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

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 × 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 × 2 array antenna is 10.5 dBi at 9.5 GHz. In addition, the measured HPBWs under the same conditions are 47.8° and 39.1° 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.

1. Introduction

Recently, with the gradual development of military ship radar systems, the demand for improved design technologies for X-band array antennas has been increasing for target search and tracking, maritime traffic monitoring, and maritime navigation [1,2,3]. In such radar systems, various antenna types, such as horn antennas [4,5], conical antennas [6,7], and Vivaldi antennas [8,9,10,11], have been implemented as X-band radar elements. However, when such antenna elements are mounted on ship radars, there is a critical disadvantage in that they are structurally unstable due to their bulky size and heavy weight. To resolve these problems, patch array antennas are typically used in X-band marine radar systems due to their light weight, ease of manufacturing, and low profile [12,13,14,15]. Since an X-band radar system must be capable of electric beam steering, a patch array antenna should be connected to transmitting receiver modules (TRMs) [16,17,18]. To stably connect the antenna and the TRMs, interface layers are often employed between the antenna and the TRM using various transmission lines, such as coplanar waveguides (CPWs) [19], microstrip lines [20], and strip lines [21]. Although some research has been conducted to develop interface layers with improved reflection and transmission characteristics, there is not enough in-depth consideration of the performance when the interface layer is mounted on the array antennas of actual radar systems.

In this paper, we propose an interface layer using a CPW between an array antenna and TRMs for X-band ship radar systems. To improve transmission characteristics, the interface layer with the CPW consists of three transition parts: Transition part A is applied 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 × 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.

2. Proposed Interface Layer Using a CPW

2.1. Geometry of the Interface Layer Using the CPW

Figure 1 presents the geometry of the interface layer using the CPW considering the connection part between the antenna and TRM in X-band radar systems. To enhance the transmission characteristics and minimize the leakage fields of the CPW, the proposed interface layer consists of three transition parts: A, B, and C. Transition part A is located between the SMP connector and the CPW line of the interface layer. The SMP is a surface-mounted connector, and it is generally recommended with this connector to use a CPW without ground [24]. However, to enhance the transmission characteristics with minimum leakage of electric fields, vias should be densely inserted into the ground plane around the CPW. X-band radar systems typically use a small connector due to the narrow array distances of the antenna elements. Thus, the CPW line mounted on the SMP connector has a thin width of w₁ and a gap of g₁, as shown in Figure 1b. In contrast, the CPW line near the SMP connector has a greater width of w₂ and a gap of g₂ to prevent dropout of the inner metallic line when the feed-pin is connected to the CPW line using solder. Transition part B is the CPW, the thickness of which gradually increases between the thin and thick CPWs in the interface layer. The width of this transition line with a length of d₁ is gradually varied from w₁ to w₂ to obtain suitable impedance-matching characteristics. Finally, transition part C is implemented between the ground of the CPW and the feed-pin, as shown in Figure 1c. The transition part C comprises the feed-pin radius (r_f) and the radius of the perforated part of the ground plane (r_d). The transmission characteristics of the interface layer can be further enhanced by adjusting the ratio of r_f to r_d. The proposed interface layer using the CPW uses an FR-4 substrate (ε_r = 4.4, tan δ = 0.027) with a thickness of h₁, and it is designed using a FEKO full electromagnetic simulator [25]. The detailed design parameters of the proposed interface layer using the CPW are listed in

Table 1. Optimal design parameters of the interface layer.

Parameter	Value (mm)
h1	0.8
w1	0.75
w2	1.5
g1	0.1
g2	0.37
d1	0.85
d2	1.35
rf	0.22
rd	1.3

.

2.2. Fabrication and Measurement

Figure 2a,b present photographs of the fabricated interface layer. The fabricated interface layer using the CPW has dense vias around the CPW line. The SMP connector and feed-pin are combined with the CPW of the interface layer by soldering. To measure its transmission coefficients, the proposed interface layer is extended to a back-to-back configuration, which allows for investigation of radar performance degradation caused by the interface layer, as shown in Figure 2c. Figure 2d illustrates a photograph of the fabricated back-to-back configuration. In this configuration, the bottoms of the CPWs face each other, and the feed-pin penetrates the two CPW layers. To verify its feasibility, the fabricated interface layer in the back-to-back configuration is measured in a full anechoic chamber. Figure 3 represents the measured and simulated reflection (S₁₁) and transmission coefficients (S₂₁) of the proposed interface layer in the back-to-back configuration. The measured (solid line) and simulated (dashed line) reflection coefficients are below −22.4 dB and −29.2 dB, respectively, in the operating frequency. In addition, the transmission coefficients for the measured results exceed −0.82 dB (dotted line), and for the simulated results exceed −0.14 dB (dashed–dotted line). The interface layer characteristics, such as reflection coefficient, target frequency, and interface layer size, are listed in Table 2 to compare them with previous studies. The proposed interface layer has good reflection characteristics and a small size in our target frequency band from 9 GHz to 10 GHz. These results demonstrate that the performance of the proposed interface layer using the CPW is suitable for X-band radar applications.

Table 2. Comparisons of the interface layer characteristics.

Research	Reflection Coefficient	Target Frequency	Size (mm)
[16]	−20 dB	8–12 GHz	13 × 16 × -
[19]	-	8.5–11 GHz	14 × 14 × 6.1
[20]	−13.3 dB	8.5–10.5 GHz	20 × 20 × 3.7
[21]	−20.8 dB	8.8–10.4 GHz	20 × 20 × 2.6
Proposed interface layer	−26.9 dB	9–10 GHz	17 × 17 × 0.8

Table 2. Comparisons of the interface layer characteristics.

Research	Reflection Coefficient	Target Frequency	Size (mm)
[16]	−20 dB	8–12 GHz	13 × 16 × -
[19]	-	8.5–11 GHz	14 × 14 × 6.1
[20]	−13.3 dB	8.5–10.5 GHz	20 × 20 × 3.7
[21]	−20.8 dB	8.8–10.4 GHz	20 × 20 × 2.6
Proposed interface layer	−26.9 dB	9–10 GHz	17 × 17 × 0.8

2.3. Analysis of the Transitions of the Proposed Interface Layer

Figure 4 shows the normalized E-field distributions according to the configurations in transition part A. The E-field distributions were investigated on the zx-plane (y = 0 mm) at an observation frequency of 9.5 GHz. To analyze the influence of the configurations, we investigated the normalized E-field to observe the leakage E-field in the ranges of 1 mm ≤ z ≤ 3 mm and −3 mm ≤ z ≤ −1 mm, which correspond to the outside of the interface layer. The mean value in the case without ground was 0.1397, as shown in Figure 4a, whereas that in the case with ground was 0.1018, as shown in Figure 4b. On the other hand, when the dense vias were inserted with ground, as shown in Figure 4c (proposed design), the mean value was observed to be 0.0876. Thus, the transition part A of the proposed interface layer using the CPW should be designed to include ground and vias to minimize the leakage fields.

Figure 5 represents the reflection coefficients at 9.5 GHz according to the variations of d₁ and d₂ applied to examine the influence of transition part B. In this analysis, d₁ varied from 0.05 to 1 mm, and d₂ varied from 0.9 to 1.85 mm. Herein, design parameters of d₁ = 0.85 mm and d₂ = 1.35 mm were chosen, taking into account the ease of fabrication and reflection coefficients of less than −30 dB. A reflection coefficient of −30.7 dB was observed when the design parameters of d₁ and d₂ were 0.85 mm and 1.35 mm, respectively. These results indicate that the transmission characteristics can be improved by adjusting transition part B.

Figure 6 illustrates the parametric study of transition part C, which consists of the feed-pin (radius of r_f) and the perforated ground (radius of r_d). The transmission characteristics can be improved by adjusting the ratios of r_f and r_d. To find the minimum reflection coefficient at 9.5 GHz, the parameter r_f was varied from 0.1 to 0.3 mm in increments of 0.02 mm, and r_d was changed from 0.8 to 1.4 mm in increments of 0.05 mm. In Figure 6, the dashed line represents the minimum reflection coefficients according to r_f, indicating a required ratio of r_f to r_d. Since the proposed interface layer uses a feed-pin with a radius (r_f) of 0.22 mm, the optimal radius of the perforated ground (r_d) is 1.4 mm. However, in our design, we chose a slightly smaller radius (r_d) of 1.3 mm to avoid overlapping with the surrounding vias while maintaining the reflection coefficient below −30 dB. The reflection coefficients were −31.4 dB at 9 GHz and −29.2 dB at 10 GHz, respectively.

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 × 2 E-shape patch array for X-band radar systems. In this configuration, the array spacing 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 × 2 array antenna to assess the performance of the proposed interface layer. The fabricated 2 × 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 × 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 × 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 ‘×’ 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 × 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° and 39.1° in the zx- and zy-planes, and simulated HPBWs were 46.5° and 40.3° in the zx- and zy-planes, respectively. Thus, the measurement results for the 2 × 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.

4. Conclusions

We investigated the interface layer using the CPW in X-band radar systems to minimize the leakage electric fields and improve the transmission characteristics. To prevent performance degradation in the radar systems, the interface layer with the CPW had three transition parts. Transition part A was between the SMP connector and the interface layer. Transition part B was the CPW in which the thickness gradually increased. Transition part C was for connecting the interface layer and the antenna. In addition, we proposed the interface layer using the CPW in the back-to-back configuration, which could observe the performance deteriorations caused by the interface layer in the radar systems. The reflection and transmission coefficients layer in the back-to-back configuration were measured to be −22.4 dB and −0.82 dB, respectively. To examine the performance when the interface layer using CPW was mounted on the actual radar systems, X-band 2 × 2 E-shape antennas with broadband characteristics were connected to the proposed interface layer. The bore-sight gain of the 2 × 2 array antenna was 10.5 dBi at 9.5 GHz. In addition, the HPBWs under the same conditions were 47.8° and 39.1° in the zx- and zy-planes, which are in good agreement with the simulated results. The results demonstrated that the proposed interface layer using the CPW was suitable for X-band marine radar systems.