## 3 Measurement Results of the Proposed Antenna  
Figure 7 illustrates the fabricated array antenna. The manufacturing process of the proposed antenna is as follows: The antenna radiators of the proposed unit cell (one S-band element and four X-band elements) are printed on the RF-60 substrate using etching techniques, as shown in Figure 7a. We then make holes for inserting the antenna connectors, and SMA connectors are mounted on the bottom face of the substrate for all elements. In addition, the inner conductor of the SMA connector is connected to the radiator through soldering. Finally, the unit cell of the proposed antenna is expanded to a 2 \(\times\) 2 array configuration. All elements of the array have their own ports that can be connected to transmitter-receiver modules for digital beamforming, as shown in Figure 7b. The average coupling characteristics in the operating frequency are examined, and these average couplings for the nearest element are less than \(-10.1\) dB in the S-band and \(-15.9\) dB in the X-band. Next, antenna performances such as reflection coefficients, bore-sight gains, and radiation patterns are measured in a full anechoic chamber, as shown in Figure 7c. Figure 8 shows the reflection coefficients of the proposed antenna. The measured and simulated reflection coefficients are indicated by the solid and dashed lines, respectively. In the S-band, the measured fractional bandwidth is 13.6% (2.87 GHz to 3.28 GHz), which is in good agreement with the simulated result of 15.6%. In the X-band, the measured fractional bandwidth is 13.4% (8.60 GHz to 9.82 GHz), which is also similar to the simulated result of 14.1%. To confirm the feasibility of the proposed shared-aperture antenna, the array antenna characteristics, such as a bandwidth, a bore-sight gain, array spacing, number of elements, and unit cell height, are compared with the other previous studies, as shown in Table 3. The results demonstrate that the proposed antenna has broad bandwidths with a small unit cell size compared with the other antennas in previous studies. Figure 9 presents the 2D radiation patterns of the individual element of the proposed antenna. In the S-band, the measured and simulated peak gains are observed in the bore-sight direction, and their values are 2.4 dBi and 3.8 dBi, respectively. In addition, the half power beam widths (HPBWs) of the S-band element in the \(zx\)- and \(zy\)-planes are 131.4\({}^{\circ}\)and 95.9\({}^{\circ}\), respectively, which are in good agreement with the simulated HBPWs of 122.9\({}^{\circ}\) and 96.1\({}^{\circ}\). The measured bore-sight gain for the X-band element is 2.6 dBi, which is the same as the simulation result. Herein, the direction of the main beam is slightly tilted, and the measured and simulated peak gains are 4.8 dBi and 2.7 dBi, respectively. The X-band element HPBWs in the \(zx\)- and \(zy\)-planes are 59.2\({}^{\circ}\) and 89.1\({}^{\circ}\), respectively, which are also similar to the simulated HBPWs of 59.4\({}^{\circ}\) and 89.1\({}^{\circ}\). Figure 6: Antenna performance variations of the X-band element according to the dielectric constant and the thickness of the substrate: (**a**) isolation and radiation efficiency according to the dielectric constant; (**b**) bandwidth and front-to-back (F/B) radiation ratio according to substrate thickness. Figure 8: Reflection coefficients of the proposed antenna: (**a**) for the S-band; (**b**) for the X-band. Figure 7: Fabricated \(2\times 2\) array: (**a**) unit cell with printed radiators; (**b**) top and bottom faces of the \(2\times 2\) array; (**c**) measurement setup.