## Abstract

A hybrid design method for broadband radar cross section (RCS) reduction is proposed and successfully demonstrated based on the coupling effects between diffuse and absorptive structures. The reflection energy is distributed into more directions away from the source direction by the one-bit diffuse coding metasurface (CM). The two-layer resistive frequency selective surface (RFSS) is employed in the one-bit CM structure, reducing the amplitude of the co- and cross-polarized reflected waves under circularly polarized wave incidence by converting it into ohmic loss. In addition, the bandwidth of RCS reduction is further broadened through the coupling effects between the metallic patterns and the two-layer RFSS. The coupling effect shows that the absorption rate of the composite structure is significantly improved compared to the only RFSS structure. A lightweight CM loaded with RFSS (the area density is 597 g/m^{2}) was fabricated, analyzed, simulated, and measured. The results show that the proposed mechanism can effectively break the bandwidth constraints of traditional diffusion and absorption methods. Furthermore, the proposed mechanism significantly expands the bandwidth of RCS reduction. The proposed metasurface can achieve a 10 dB RCS reduction in an ultra-wideband from 7.3 to 44.2 GHz with about 143.3% fractional bandwidth. Moreover, the metasurface also has good performances under wide-angle oblique incidences. Under the condition of maintaining lightweight, the design provides an idea for broadening the frequency band.

© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

## 1. Introduction

Electromagnetic (EM) metasurfaces are artificial structures that comprise subwavelength unit cells on two-dimensional surfaces [1–3]. Metasurface can flexibly control the amplitudes and phases of EM wave scattering either individually or simultaneously. Thus, it has attracted extensive interest due to its powerful capabilities in wave manipulation. Some intriguing applications have been realized based on metasurfaces, such as perfect lenses [4], invisible cloak [5], wave-front control [6] and polarization converter [7]. One application that has notably been advanced is the radar cross section (RCS) reduction where the introduction of metasurfaces has overcome the limitations and improved the performance of conventional methods [8–13].

To effectively reduce the RCS of the target is challenging, and it is well known that the RCS reduction methods can fall into four main approaches. The first approach is based on the absorbing-type metasurface, which transforms the EM energy into heat. Landy et al. [14] firstly proposed the perfect absorber. However, the absorption is restricted at a narrow band. Later on, broadband absorption can be achieved using the resistive frequency selective surface (RFSS) or the metallic FSS loaded with lumped resistors [15,16]. Shang et al. [16] proposed simple guidelines for designing a double-square-loop absorber, and the fractional bandwidth of 126.8% is realized for at least 10 dB reflectivity reduction under the normal incidence. The second approach is the opposite phase cancellation, which exploits the phase difference between the corresponding reflection coefficients. Paquay et al. [17] firstly proposed a thin artificial magnetic conductor (AMC) structure, and the bandwidth of RCS reduction is very limited. Subsequently, significant efforts have been made to broaden the RCS reduction bandwidth with the phase cancellation metasurface. Su et al. [18] designed a novel checkerboard metasurface based on optimized multielement phase cancellation. It can achieve a 10 dB RCS reduction in a superwide frequency band from 5.5–32.3 GHz with a bandwidth of 141.8%. However, it is not a planar metasurface. In [19], Sang et al. proposed a novel methodology of increasing the number of AMC bands while maintaining the linear reflection phase, obtaining the 10 dB RCS reduction with about 91.5% fractional bandwidth.

Similarly, the third approach is where coding metasurfaces (CMs) are designed by using digital coding elements “0” and “1” for realizing diffusion. In order to obtain the lower RCS, the reflected energy is distributed into more directions away from the source direction by elaborately arranging the coding sequences. In [1], coding, digital and programmable metamaterials with excellent abilities for manipulating EM waves were presented, and a 10 dB RCS reduction bandwidth of 66.67% was achieved. A broadband and broad-angle polarization-independent random coding metasurface structure has been proposed in [20], the 10 dB RCS reduction bandwidth of 84.75% is realized by utilizing an efficient genetic algorithm.

The last approach is the combination of absorption and diffusion mechanisms [21–24]. Lossy material is applied to absorb EM waves in the composite structure. In [25], Li et al. investigated an array of randomly distributed lossy scatters to achieve broadband backscattering reduction with about 140.4% fractional bandwidth, but it cannot modulate phase flexibly. In [26], Ji et al. proposed using two structural layers to achieve the RCS reduction, the upper layer through random phase distribution design to realize EM diffusion, and the other layer is made of an RFSS that can absorb EM waves by converting them into ohmic loss. This work accomplished an 84% fractional bandwidth, but there are no strong coupling effects between RFSS and geometric phase cell. Recently, Feng et al. [27] proposed a novel spin-selectivity absorbing coding phase gradient metasurface, which could achieve absorption and diffusion through introducing the lumped resistor into the spiral of the metal structure. As for this work, the bandwidth of 10 dB RCS reduction in the frequency of 12.5–28.4 GHz is about 77.8%. In this work, the lumped resistors solely reduce the amplitude of one reflected circularly polarized (CP) wave, and it doesn't take absorption as an effective way to expand the bandwidth of RCS reduction. The above-mentioned works use the combination of absorption and diffusion mechanisms to reduce RCS. However, the coupling effects between absorptive and diffuse structures have been neglected.

In this work, the integral structure is composed of CM and two-layer RFSS. The ultra-wideband RCS reduction is achieved based on coupling effects between the diffuse and absorptive structures. As for integral structure, the coupling effects exits between the adjacent metallic pattern and two-layer RFSS, and the coupling effects exits between metallic patterns as well. Compared with the only RFSS structure, the coupling effects between metallic pattern and two-layer RFSS further increases the absorption of the integral structure and thus improves the bandwidth. The working frequency band of the original one–bit CM covers the range from 8.32 to 41.37 GHz. After introducing two-layer RFSS, the amplitudes of the cross- and co-polarized reflected waves are significantly reduced through rational structural layout and parameters optimization. Furthermore, a prototype of the integral metasurface combing the above hybrid mechanism has been fabricated and measured. It can achieve the 10 dB RCS reduction between 7.3–44.2 GHz with about 143.3% fractional bandwidth. The physical mechanism is explained by examining the power loss density and scattering patterns. Moreover, our design is simple without complicated optimizations and promises extensive applications in the future.

## 2. Design Principles

Figure 1 illustrates the proposed ultra-wideband RCS reduction metasurface based on the strong coupling mechanism of diffuse and absorptive structures. As shown in Fig. 2(a)-(b), the integral structure comprises two layers of structured patterns. The two-layer RFSS is employed to reduce the amplitude of incident EM waves. The fabrication process of RFSS can be seen in the experimental verification of part 4. The bottom layer employs the S-shaped metallic pattern as the geometric phase unit cell to modulate the reflection phase by rotating its orientation. By rotating the S-shaped metallic pattern of the “0” unit cell by 90°, the “1” unit cell can be obtained. The one-bit CM loaded with two-layer RFSS is arranged according to the random coding sequence generated by MATLAB [as shown in Fig. 2(e)]. It can be seen from Fig. 2(c)-(d) that the coding element “0” is composed of a 6×6 array of “0” unit cells, and the coding element “1” is composed of a 6×6 array of “1” unit cells. As shown in Fig. 2(f), the one-bit CM loaded with two-layer RFSS comprises an 8×8 array of coding elements. The one-bit CM achieves diffuse reflection, and two-layer RFSS reduces the amplitude of incident EM waves by converting it into ohmic loss. The RCS reduction bandwidth could be further broadened through the coupling effects between the metallic pattern and the two-layer RFSS. Due to the metal plane of the integral structure, *T* (*w*) = 0, the specular reflectivity |*S*_{11}|^{2} can be expressed as:

*T*(

*w*),

*A*(

*w*), and

*D*(

*w*) are transmission coefficient, absorption rate, and diffusion rate, respectively. It can be seen from Eq. (1) that when there exit coupling effects between the absorptive and diffuse structures, the RCS can be further reduced. The coupling effect can be promoted by optimizing the structural layout and parameters.

In CP fields, ultra-thin metasurface can optimally control the phase of cross-polarized transmission [28,29]. A similar situation exists for the reflection of EM waves. When the circularly polarized wave illuminates on the reflective metasurface, the matrix can be extended to describe the relationship between the incident and the reflected electric fields as:

*R*means that the Jones matrix is expressed in the circular base.

_{circ}*R*and

_{RR}*R*represent co-polarized and cross-polarized reflection coefficients of right-handed circularly polarized (RCP) waves, respectively.

_{LR}*R*and

_{LL}*R*represent co-polarized and cross-polarized reflection coefficients of left-handed circularly polarized (LCP) waves, respectively.

_{RL}The integral structure can realize phase modulation and amplitude reduction simultaneously, and it can achieve a 180° phase difference in the wideband range so that the combined mode of diffusion and absorption can be realized. *f _{m}*

_{,n}(

*θ*,

*j*) is the primary pattern which expresses the vector properties of far-field like polarization and directional pattern [27]. As shown in Fig. 2(c)-(d), since the interior of the two coding elements are composed of the same phase response “0” or “π”,

*f*

_{m}_{,n}(

*θ*,

*j*) can be considered as a constant in the calculation process. Therefore, the far-field function can be expressed as [27]:

*q*and

*j*are the elevation and azimuth angles of the reflected wave. ${S_a}(\theta ,\varphi )$ is the array pattern, and it can be represented by:

*d*is the distance between the basic coding elements,

*k*= 2π/

*λ*,

*f*(

*m*,

*n*) is the initial phase of the coding element. In order to distribute the incident wave uniformly among all possible directions, the coding sequence is optimized by the genetic algorithm, as shown in Fig. 2(e)-(f). Here, the objective function is defined as fitness = max [

*S*(

_{a}*θ*,

*j*)], and our previous work provide more details about the optimized process of the coding sequence [30]. The optimization is carried out for a single frequency (central frequency of one-bit CM). The coupling effect between structures varies with frequency, so the absorption rate at different frequencies varies greatly. For the sake of simplicity, the optimization process does not consider the amplitude

*A*

_{m}_{,n}.

As for transmissive metasurfaces, only the phase of cross-polarized transmitted waves can be arbitrarily controlled, whereas the co-polarized transmission phase is severely limited [28]. Due to the metal backplate, the reflective metasurface is just the opposite. The co-polarized CP waves can achieve a reflection phase controllable over the whole 2p range. When absorptive and diffuse structures are in the same layer [21], according to Eq. (2), the amplitudes of the co- and cross-polarized reflected waves can be close to the minimum when the amplitude of the incident electric field is close to zero. Thus, the remarkable absorbing property could be achieved. As for the two-layer structure we proposed, the upper layer is covered with the resistive film. When the upper layer keeps good absorption, its transmission rate is poor, which further affects the phase modulation performance of the S-shaped metallic pattern in the lower layer. Therefore, the inappropriate parameter of two-layer RFSS affects the working performance of the S-shaped metallic pattern. The most straightforward idea of combining the two structures of RFSS and CM to achieve ultra-wideband RCS reduction is first to optimize the best working performance of each of the two structures and then realize the superposition of frequency bands [26]. This idea can be further improved. The resonance frequency of the phase modulation structure is distributed in a wide frequency range in our design process, even though its cross-polarization reflection is not strictly controlled. By optimizing the structure, the amplitudes of the reflected co- and cross-polarized waves are reduced in the operating frequency band of the geometric phase cell through the strong coupling effects between the metallic pattern and two-layer RFSS. In addition, the operating bandwidth of the integral structure in the low- and high-frequency bands is widened. In this way, the two mechanisms of diffusion and absorption are perfectly combined.

## 3. Design and analyze of metasurface

#### 3.1 Unit cell design of one-bit CM for achieving diffusion

In this part, we explained the unit cell design of one-bit CM without two-layer RFSS loaded. In Section II, we have introduced the one-bit CM arrangement method. The reflection energy could be distributed into more directions away from the source direction by the designed one-bit CM. As for reflective metasurfaces, according to Pancharatnam-Berry (P–B) geometric phase manipulation concept, the need to work with co-polarized beams is associated with the general issue that only the phase of the co-polarized reflected wave can be arbitrarily controlled [28].

In order to expand the bandwidth of the integral structure, when designing the unit cell of the one-bit CM, the principle is to distribute the resonance frequency points in the broader frequency band. By optimizing the structural parameters and rotating the metallic pattern, resonance frequency points distributed in broadband can be obtained. However, the cross-polarized reflection cannot be strictly limited in this case. Therefore, we introduced a two-layer RFSS structure in integral structure to absorb cross- and co-polarized reflected waves simultaneously. In this way, the cross-polarized reflection can be reduced.

In this part, we mainly introduce the design of the S-shaped metallic pattern. The integral structure adopts a two-layer structure, and the S metallic pattern is placed on the bottom layer. Here, the S-shaped metallic pattern is employed to modulate the P-B phase. As shown in Fig. 3(a), the dielectric substrate layers from top to bottom are FR-4 (*e _{r }*= 4.3, tan

*d*= 0.025), PMI foam (

*e*= 1.05), FR-4 and PMI foam, respectively, and their thickness are

_{r }*t*,

*d*

_{1},

*t*, and

*d*

_{2}, respectively. The unit cell period is

*p*, and the metallic patterns are printed on FR-4 substrate, whose geometrical parameters are

*w*,

*r*, and

*q*, as illustrated in Fig. 3(b). The optimal parameters of structure are

*d*

_{1 }=

*d*

_{1 }= 2 mm,

*t*= 0.1,

*w*= 1.2 mm,

*r*= 3.1 mm,

*p*= 6.5 mm, and

*q*= 1.6 mm.

Based on the theory of the P-B phase, the phase can be obtained as D*j* = ±2*a*, where *a* is the rotation angle of the S-shaped metallic pattern, and “+” is for LCP wave, “-” is for RCP wave. The simulated amplitude of reflection coefficient is shown in Fig. 3(c) and Fig. 3(d) when the rotation angle *a* varied from 10° to 70° with a step of 20°, where *r _{RR}*,

*r*,

_{LR}*r*, and

_{LL}*r*represent the amplitude of co- and cross-polarization reflection under RCP and LCP wave normal incidence. Figure 4 gives the simulated phase of the co-polarization reflection coefficient with different rotation angles under RCP wave incidence, respectively. Simulations are computed by frequency-domain solver in the CST. The boundary conditions along the

_{RL}*x*- and

*y*- directions are unit cell boundaries, while that along

*z*-direction is open added space.

The coupling effect exists between adjacent S-shaped structures. When the rotation angle *a* of the S-shaped metallic pattern changes with a step of 20°, the coupling effect of adjacent S-shaped structures also changes. As shown in Fig. 3(c)-(d), the working frequency distribution range of the unit cell is the widest when *a* = 10°, reaching 8.32–41.37 GHz. Obviously, there is a new resonant point at high frequency when *a *= 10°, due to the coupling effect between the adjacent S-shaped metallic patterns. Therefore, for the one-bit CM in our design, the unit cell with rotation angles *a *= 10° and 100° can be selected to construct the coding elements “0” and “1”. The geometric phase cell is the basis for achieving diffusion. When S-shaped metallic pattern combing with the two-layer RFSS, the co- and cross-polarization reflection amplitudes reduce.

#### 3.2 Unit cell design of two-layer RFSS for absorption

This part mainly introduces the design of the two-layer RFSS structure, which simultaneously reduces the amplitude of cross- and co-polarized waves under CP wave incidence. It has been mentioned that the S-shaped metallic pattern is placed at the bottom of the two-layer structure. To better couple the two-layer RFSS structure with the S-shaped metallic pattern, the resistive film is set at the four corners of the two-layer structure. The dimensions of the upper and lower resistive films are *c* and *f*, respectively. There are two design guidelines for two-layer RFSS: (1) According to the formulas available in [31,32] for calculating equivalent capacitances and resistances of the resistive films from given parameters, the parameters (*c*, *f*, *R _{s}*) of two-layer RFSS can then be determined.

*R*is the surface impedance of resistive films. (2) Using the dimensions obtained from the circuit analysis for the absorption structure to be designed as the initial values, the full-wave simulator, CST, is used to fine-tune the structure. In this step, the coupling effect between the S-shaped metallic pattern and two-layer RFSS must be considered comprehensively. Finally, the parameters of the two-layer RFSS that enable the integral structure to achieve broadband RCS reduction are obtained.

_{s}The equivalent circuit model is constructed to predict the optimal absorption of the structure, as shown in Fig. 5(b). According to transmission line theory, the dielectric layer can be equivalent to a fixed-length transmission line, and the metal backplane is equivalent to a terminal short circuit. The characteristic admittance corresponding to the upper and lower RFSS is *Y*_{RFSS1} and *Y*_{RFSS2}, respectively. *R*_{1} and *C*_{1} represent the upper RFSS while *R*_{2}, *C*_{2} corresponding to the lower one. The transmission line sections of length *d*_{1} and *d*_{2} represent the upper- and lower-layer PMI foams, respectively, and their characteristic admittances are, respectively, represented by *Y*_{1} and *Y*_{2}. In this way, the reflection coefficient of the absorption structure can be expressed as [31]:

And due to the full metallic plate on the bottom layer, the frequency dependent transmitted power is zero, and the absorption rate of the two-layer RFSS can be simplified by:

As shown in Fig. 6, to verify the validity of the equivalent circuit model of the absorption structure based on the two-layer RFSS, we compare the simulated reflection performance of the absorption structure by the circuit model against the full-wave simulation software CST Microwave Studio. The lumped capacitances (*C*_{1}, *C*_{2}) values in the circuit model are then determined from the two-layer RFSS dimensions (*c*, *f*) [31]. The square patch resistance *R*_{1}, *R*_{2} in the equivalent circuit model is related to the surface impedance of its material [31]. The optimized circuit parameters are: *R*_{1 }= 67.01 W, *C*_{1 }= 3.01 pF, *R*_{2 }= 361.86 W, *C*_{2 }= 1.31 pF. After the full-wave simulation in CST, the optimized parameters are: *c *= 1.5, *f *= 1 and *R _{s}* = 100 W/m

^{2}. It is observed that the results from the circuit model and CST simulations are in good agreement.

Next, the effect of parameter optimization on phase modulation and absorption is explained in essence. Since the upper layer RFSS partially covers the S-shaped metal structure, the transmission of the upper layer RFSS must be considered under CP incidence. Therefore, according to the design principles, the upper layer RFSS should maintain good transmittance at the frequency band where the S-shaped metallic pattern performs phase modulation and maintain a specific absorption rate. In this way, the amplitude of the cross-polarized reflected wave would be strictly limited in the working frequency band through the absorption of the two-layer RFSS, and the amplitude of the co-polarized reflected wave would also decrease at the same time.

The dimension of *c* would greatly influence the performance of the integral structure, which affects the transmittance of the upper RFSS. As for two-layer RFSS, there are only co-polarization reflection and transmission under the illumination of the CP wave. The transmission of the upper RFSS is calculated as | *T*_{CP} |^{2 }= 1- *A*_{CP} - |*R*_{CP}|^{2}, where *T*_{CP}, *A*_{CP}, and *R*_{CP}, respectively represent the transmission coefficient, absorption rate and reflection coefficient of the upper layer RFSS. As shown in Fig. 5(a), the width of the resistive film layer placed at the four corners of the upper layer RFSS is *c*, and the width of a single resistive patch would reach 2*c* after forming the array. When *c *= 2.5, the EM wave absorption rate and reflection coefficient values are relatively large, but the transmittance deteriorates, as shown in Fig. 5(c). Because the upper resistive film occupies a relatively small area in the upper layer when *c* = 0.5, the value of *T*_{CP} is relatively large. However, *c* = 0.5 cannot guarantee that the upper layer RFSS has a required absorption rate. *c *= 1.5 ensures that the transmittance of the upper RFSS is relatively large in the operating frequency band of the geometric phase cell, and the absorption performance meets the design requirements. The lower-layer RFSS also plays a role in reducing the amplitude of cross-polarized and co-polarized reflected waves. The equivalent circuit and simulation reveal that as *f* increases, the absorption rate at high-frequency bands gradually increases, as shown in Fig. 5(d). The lower RFSS cannot contact the S-shaped metallic pattern, so the optimal parameter of *f* is 1.

#### 3.3 Unit cell design of the integral structure for achieving diffusion and absorption

The integral structure is a composite structure consists of the S-shaped metallic pattern and two-layer RFSS. The diffusion can be realized by one-bit CM in the broadband. By loading the two-layer RFSS in one-bit CM reasonably, the RCS can be further reduced through the coupling effects between structures. This coupling effect shows that the absorption performance of the composite structure is better than that of the only two-layer RFSS absorption structure. This part analyzes this coupling effect through simulated curves and simulated power loss density.

After the geometric phase cell and the resistive films are combined, it can be seen from Fig. 7(a) that the amplitude of the cross-polarized reflected wave is limited to less than 0.3 in the frequency band of 7.3–44.2 GHz. The amplitude of the co-polarized reflected wave also drops dramatically and stays below 0.3 in the 25.8–44.2 GHz band. It shows that the two-layer RFSS significantly reduces the amplitude of the co- and cross-polarized reflected waves through ohmic loss. To further analyze the phase modulation property of the integral structure, the unwrapped phase of co-polarized reflected waves of the integral structure with two rotation angles of the metallic pattern is given in Fig. 7(b) under the incidence of CP wave. After the two-layer RFSS is loaded, the “0” and “1” element of the integral structure maintains stable 180° phase difference in the ultra-wide frequency band. The coupling effects between the adjacent S-shaped metallic patterns as well as the S-shaped metallic pattern and the two-layer RFSS vary as *a* changes. Hence the cross-reflection performance of the integral structure would be diverse, as shown in Fig. 7(c). The strong absorption band is revealed between 28.7–44.2 GHz, where the absorptivity reaches about 80%, and the results are given in Fig. 7(d). The absorption is relatively weak in the lower frequency band, and energy dissipation mainly relies on diffusion through the surface phase modulation.

Figure 8(a), it is evident that the inroduction of two-layer RFSS not only effectively absorbs cross-polarized reflected waves but also expands the bandwidth of the integral structure at low and high frequencies. It is seen in Fig. 8(b) that the absorption rate of the integral structure is greatly improved in the range of 5.0–12.7 GHz and 21.0–42.0 GHz compare with the structure only loaded with two-layer RFSS. At 7.5 GHz and 30.0 GHz, the increments of absorption are D*A *= 46.5% and D*A *= 36.6%, respectively. To better understand the mutually reinforcing mechanism of the proposed metasurface, the power loss density distributions of different cases at 7.5 GHz have been illustrated in Fig. 8(c)-(f), respectively. As Fig. 8(c) shows, strong coupling exists between adjacent metallic patterns. The broadband phase modulation could be obtained when *a*=10^{°}, it has already been mentioned in Part A. Figure 8(d) shows the coupling between the metallic pattern and lower RFSS, energy loss distributed on the gaps of the structures. The power loss densities of the upper RFSS in two cases are compared in Fig. 8(e)-(f), which shows that the energy loss of the integral structure is enhanced through the coupling of the upper and lower structures. Therefore, the coupling between the metallic pattern and the two-layer RFSS mainly contributes to improving the absorption of EM waves in most frequency bands. The energy distributions at other frequency points are similar.

## 4. Simulation and experiment verification

#### 4.1 RCS Simulation

To further verify the performances of the one-bit CM with two-layer RFSS loaded, we fabricated a metasurface based on the optimized coding sequences generated by MATLAB, which contains an 8×8 array of coding elements. The sizes of the composite metasurface are 312×312 mm^{2}, the sizes of each coding element are 39×39 mm^{2}. Now we discuss the polarization and incidence sensitivity of the designed metasurface. In this regard, we calculate the RCS of the integrated metasurface under the normal incidence of *x*-, *y*- and two CP waves, and the results are given in Fig. 9(a)-(c). Nearly the same scattering behavior is manifested in all four cases, which is clear evidence of polarization insensitivity. The scattering properties of the metasurface under oblique incidences for both transverse-electric (TE) and transverse-magnetic (TM) polarizations are provided in Fig. 9(d). The simulation results illustrate the excellent performance of RCS reduction under oblique incidences.

In order to get an insight into the RCS reduction mechanism, the 3D scattering patterns of the proposed integrated metasurface under normal incidence are displayed in Fig. 10. In addition, Fig. 10 includes the 3D scattering patterns of the metasurface without RFSS with the same size as a comparison. As Fig. 10(a)-(c) show, the backward scattering waves are relatively stronger along normal at 12, 24, and 40 GHz without the absorption of the RFSS. After the two RFSS are introduced to composite with one-bit CM, the incident EM energy is significantly reduced at three frequency points, resulting in the EM diffusion and absorption phenomenon, as seen in Fig. 10(d)-(f). The 3D far-field scattering pattern shows that the coupling effects between absorptive and diffuse structures can effectively expand the RCS reduction bandwidth.

#### 4.2 Experiment Verification

In order to experimentally verify the RCS reduction performance of the proposed metasurface, a design example is fabricated and tested, as shown in Fig. 11(a). In our design, the geometric phase layer is fabricated by printed circuited board technology. The schematic diagram of the fabrication process of the resistive frequency selective surface (RFSS) layer is shown in Fig. 11(b). The method of making RFSS is very simple, and the cost is low. First, the conductive carbon paste was poured uniformly onto one side of the silkscreen. Next, a bamboo board was used to brush the carbon paste on the FR-4 with even force. After that, the FR-4 printed with RF was dried in the drying closet. And then, the resistance tester was used to test the resistance value of RF. Finally, we used epoxy glue to adhere the PMI foams and FR-4 films together to reduce the air gap.

Measured results of the metasurface with or without RFSS are compared in Fig. 11(c)-(d). which show the design can achieve broadband RCS reduction in a continuous wideband. The frequency response curve of the measured RCS reduction over the frequency band of 7.4–40 GHz for the *x*-polarized and 7.3–40 GHz for the *y*-polarized under normal incidence. Because the testing equipment is limited, the performance at frequencies above 40 GHz cannot be measured. The slight discrepancy between measurement and simulation can be attributed to two reasons. First, the fabrication error and the roughness of the film cannot be fully removed in our process, which unavoidably leads to some deviations. Second, epoxy glue for adhering layers in the experiment is not considered in the simulation. The area density of the prototype was measured and can achieve as small as 597 g/${\textrm{m}^\textrm{2}}$. Therefore, the prototype has the advantage of being lightweight while ensuring the broadband RCS reduction performance. A comparison between the previous researches and this work is provided in Table 1 [33–35]. Our work has an overwhelming advantage and offers the idea to extend the bandwidth of RCS reduction.

## 5. Conclusion

In summary, a hybrid design method based on the coupling effects between diffuse and absorptive structures for ultra-wideband RCS reduction has been comprehensively investigated. The metasurface is composed of one-bit CM and two-layer RFSS, modulating the phase and reducing the amplitude of EM waves simultaneously in an ultra-wideband. After optimization, RFSS can absorb both reflected cross-polarized and co-polarized waves, and it does not affect the working performance of the geometric phase cell. Compared to the single structure, the performance of the integral structure is significantly improved through the absorption of the RFSS and the strong coupling effect between two-layer RFSS and S-shaped metallic patterns. The metasurface can achieve more than 10 dB RCS reduction in an ultra-wideband ranging from 7.3 to 44.2 GHz with a ratio bandwidth (*f*_{H}/*f*_{L}) of 6.05:1. Compared with the traditional single RCS-mechanism, the use of hybrid mechanisms can provide more freedom to achieve ultra-broadband or multi-spectral RCS reduction, which are welcome for stealth platforms.

## Funding

National Natural Science Foundation of China (61971437, 61971435).

## Disclosures

The authors declare no conflicts of interest.

## Data availability

The datasets presented in this paper are available from the corresponding author upon reasonable request.

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