## 3 Verification Using an X-Band Radar Array Antenna  
To verify the performance of the proposed interface layer using the CPW when it is mounted on an actual radar system, the proposed interface layer was combined with X-band E-shape antennas [26] that can operate in broadband. Figure 7a-c show the geometry of the 2 \(\times\) 2 E-shape patch array for X-band radar systems. In this configuration, the array spacing  
Figure 5: The reflection coefficients in accordance with \(d_{1}\) and \(d_{2}\) in transition part \(B\).  
Figure 6: The reflection coefficients in accordance with \(r_{\text{f}}\) and \(r_{\text{d}}\) in transition part \(C\).  
was determined to be a half-wavelength at 9.5 GHz, and the interface layer using the CPW was placed in the bottom layer. Figure 7d,e show photographs of a fabricated 2 \(\times\) 2 array antenna to assess the performance of the proposed interface layer. The fabricated 2 \(\times\) 2 array antenna with the interface layer was measured in a full anechoic chamber to obtain the reflection coefficients of the array elements and the array beam patterns. Figure 7e does not include a feeding network, and during measurements, a feed line was connected to each port of the array to obtain the array gain. Figure 8 represents the reflection coefficients of the 2 \(\times\) 2 array antenna in the X-band. The measured reflection coefficients for each element had a broad bandwidth greater than the frequency range from 8.6 GHz to 10.2 GHz and were consistent with the simulation results. Figure 9 shows the measured and simulated bore-sight gains of the 2 \(\times\) 2 array with and without the interface layer. The solid line is the simulation result with the interface layer, and the dashed line is the result without the interface layer. The '\(\times\)' markers indicate the measurement with the interface layer. The measured results agreed well with the simulations. The array antenna gains with and without the interface layer were greater than 10.1 dBi in the target frequency range from 9 GHz to 10 GHz. Figure 10 illustrates the measured and simulated 2-D array beam patterns of the 2 \(\times\) 2 array antenna in the _zx_- and _zy_-planes at 9.5 GHz. The bore-sight gains were 10.5 dBi for the measurement and 10.6 dBi for the simulation. In addition, the half-power beamwidths (HPBWs) were 47.8\({}^{\circ}\) and 39.1\({}^{\circ}\) in the _zx_- and _zy_-planes, and simulated HPBWs were 46.5\({}^{\circ}\) and 40.3\({}^{\circ}\) in the _zx_- and _zy_-planes, respectively. Thus, the measurement results for the 2 \(\times\) 2 arrays with the interface layer were in good agreement with the simulation results. These results demonstrate that the proposed interface layer using the CPW can be applied to X-band marine radar systems.  
Figure 7: Geometry and fabrication of the 2 \(\times\) 2 E-shape array antennas with the proposed interface layer: (**a**) isometric view; (**b**) top view; (**c**) side view; (**d**) top view photograph; (**e**) bottom view photograph.  
Figure 8: Reflection coefficients of the 2 \(\times\) 2 X-band array.  
Figure 10: Array beam patterns of the 2 \(\times\) 2 X-band array.  
Figure 9: Bore-sight gain of the 2 \(\times\) 2 array.