A Wideband Polarization-Insensitive Bistatic Radar Cross-Section Reduction Design Based on Hybrid Spherical Phase-Chessboard Metasurfaces

Abstract

A wideband polarization-insensitive bistatic radar cross-section (RCS) reduction design under linear and circular polarization incidence is proposed based on spherical-chessboard metasurfaces. A new metasurface element with wideband characteristics was designed, including a double split-ring structure, single-layer media, and metal board. In the proposed RCS-reduction design, the Pancharatnam–Berry (P-B) phase theory is applied with the designed metasurface element to realize phase distribution mimicking the low-scattering sphere, and thus realizing RCS reduction. In addition, the chessboard configuration is combined with spherical phase distribution to further improve the performance of monostatic and bistatic RCS reduction. Finally, the proposed RCS reduction design can not only realize wideband RCS reduction but also exhibit polarization-insensitive characteristics. It realized 10 dB monostatic and bistatic RCS reduction in a frequency band ranging from 8.5 to 21 GHz (84.8% relative bandwidth) under linear polarization (LP) and circular polarization (CP) incidence. The straightforward and efficient design method of the hybrid spherical chessboard can effectively avoid the complex and time-consuming optimization process in RCS-reduction design.

1. Introduction

In the past two decades, metamaterials have attracted much attention due to their remarkable electromagnetic properties. Metamaterials are rationally designed arrays of anisotropic scattering elements with some special electromagnetic properties, such as negative refractive index [1], zero refractive index [2], and ultrahigh refractive index [3]. The two-dimensional structure of metamaterials, the metasurface, consists of subwavelength structures on a two-dimensional surface that can effectively manipulate electromagnetic waves, including wavefront shaping [4], polarization switching [5], radiation control, and beam concentration [6]. Due to these versatile electromagnetic properties, metasurfaces have attracted a great deal of attention from the research community and have been used in several applications in the last decade, such as meta-lenses [7], invisibility cloaks [8], absorbers [9], vortex beam generators [10], holography [11], etc.

Radar cross section (RCS) reduction is of great significant for the survival of targets under various detectors. Metasurfaces, as emerging 2-D structures with the characteristics of low profile, low cost, and easy fabrication, can flexibly manipulate the electromagnetic wave which is widely applied in RCS-reduction designs [12,13,14,15,16]. The main principle of designing RCS-reduction-exploiting metasurfaces is to combine metasurfaces with different reflection coefficient phases to form the phase difference in the reflected field, thereby cancelling the reflected field in a certain direction. For monostatic RCS reduction, many RCS-reduction designs composed of different metasurfaces in a chessboard configuration have been proposed [17,18,19,20,21]. In these RCS-reduction designs, even though the monostatic RCS reduction is achieved by combining different metasurfaces for phase cancellation of the boresight reflected field, there is still strong scattering in other directions which degrades the bistatic RCS-reduction performance.

In order to improve the bistatic RCS reduction, the arrangement of the chessboard is further optimized to form phase cancellation in more directions of the overall space [22,23,24,25,26]. In addition, coding metasurfaces such as 1-bit, 2-bit, 3-bit, and multi-bit are also used to design bistatic RCS reduction [27,28,29,30], while the phase distribution of coding metasurfaces also needs to be optimized for bistatic RCS reduction. In a word, the chessboard configuration and coding metasurfaces can be effectively used for bistatic RCS reduction. However, in order to obtain the ensured chessboard arrangement and phase distribution corresponding to bistatic RCS reduction, the complex and time-consuming optimization algorithm is inevitable. Therefore, it is necessary to find a straightforward and efficient bistatic RCS-reduction-design method.

In the paper, a wideband polarization-insensitive bistatic RCS-reduction-design method is proposed. The proposed design method can efficiently realize wideband monostatic and bistatic RCS reduction without the complex and time-consuming optimization algorithm. Based on the low-scattering characteristics of a sphere, the Pancharatnam–Berry (P-B) phase theory is combined with the metasurface to mimic the spherical phase distribution for RCS reduction. Further, the chessboard configuration is combined with spherical phase distribution to realize better bistatic RCS-reduction performance. In addition, a new metasurface element with wideband characteristics was also designed for wideband RCS reduction. Finally, under linear polarization (LP) and circular polarization (CP) incidence, the proposed design realized 10 dB monostatic and bistatic RCS reduction ranging from 8.5 to 21 GHz (84.8% relative bandwidth).

The rest of this paper is organized as follows: The design of metasurfaces with wideband characteristics is presented in Section 2. Section 3 provides the wideband monostatic and bistatic RCS-reduction design based on the hybrid spherical-chessboard metasurfaces. The RCS-reduction results are proposed in Section 4. Finally, conclusions are summarized in Section 5.

2. The Design of the Wideband Metasurface

The designed metasurface is composed of a metal structure and a substrate (thickness = 3 mm, dielectric constant = 2.2) backed with a perfect electric conductor (PEC). The metal structures consist of a Y-shaped structure and multiple split rings, and are shown in Figure 1a. The metasurface has a structural symmetry axis along the Y-axis direction. The specific structural parameters were set as: the period P = 8 mm, the width of Y-shaped structure w₂ = 1.6 mm, the cross angle α₄ = 20°, the width of split ring w₁ = 0.2 mm, the gap between the different split rings g = 0.3 mm, and the open angles of the split rings were α₁ = 30°, α₂ = 40°, and α₃ = 60°.

The reflection coefficient of the designed metasurface was simulated by CST Microwave Studio with the unit cell boundary conditions under different polarization incidences. The amplitude of the co-polarization and cross-polarization reflection coefficients was proposed as shown in Figure 1b. Under linear polarization (LP) and circular polarization (CP) incidences, the amplitude of the co-polarization reflection coefficient was close to 0 dB in wideband and the amplitude of the cross-polarization reflection coefficient was kept less −10 dB in the wideband ranging from 8.1 to 23.4 GHz (97.1% relative bandwidth).

Then, based on the Pancharatnam–Berry (P-B) phase theory, the phase of the co-polarization reflection coefficient could be adjusted by changing the rotation angle θ at 15 GHz (center frequency), which is shown in Figure 2b. With the increase in the rotation angle θ from 0° to 180°, the phase of the reflection coefficient also changed, ranging from 0° to 360°. In addition, with the change in rotation angle, wideband characteristics could be maintained. The required aperture field phase distribution could be realized by rotating the wideband metasurface, thereby realizing the wideband RCS-reduction design.

3. The RCS-Reduction Designs

A hybrid spherical-chessboard metasurface was designed to realize wideband monostatic and bistatic RCS reduction. In order to highlight the advantages of hybrid spherical-chessboard metasurfaces, three 240 mm × 240 mm RCS-reduction surfaces, termed as MS1, MS2, and MS3, respectively, were designed according to different phase distributions. Based on the low-scattering characteristics of a sphere, MS1 was designed by mimicking the spherical phase distribution. MS2 was the chessboard configuration. Finally, MS3 was the hybrid design of spherical phase distribution and chessboard configuration. The phase-compensation diagrams and array compositions for the three different RCS-reduction arrays, are shown in Figure 3.

Utilizing the sphere as the typical low-scattering structure, the designed MS1 achieved RCS reduction by exploiting the P-B phase of the metasurfaces to mimic the spherical phase distribution. The specific formula of the spherical phase distribution is as follows:

$${\varphi }_{sphere}(x,y)=k\sqrt{0.5\cdot D^2-x^2-y^2}$$

(1)

where the k is the free space wavenumber, D is the length of the side of the surface, and x and y are the coordinates on the surface. Then, based on Equation (1) and the center frequency point of 15 GHz, the specific phase distribution is calculated. According to the P-B phase theory, the designed MS1 is also proposed. In addition, the phase distribution of chessboard configuration is also proposed and is shown in Figure 2. The 240 mm × 240 mm surface is divided into 6 × 6 sub-chessboards where each sub-chessboard consists of 5 × 5 identical metasurfaces. The chessboard configuration alternates between 0° and 180° to realize monostatic RCS reduction. The designed RCS-reduction surface MS2 is also presented and was based on the P-B phase theory [31]. Finally, based on the spherical phase distribution and chessboard configuration, the RCS-reduction surface MS3 was designed by combining the spherical phase distribution and chessboard configuration.

4. Fabrication and Simulation

The three designed RCS-reduction surfaces were simulated, measured, and compared. The proposed designs were simulated in CST Microwave Studio and measured in and anechoic chamber. Setting the plane wave to vertical incidence, the observation angle of the monostatic RCS was θ = 0°, and θ = 90° for the bistatic RCS.

The simulated farfield pattern of the three designed RCS-reduction surfaces and the PEC of the same size at 14 GHz and 20 GHz are shown in Figure 4 and Figure 5. In Figure 4, compared with the PEC plate, the three RCS-reduction surfaces have the obvious characteristic of dispersing the incident energy under linear polarization (LP) incidence. MS1 and MS2 could only diffuse the incident energy within a certain range, while MS3, with a hybrid of spherical phase distribution and chessboard, could diffuse the incident energy into the whole space. The farfield-pattern results under the circular polarization (CP) incidence are also presented in Figure 5. Similarly, MS3 could more effectively diffuse incident energy.

The simulated monostatic RCS-reduction bandwidth characteristics of the three surfaces under different polarization incidences are shown in Figure 6. Under the linear polarization and circular polarization incidences, the designed MS1 and MS2 could not achieve 10 dB RCS reduction with a certain frequency band, and thus lacked the broadband characteristic of 10 dB RCS reduction, while MS3, with a hybrid of spherical phase distribution and chessboard, could realize 10 dB monostatic RCS reduction in the broadband range from 8.5 GHz to 21 GHz (84.8% relative bandwidth).

The bistatic RCS reductions of the designed surfaces under different polarization incidences are also presented in Figure 7. Bistatic RCS reduction is defined as [32].

$${\sigma }_R^{B\mathrm{i}}={\displaystyle \frac{\max (\mathrm{Bistatic} \mathrm{RCS} \mathrm{with} \mathrm{metasurface})}{\max (\mathrm{Bistatic} \mathrm{RCS} \mathrm{same} \mathrm{sized} \mathrm{PEC})}}$$

(2)

From the results, MS1 could also not realize 10 dB bistatic RCS reduction within a certain frequency band and MS2 can only realize the about 8 dB bistatic RCS reduction in the frequency band, while MS3 with a hybrid of spherical chessboard finally achieved the 10 dB bistatic RCS reduction in wideband ranging from 8.5 GHz to 21 GHz (84.8% relative bandwidth) by combining the advantages of both spherical phase distribution and chessboard configuration. Then, the RCS-reduction performance of MS3 under oblique incidence was analyzed. As shown in Figure 8, the farfield pattern of MS3 under different incident polarizations and angles are presented. Compared with the same sized PEC, the designed MS3 could also effectively diffuse the incident energy throughout the entire space under oblique incidences.

The bistatic RCS reduction in MS3 under oblique incidences at different frequency points is also shown in Figure 9. Under different incidences, the designed MS3 could basically achieve 10 dB bistatic RCS reduction in broadband, except for a small performance degradation near 12 GHz which may have been due to the phase errors and performance degradation of the designed metasurfaces under oblique incidence.

A comparison of the proposed RCS-reduction surface MS3 and other designs is also given in

Table 1. Comparison with other designs.

Refs.	10 dB RCSR BW (GHz, %)	h (mm)	Angular Stability RCSR θin	Working Polarization	With/Without Optimization
[32]	9.3–15.5 (50%) Monostatic	3.3	${50}^{\circ }$	LP	Without
[33]	5.4–7.4 (31.2%) Bistatic	3	${45}^{\circ }$	LP	With
[34]	6.9–9.2 (28.3%) Monostatic	2	${40}^{\circ }$	LP	With
[35]	8–15.8 (68.6%) Monostatic	3.17	${15}^{\circ }$	LP	Without
[36]	9–16 (56%) Monostatic	2	N.A.	LP	Without
[37]	3.9–4.05 (3.8%) Monostatic	2	N.A.	LP	With
This work	8.5–21 (84.8%) Monostatic, bistatic	3	${30}^{\circ }$	LP/CP	Without

. From the comparison, the proposed RCS-reduction design simultaneously had the characteristics of broadband and polarization insensitivity which could realize monostatic and bistatic RCS reduction under linear polarization (LP) and circular polarization (CP) incidences. In addition, the proposed design also had a lower profile than other designs and could maintain performance stability under oblique incidence. Therefore, the proposed RCS-reduction surface is more suitable for various detectors in practical applications. More importantly, the proposed RCS-reduction surface can be directly designed by combining the spherical phase distribution and chessboard configuration without a complex and time-consuming optimization process.

5. Conclusions

A wideband polarization-insensitive bistatic RCS-reduction design combining the spherical phase distribution and chessboard configuration is proposed. A metasurface element with wideband characteristics was designed. Based on the designed metasurface, the Pancharatnam–Berry (P-B) phase theory is applied to realize the corresponding phase distribution without complex and time-consuming optimization. Finally, the designed RCS reduction could realize monostatic and bistatic RCS reduction in broadbands ranging from 8.5 to 21 GHz (84.8% relative bandwidth) under linear polarization (LP) and circular polarization (CP) incidences. The simulated results confirmed the effectiveness of the proposed RCS-reduction surface. This wideband monostatic and bistatic RCS-reduction design could be widely useful in practical applications.