A Low-RCS 2D Multi-Layer Van Atta Array at X-Band

Abstract

This paper presents a novel approach to reducing radar cross section (RCS) using a 2D multi-layer Van Atta array based on the phase cancellation principle. By controlling the phase of transmission lines using wideband phase shifters, the proposed array can achieve significant RCS reduction at a wide frequency range of 8 GHz to 11 GHz. Both theoretical calculations and experimental measurements were conducted to evaluate the performance of the Van Atta array with phase shifters. Results showed significant RCS reduction from various incident angles, demonstrating the effectiveness of the proposed design in achieving wideband RCS reduction at the X-band.

1. Introduction

Radar cross section (RCS) reduction of antennas has attracted much attention in recent years and has found wide application in different fields, especially in stealth technology such as fighter planes. Considerable research interest in radar cross section reduction has been triggered by the development of modern stealth technology. The methods of using radar-absorbing materials and phase cancellation screens are studied. Absorbing materials could realize RCS reduction and are applied to some applications due to their advantages of being flexible and convenient, such as microwave absorbing materials (MAMs) [1,2,3] and high impendence surfaces (HISs) [4,5]. However, their effectiveness is limited by narrow bandwidth, frequency, and incident angle dependence. Distinct from the approach of using absorbing materials to absorb electromagnetic waves, another method of achieving RCS reduction is by using phase cancellation theory to redirect or cancel the incident wave by modifying the characteristics of the reflected wave, such as frequency-selective surfaces (FSSs), artificial magnetic conductors (AMCs), and polarization rotation reflective surfaces (PRRSs). They can achieve wideband RCS reduction and high reduction rates [6]. An FSS is a periodic surface with identical 2D arrays of elements arranged on a dielectric substrate, exhibiting transmission and reflection at certain resonant frequencies [7,8,9,10]. As for AMC, the RCS reduction is realized with the combination of AMC units and perfect electronic conductors [11,12,13,14]. PRRS could reflect the incoming wave with a 90° polarization rotation to realize RCS reduction [15,16]. Recently, a low-wideband RCS antenna co-designed with a high-performance AMC-FSS radome has been formed, and the monostatic RCS for a normal incidence is greatly reduced from 4–18 GHz [17]. However, these methods mentioned above generally need a large space, and the RCS reduction rate of these methods is rapidly decreased depending on the angle of the incident wave.

Addressing these challenges, a method was developed to obtain angular independence in RCS reduction by adjusting the length of transmission lines in the Van Atta array. The RCS of this structure is reduced at the angle of incidence by making a null [18]. The simulation result indicated that the Van Atta array exhibited a lower RCS level than the identical-sized metal plate. Reference [19] used the adjustable transmission line switch from 0 to $\pi$ phase to add phase modulation of the retro-reflected signal, and the results show the Van Atta array can realize phase modulation in a large range of the incident angle. However, it is important to note that the RCS reduction achieved through this method is limited to a narrow frequency band due to the variations in the phase of the same-length transmission lines at different frequencies.

This paper discusses a low-RCS 2D multi-layer Van Atta array based on a microstrip antenna operating at X-band. The array’s construction is shown in Figure 1; the line connection pattern of the array is shown in Figure 1a, and the side view of the array is shown in Figure 1b. The antennas on the top layer with each unit are connected to the feeding network on the bottom layer by the probe, and the ground plane is between the top layer and the bottom layer. Wideband RCS reduction could be realized by using wideband phase shifters to adjust the phase of the feeding network. Table 1 shows the comparison of different RCS reduction methods, indicating that our proposed array performs well in both bandwidth and angle independence compared to other RCS reduction methods.

Figure 1. A 2D multi-layer Van Atta Array: (a) line connection pattern of the array; (b) side view of the array.

Table 1. Comparison of different RCS reduction methods.

Methods	Ref.	Frequency (GHz)	RCS Reduction Level	Incident Angles	Bandwidth	Angle Independence
absorbing material	[2]	2–18	−10 dB	±30°	wideband	no
	[3]	0.878 and 0.965	−25 dB	not mentioned	narrowband	no
FSS, AMC, and PRRS	[11]	15–15.4	−20 dB	±15°	narrowband	no
	[13]	5.4–14.2	−10 dB	±10°	wideband	no
	[14]	3.77–10.14	−10 dB	±20°	wideband	no
	[16]	7.5–22.5	−10 dB	±30°	wideband	no
	[17]	4–18 GHz	−13.7 dB	±25°	wideband	no
Van Atta array	[18]	not mentioned	−25 dB	over ± 40°	narrowband	yes
	[19]	9.75–10.75	not mentioned	large range angle	narrowband	yes
proposed array	-	8–11	−20 dB	maximum ± 90°	wideband	yes

2. Theory and Design

The Van Atta array is a retrodirective array composed of a pair of antennas connected by equal-length transmission lines and symmetrically located around the array center. As shown in Figure 2, it can reverse the electromagnetic wave incident from an unknown angle. Generally, because of the retrodirective characteristic, the Van Atta array can maintain a relatively larger RCS over a large range of incidence angles [20,21].

Figure 2. Van Atta array function.

According to the principles of phase cancellation theory, two signals that possess equal amplitude but are in phase amplify each other, while signals that are out of phase by 180° could cancel each other out [22]. Therefore, adding a 180° phase shift to half of the Van Atta array transmission lines can achieve significant phase cancellation [18]. To reduce RCS, four transmission lines out of eight were selected to add a 180° phase shift, resulting in 35 different combinations. In this paper, the selected feeding method is shown in Table 2.

Table 2. Transmission line phase selection.

Transmission Lines Number	Phase [deg]
line1	180
line2	180
line3	0
line4	0
line5	180
line6	0
line7	180
line8	0

The theoretical calculation result for phase cancellation of Table 2 combination at 10 GHz is shown in Figure 3, which shows that this method can realize phase cancellation, leading to a significant reduction in the reflected wave at the incident angle. The RCS reduction can achieve more than 40 dB.

Figure 3. Phase cancellation of the array.

Conventionally, the Van Atta array reduces the RCS level by changing the length of transmission lines, which is only effective in narrowband. To achieve wideband RCS reduction, the Van Atta array element must be wideband, and the transmission line needs to realize a 180° phase shift across the wide frequency range. This paper employs a wideband U-slot antenna as the array element and utilizes wideband phase shifters to control the phase of transmission lines. This enables effective RCS reduction across a broad frequency range. The phase shifter structure and the phase difference are shown in Figure 4. Apparently, the phase shifter is able to achieve a phase shift of 180° ± 10° over 8 to 11 GHz.

Figure 4. Wideband phase shifter and the phase difference.

The construction of the transmission lines as the feeding network of the low-RCS Van Atta array was designed. Based on Table 2, four phase shifters were used on eight transmission lines. The phase differences between line1 and other transmission lines are shown in Figure 5a,b, indicating that all phase differences are kept within the range of ±10°. In Figure 5c, variations in RCS reduction with different phases are presented, which indicates that the RCS reduction of 180° is more than 25 dB from 8 GHz to 11 GHz, whereas it is less than 18 dB when the phase deviates 10° from 180° (i.e., 170° and 190°).

Figure 5. Phase difference between transmission lines and its impact on RCS reduction. (a) Phase difference between line1 and line2, line5, and line7; (b) phase difference between line1 and line2, line4, line6, and line8; (c) RCS reduction with different phases.

In this paper, a U-slot wideband microstrip antenna unit is designed to form a 16-element array. The configuration of the antenna unit and the VSWR of the antenna are shown in Figure 6, which demonstrates that the antenna could work in the frequency band from 8 GHz to 11 GHz.

Figure 6. Antenna unit and VSWR.

Two types of 16-element Van Atta arrays were manufactured, including a regular retrodirective multi-layer array named Array1 and a low-RCS multi-layer array named Array2. The structure of the top layer is identical for both arrays, as shown in Figure 7a. The size of the top layer is 135 mm × 115 mm × 3 mm (${\epsilon }_r=3.5$). The bottom layer of Array1 as the feeding network consists of eight transmission lines of the same length, whereas the bottom layer of Array2 consists of eight equal-length transmission lines with four wideband phase shifters whose phase shift is 180° ± 10° over the band of 8–11 GHz. The bottom layers of Array1 and Array2, as shown in Figure 7a,b, have the same total dimension of 135 mm × 115 mm × 0.6 mm (${\epsilon }_r=6.15$). The top and bottom layers of both Array1 and Array2 are integrated using metal vias.

Figure 7. Photos of fabricated antenna arrays: (a) top layer of the arrays; (b) bottom layer of Array1; (c) bottom layer of Array2.

3. Measurement Setup and Results

Figure 8 illustrates the setup used for monostatic RCS measurement. It consists of two horn antennas and a turntable. One of the horn antennas functions as the transmitter, and the other operates as the receiver, both connected to a vector network analyzer (VNA). These two horn antennas are placed side by side and directly point at the proposed antenna array. To ensure a plane wave incidence, the distance between the antenna array and horn antennas is set to be greater than $\frac{2D^2}{\lambda }$, where D represents the maximum dimension of the antenna array, and $\lambda$ denotes the wavelength. As the turntable rotates, the incident angle changes accordingly. When the top layer of the antenna array is aligned with the horn antennas, the incident angle is set to 0°.

Figure 8. Monostatic RCS setup.

Comparing the retrodirective antenna array (Array1) with the low-RCS antenna array (Array2), we conducted monostatic RCS measurements in an anechoic chamber. The measurement results of the arrays are presented in Figure 9 and Figure 10, showing the RCS reduction at different incident planes of 0° and 90°. The monostatic RCS of the retrodirective antenna array (Array1) is higher than that of the Van Atta array with phase shifters (Array2). Specifically, the RCS level of Array2 is approximately 20 dB lower than that of Array1 at an initial angle of 8 GHz to 11 GHz. These results demonstrate the effectiveness of the proposed Van Atta array in achieving RCS reduction across a wide frequency range.

Figure 9. Monostatic RCS of Array1 and Array2 from 8 GHz to 11 GHz (incident plane = 0°): (a) 8 GHz; (b) 9 GHz; (c) 10 GHz; (d) 11 GHz.

Figure 10. Monostatic RCS of Array1 and Array2 from 8 GHz to 11 GHz (incident plane = 90°): (a) 8 GHz; (b) 9 GHz; (c) 10 GHz; (d) 11 GHz.

As shown in Figure 9 and Figure 10, the performance of the incident plane at 90° surpasses that of the incident plane at 0°. This discrepancy can mainly be attributed to the non-symmetrical construction of the U-slot antenna unit. Notably, at the frequency of 11 GHz, the RCS reduction results are less significant compared to other frequencies due to the increased phase deviations among transmission lines.

4. Conclusions

In this study, we designed a 2D multi-layer Van Atta array with phase shifters to reduce monostatic RCS effectively. By controlling the phase of transmission lines, the proposed array achieved phase cancellation across a wide frequency range, resulting in a significant RCS reduction. Both simulation and experimental results demonstrated that the Van Atta array with phase shifters effectively reduced RCS from 8 GHz to 11 GHz at various angles of incidence. Overall, the proposed approach presents a promising solution for designing low-RCS antennas for radar and communication systems.