results
### Antenna Prototype Fabrication and Measurement  
A prototype of the proposed shared-aperture phased array antenna is developed, and the scanning performance is measured in a near-field anechoic chamber, as shown in Figure 10. The excitation of each element by the coaxial Sub Miniature version A (SMA) is fed through the beam controller. The array scanning can be achieved by changing the phase difference of each element. Due to the limitations of test time and venue, the array performances at 1.97 GHz for the S-band and at 3.55 GHz for the C-band were tested. These measured results are sufficient to verify the design validity of the proposed phased array antenna. Figure 11a shows the simulation and measurement S\({}_{11}\) of the proposed antenna array at the S-band. As shown, a good agreement between the simulated and measured is attained, showing a relative bandwidth of about 5.3% from 1.93 to 2.03 GHz with S\({}_{11}\) less than \(-\)10 dB. The simulation and measurement S\({}_{11}\) at the C-band are depicted in Figure 11b. The measurement agrees well with the simulated results, which show a relative bandwidth of about 11% from 3.36 to 3.75 GHz with S\({}_{11}\) less than \(-\)10 dB. The isolation between the nearest two different-frequency elements is shown in Figure 12. For the convenience of analysis, the port sequence is marked in Figure 12. P1 and P2 are two adjacent C-band ports, respectively, while P3 and P4 are two adjacent S-band ports, respectively. The simulated and measured isolations (S\({}_{34}\) and S\({}_{12}\)) between the nearest S-/C-band elements are higher than 20 dB across the operating S-band and C-band. Similarly, the simulated and measured isolations (S\({}_{31}\) and S\({}_{13}\)) between the S-band element and its nearest C-band elements are higher than 20 dB across the operating S-band and C-band. The results represent the proposed S-band and C-band antenna arrays that can effectively work at the same time without serious coupling and interference. In addi  
Figure 10: Prototype of the fabricated as-proposed antenna in a near-field anechoic chamber. Figure 9: The simulated ARs of the S-/C-band CP antenna array. tion, the S\({}_{11}\) and isolations of the proposed phased array were measured through a vector network analyzer, the Rohde & Schwarz ZNB 40. The simulated and measured normalized radiation pattern of the S-band array is shown in Figure 13. Notably, \(\theta\) and \(\varphi\) are the scanning angles of the array (\(\theta\) represents the elevation angle and \(\varphi\) represents the azimuth angle). In addition, the excitations of all ports are of equal amplitude, and the scanning of the array is achieved by changing the phase differences among the ports. A maximum gain drop of approximately 4.5 dB is observed when the scan angle is up to 50\({}^{\circ}\) (\(\theta=50^{\circ}\), \(\varphi=0^{\circ}\)). As can be seen, an obvious grating lobe appeared when the array pattern pointed down to 50\({}^{\circ}\) as the element spacing is exactly 0.58 \(\lambda\) at 1.97 GHz, which is close to the minimum spacing of generating grating lobes. Because the gain drop is less than 4.5 dB, the above grating lobe is still acceptable. The simulated and measured radiation pattern of the C-band array at the center frequency of 3.55 GHz is shown in Figure 14a. The maximum gain drop of 3.4 dB is observed when the beam is scanning up to 50\({}^{\circ}\) (\(\theta=50^{\circ}\), \(\varphi=0^{\circ}\)), the measurement result agrees well with the simulated results. Limited by the number of phase-shifter channels of the testing equipment, only the scanning properties of the 64-element subarray located in the center of the 256-element array are measured. However, the sim  
Figure 11: Simulated and measured S-parameters of the antenna (a) at S-band; and (b) at C-band. Figure 12: Simulated and measured isolations between the S-/C-band. 256-element array at 3.55 GHz, as presented in Figure 14b, show that a larger phased array has a better scanning performance at a large angle with gain drops less than 3 dB.