###### Abstract  
In this paper, we propose an interface layer using the coplanar waveguide (CPW) between an array antenna and transmitting receiver modules (TRMs) for X-band military ship radar systems. To improve transmission characteristics, the interface layer with the CPW has three transition parts: Transition part \(A\) is between the sub miniature push-on (SMP) connector and the interface layer. Transition part \(B\) is the CPW in which the thickness gradually increases. Transition part C is for connecting the interface layer and the antenna. The measured reflection and transmission coefficients of the fabricated interface layer are \(-\)22.4 dB and \(-\)0.82 dB, respectively. To verify the proposed interface layer, we then apply the layer to a 2 \(\times\) 2 X-band array antenna and measure the array antenna characteristics, such as reflection coefficients, array antenna gain, and half-power beam widths (HPBWs). The measured reflection coefficient is under \(-\)10 dB from 8.6 GHz to 10.2 GHz, and the bore-sight array gain of the 2 \(\times\) 2 array antenna is 10.5 dBi at 9.5 GHz. In addition, the measured HPBWs under the same conditions are 47.8\({}^{\circ}\) and 39.1\({}^{\circ}\) in the \(zx\)- and \(zy\)-planes, respectively. The results demonstrate that the proposed interface layer using the CPW is suitable for X-band radar systems.  
X-band radar; coplanar waveguide; interface layer; array antennas Article  
Design of an Interface Layer Using CPW between an Array Antenna and TRM in X-Band Radar Systems to Minimize Leakage Fields and Improve Transmission Characteristics. _Appl. Sci._**2022**, _12_, 8514. [[https://doi.org/10.3390/app12178514](https://doi.org/10.3390/app12178514)]([https://doi.org/10.3390/app12178514](https://doi.org/10.3390/app12178514))  
Academic Editor: [PERSON]  
Received: 23 July 2022  
Accepted: 24 August 2022  
Published: 25 August 2022  
**Published:**  
**Published:**  
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between the sub miniature push-on (SMP) connector and the interface layer. Transition part \(B\) is the CPW in which the thickness gradually increases between the thin and thick CPWs in the interface layer. Transition part \(C\) is implemented between the interface layer and the antenna. Through the optimal design of these transition parts, the transmission characteristics of the interface layer are improved, while the leakage fields that cause power loss are minimized. This can achieve enhanced array antenna performance when the interface layer is mounted on actual radar systems. Herein, the target frequency band is determined from 9 GHz to 10 GHz, which is typically used in actual X-band radar systems [22; 23]. For performance verification, the proposed interface layer is fabricated, and the reflection and transmission coefficients are measured in a full anechoic chamber. Moreover, to examine the interface layer's performance when it is mounted on actual radar systems, a fabricated 2 \(\times\) 2 X-band array antenna is connected to the proposed interface layer. Array characteristics including the interface layer, such as the reflection coefficients of array elements and the array beam patterns are measured and compared with simulation results. The results demonstrate that the proposed interface layer using the CPW is suitable for X-band marine radar systems.