Radar cross section (RCS) is a key factor in evaluating the radar visibility of stealth objects. To achieve radar stealth, the RCS of various stealth objects must be reduced as much as possible. The application of coding metasurfaces to achieve RCS reduction has attracted widespread attention in recent years. A coding metasurface usually consists of numerous coding elements, which can be divided into N types with many different reflection phases. So the coding metasurface can work as a reflective phased array antenna, and its far-field scattering patterns can be controlled by changing the array pattern. In this way, through the optimization of the array pattern, the coding metasurface can diffuse its incident electromagnetic wave in various directions to achieve RCS reduction.

In recent years, a large number of coding metasurfaces have been proposed [1–16]. Most of the wideband coding metasurfaces were proposed using Pancharatnam-Berry (P-B) phase [5–16]. P-B phase is a geometric phase that stems from the rotation of the unit cell structure in a metasurface. According to the P-B phase principle, when the unit cell structure of a metasurface is rotated by an angle ψ, almost ±2ψ of the P-B phase will be generated in the co-polarized reflection coefficient under circular polarized (CP) incidence [5]. The phase of the cross-polarized reflection coefficient does not change due to the rotation. However, a reflective circular polarization conversion metasurface (PCM) can achieve CP-maintaining reflection to make its co-polarized reflection coefficient under CP incidence close to 1.0 in magnitude. This would allow its main reflection phase to be freely changed by the P-B phase. Thus, based on a proper reflective circular PCM, a coding metasurface, which can achieve RCS reduction by means of diffusion, can be conveniently proposed. However, the methods for achieving RCS reduction include not only absorption but also diffusion [17, 18]. Based on proper lossy reflective circular PCMs, a number of absorptive coding metasurfaces (ACMs) that can achieve RCS reduction by means of diffusion and absorption have been proposed [19–28].

To achieve a wider band RCS reduction, a lossy reflective circular PCM is proposed firstly in this paper. The lossy PCM can achieve CP-maintaining reflection and absorption to make the magnitude of its cross-polarized reflection coefficient under CP incidence less than −10 dB in the ultra-wide frequency band of 6.7–40.9 GHz. Thus, based on the lossy PCM, this paper also proposed a 2-bit ACM that uses the P-B phase. The simulation and experimental results showed that the ACM can achieve ultra-wideband RCS reduction under an arbitrary polarized incidence with an incident angle less than 45°.

The proposed lossy PCM is a two-dimensional periodic structure. One of its unit cells is shown in Fig. 1. The unit cell structure is composed of a metallic patch and two resistive patches mounted on a grounded dual-layer dielectric substrate. As shown in Fig. 1(a), the metallic patch is an anisotropic structure with a pair of mutually perpendicular symmetric axes u and v, which implies that the reflection phase of the lossy PCM will differ under u-polarized (UP) and v-polarized (VP) incidences. In addition, the two resistive patches, which are of the same shape but on different layers, can achieve effective absorption at different frequencies when their surface resistances are appropriate. Absorption performance would not be significantly different under the UP and VP incidences because the resistive patches are not anisotropic. Therefore, when the lossy PCM is illuminated by an arbitrary CP wave, which can be regarded as a composite wave composed of a pair of equal-magnitude UP and VP components with a phase difference of ±90°, the UP and VP components would basically be equal in magnitude after being reflected and absorbed by the lossy PCM. However, the phase difference between them would be changed. If the metallic patch has proper anisotropy, which can make the phase difference Δϕ_uv between the reflection coefficients r_uu and r_vv close to 180°, the phase difference between the UP- and VP-reflected components would be changed to approximately ∓90°. In this way, the lossy PCM can simultaneously achieve CP-maintaining reflection and absorption. Thus, according to the P-B phase principle [5], the lossy PCM can be used as one of the four types of coding elements for a 2-bit ACM when its unit cell structure is rotated by 0°, 45°, 90°, or 135°. The rotation angle ψ of its unit cell structure is defined as the angle between the positive x-axis and positive u-axis. As shown in Fig. 1(b), the unit cell structure was rotated by 45°. To achieve the desired results, through repeated simulations, the following structural parameters of the unit cell structure were chosen: P = 6.00 mm, r = 2.80 mm, w = 0.30 mm, t = 1.50 mm, g = 0.20 mm, k = 0.90 mm, l = 2.485 mm, h₁ = h₂ = 2.00 mm, R_s1= 80 Ω/m², and R_s2 = 160 Ω/m². In addition, both dielectric layers were polytetrafluoroethylene (PTFE) with a relative permittivity of ɛ_r= 2.00.

To numerically analyze the performance of the lossy PCM, we have simulated it under different incidences using CST Microwave Studio. Since the PCM is a periodic structure, only one unit cell structure surrounded by periodic boundary was used as the simulation object, and the incident wave was introduced through a Floquet port. Firstly, the lossy PCM was simulated under UP and VP incidences. The simulation results, shown in Fig. 2(a), indicated that the phase difference Δϕ_uv between r_uu and r_vv was close to 180° in the frequency range of 7 to 39 GHz, implying that the PCM could achieve CP-maintaining reflection in the ultra-wide frequency range. To verify the theoretical prediction, we have simulated the PCM under right-handed CP (RCP) and left-handed CP (LCP) incidences. However, in the default Floquet port, only TE and TM waves can be introduced. To introduce LCP and RCP waves through the Floquet port, we chose the following option in the detail settings of the Floquet port: “circular polarization (LCP, RCP with respect to +z axis instead of TE₀₀, TM₀₀).” According to the simulation results shown in Fig. 2(b), the reflection coefficients under the RCP and LCP incidences were almost completely the same in magnitude, and the magnitude of the copolarized reflection coefficients r_RR and r_LL was much larger than that of the cross-polarized reflection coefficients r_LR and r_RL in the frequency range from 7 to 39 GHz, indicating that the anticipated ultra-wideband CP-maintaining reflection was realized. In addition, in the frequency range from 39 to 41 GHz, the magnitudes of r_RR, r_LL and r_LR, r_RL were all very small, indicating that the lossy PCM had good wave-absorbing property in this frequency range. These simulation results showed that the lossy PCM can achieve both CP-maintaining reflection and absorption. Polarization conversion efficiency was calculated using the |r_RR|² or |r_LL|² formulas, and the absorption efficiency was calculated using the 1−|r_RR|²−|r_LR|² or 1−|r_LL|²−|r_RL|² formulas. As shown in Fig. 2(c), the sum of the polarization conversion efficiency and the absorption efficiency can be kept at greater than 0.9 in the ultra-wide frequency band between 6.7 and 40.9 GHz. Thus, r_LR, r_RL magnitude of the lossy PCM can be kept at less than 0.316 (−10 dB) in the ultra-wide frequency band, as shown in Fig. 1(b).

Moreover, to verify that the lossy PCM can be used as one type of coding elements for a 2-bit ACM when its unit cell structure is rotated by 0°, 45°, 90°, or 135°, repeated simulations were conducted under RCP and LCP normal incidences when the unit cell structure was rotated by these angles. As shown in Fig. 3, the phase of r_RR is gradually increasing, but the phase of r_LL is gradually decreasing at all frequencies along with the increase of the rotation angle. The phases of r_RR and r_LL were both altered by almost 90° each time, indicating that the lossy PCM in the four states can be used as the four types of the coding elements for a 2-bit ACM.

The above simulation results suggested that the proposed lossy PCM can be used for the design of an ultra-wideband ACM. Thus, based on the lossy PCM, we proposed a 2-bit ACM that consisted of 6×6 coding elements. Each coding element was composed of 5×5 sub-unit cells. In the ACM, the four types of coding elements, whose codes were 00, 01, 10, and 11, were the lossy PCM at the four states in which the unit cell structure was rotated by 0°, 45°, 90°, and 135°, respectively. To achieve better RCS reduction, we proposed a number of appropriate coding sequences based on the working principle of the coding metasurface. In addition, the ACM was repeatedly simulated when all of its elements were arranged according to these coding sequences. Fig. 4 shows the proposed ACM and its coding sequence, which were determined based on these simulation results.

According to the design process for the proposed ACM, we know that the ACM would achieve ultra-wideband RCS reduction under RCP and LCP incidences. Since an arbitrary polarized wave can be regarded as a composite one composed of a pair of RCP and LCP components, the ultra-wideband RCS reduction would be achieved under arbitrary polarized incidences.

To analyze the performance of the ACM, we simulated it using CST Microwave Studio under plane wave incidences with different incident angles and polarizations. As a reference, a pure metal plate with the same size was simulated in the same way. First, the ACM and the metal plate were simulated under RCP, LCP, x-polarized (XP), and y-polarized (YP) normal incidences. As shown in Fig. 5(a), the monostatic RCS of the ACM under different polarized incidences was almost completely the same; compared to the metal plate, the monostatic RCS of the ACM was significantly reduced in the frequency range from 7.0 to 41.0 GHz. In addition, Fig. 5(b) shows that the RCS reductions of the ACM under these incidences were all kept at more than 10.0 dB in the ultra-wide frequency band from 6.8 to 41.4 GHz except for 7.6–9.1 GHz. Furthermore, the minimum RCS reduction in the frequency range of 7.6–9.1 GHz still reached 8.1 dB. These simulation results indicate that the ACM can achieve ultra-wideband RCS reduction under arbitrary polarized incidences and show that the relative bandwidth of its working band is up to 143.6%.

Furthermore, to analyze the angle insensitivity, the proposed ACM and the pure metal plate were simulated under different oblique incidences. The simulated results, shown in Fig. 6(a), indicate that the specular RCS reduction of the ACM under RCP incidence was kept at more than 10 dB in the ultra-wide frequency band of 11.1–43.8 GHz when the incident angle increased to 45°. In particular, when the incident angle was equal to 60°, the specular RCS reduction was still more than 8 dB in the frequency range of 11.8–46.6 GHz. In addition, Fig. 6(b) shows that, when the incident angle was set at 45°, the RCS reductions under the RCP and LCP incidences were almost the same. Although the RCS reductions under the TE and TM incidences were different, the RCS reductions under these oblique incidences could all be kept at more than 10.0 dB in the frequency range of 11.6–43.3 GHz. These simulation results showed that the ACM had good angular stability under an arbitrary polarized incidence.

Fig. 7 shows the simulation results for the 3D far-field scattering patterns of the proposed ACM and the metal plate at 15.0 and 35.0 GHz under normal and oblique incidences with different polarizations. The far-field scattering characteristics of the ACM under these different polarized incidences were basically the same. However, compared with the strong specular reflection of the metal plate, the ACM could always diffuse its scattering wave in many directions and greatly reduce its monostatic and specular RCS simultaneously under all of these incidences. These simulation results show that the ACM can achieve ultra-wideband diffusion-like scattering under arbitrary incidence. Therefore, the ACM can achieve ultra-wideband RCS reduction. Moreover, it has the advantages of polarization-insensitivity and wide incident angle.

To experimentally verify the performance of the 2-bit ACM, a 180 mm×180 mm experimental prototype was fabricated and measured. During the fabricating process, the metallic patch array shown in Fig. 8(a) was printed on the grounded dielectric substrate using printed circuit board (PCB) technology. However, the two resistive patch arrays shown in Fig. 8(b) were screen-printed on the front and back sides of the upper dielectric layer using carbon resistive paste. The whole structure was fabricated by hot pressing the grounded dielectric substrate and the upper dielectric layer together.

The experimental prototype and a pure metal plate of the same size were measured using a near-field measurement technique in a microwave anechoic chamber. Fig. 8(c) shows the measurement setup in which the two horn antennas were connected to the two ports of an Agilent E8363B network analyzer. The experimental prototype and the metal plate were successively measured under normal incidence and oblique incidence with an incident angle of 45°. However, the incident wave was only chosen as an LP one because no ultra-wideband CP antenna exists in common microwave experimental equipment. In this way, the experimental results for the monostatic RCS reduction under the normal incidence, shown in Fig. 8(d), and the specular RCS reduction under the oblique incidence, shown in Fig. 8(e), were obtained.

As shown in Fig. 8, these experimental results were all reasonable agreement with the above simulation results, except for the small deviation caused by fabrication error and measurement tolerance, which further verified that the proposed ACM can achieve ultra-wideband RCS reduction under all of these incidences.

To better understand the performance of the proposed ACM, Table 1 shows a comparison between this ACM and the typical designs proposed in [9–16, 21–27], in which the thickness is an electrical size relative to the free space wavelength (λ_L) at the lowest operating frequency. The results revealed that the proposed ACM is not very thick, but it has a significant advantage in terms of bandwidth expansion. As such, the proposed ACM can be considered an appropriate design.

This paper proposes an ultra-wideband 2-bit ACM based on an appropriate lossy PCM. The lossy PCM is a novel design, its cross-polarization reflection coefficient r_LR, r_LR under CP incidence can be kept at less than −10 dB in the ultra-wide frequency band of 6.7–40.9 GHz due to polarization conversion and absorption. Therefore, the proposed ACM can achieve ultra-wideband RCS reduction under arbitrary incidence by means of diffusion and absorption. The simulation and experimental results demonstrate that the ACM can effectively reduce its RCS under normal incidence with arbitrary polarization in the ultra-wide frequency band from 6.8 to 41.4 GHz. Furthermore, it can achieve an ultra-wideband RCS reduction under oblique incidence when the incident angle increases to 45°. Compared with many previous designs, the ACM has a significant advantage in the bandwidth expansion of RCS reduction. The ACM also has the advantages of polarization insensitivity and wide incident angle, making it very practical.