Abstract
In this paper, it is proposed to employ phase cancellation metasurface (PCM) to substantially reduce the RCS of an electrically large open-ended cavity (EOC) by scattering the energy to the nonthreatening region, achieving broadband and ultra-wide-angle RCS reduction of the cavity. The PCM, constituted by the collection of polarizing reflectors, is loaded on the inner and outer walls of the composed cavity structure in accordance with the orthogonality principle to obtain a low-RCS cavity. The main benefits of using the polarizing reflector to build a PCM are as follows: On the one hand, it reduces the number of units that make up the PCM, which in principle only requires half of the quantity. On the other hand, it is possible to form a phase difference of 180 degrees that is completely stable. Both the simulation and measured results show that the RCS reduction of 10 dB for normal incidence relative to the original state of the cavity (that is, the metal cavity) over a frequency range of 6.3 GHz to 13.2 GHz under TE polarization and 7.8 GHz to 13.4 GHz under TM polarization has been obtained. Furthermore, the RCS reduction performances of the cavity loaded with the composed PCM under variable azimuth angles are also studied. Numerical and experimental results demonstrate that the ultra-wide-angle RCS reduction from -180° to 180° is acquired. To the best of our knowledge, this is the first time that the PCM, which is made up of a number of polarizing reflectors, has been used to get broadband and ultra-wide-angle RCS reduction for the EOC. This method, which is simple, effective, and low cost, has great application prospects in the RCS reduction design of the cavity.
1. Introduction
Radar cross-section (RCS) can be used to evaluate a flight vehicle’s electromagnetic system’s performance [1, 2]. By reducing the RCS of aircraft vehicles, the probability of them being detected and tracked can be reduced, thereby enhancing their survivability [3]. The electrically large open-ended cavity (EOC) structure of the aircraft, which includes the jet engine inlet ducts, nozzle, and cockpit [4–8], is a major scattering source that contributes significantly to the RCS of the target. Hence, it is crucial to achieving the RCS reduction of the EOC, which can significantly decrease the aircraft’s backward electromagnetic scattering and improve the target’s battlefield survivability.
Currently, the major methods for reducing the RCS of the cavity involve shaping techniques [9–13] and loading absorbing material [14–20]. For shaping technology, the strong scattering source can be converted into a weak one by designing the target’s shape, thereby effectively reducing its RCS. However, it has a significant impact on the aircraft’s mechanical strength and aerodynamic performance. With regard to absorbing material, the mechanism transforms electromagnetic energy into heat via electric loss or magnetic loss, which reduces the scattering energy of the cavity. It benefits from being thin and simple to repair and assemble. However, absorbing materials are typically expensive and tend to fail at high temperatures. The metasurface offers a novel technique for RCS reduction that distributes energy in the direction away from the threat by manipulating the wavefront [21–32]. However, metasurfaces are relatively rarely used to regulate the scattering of cavities [23]. Zhou et al. presented a novel approach to lowering the RCS of the EOC by employing metasurfaces with random distribution cross-like patterns [23]. In the X-band, they achieved the RCS reduction of the cavity from 10° to 80° incidence using the principle of diffuse reflection. The phase cancellation metasurface (PCM), which is constituted by a set of polarizing reflectors, offers excellent RCS reduction performance as one of the main metasurfaces. By creating a broadband and efficient polarization reflector and then rotating it 90 degrees, a unit cell with the same amplitude as the original unit and a 180° phase difference can be obtained. After that, the units before and after rotation are arranged alternately or placed in a checkerboard pattern to accomplish effective RCS reduction. Despite extensive research on PCM [28–32], there has been no research on the application of PCM to the field of EOC’s RCS reduction, according to our investigation.
In this research, we present the design concepts and methods for controlling the scattering characteristics of the EOC by employing PCM technology. The PCM, which is made up of a group of polarizing reflectors, is applied to the inner and outer walls of the cavity structure based on array theory and the orthogonality principle. Both simulations and measurements confirm that the proposed low-RCS EOC has excellent RCS control under normal incidence from 6.3 GHz to 13.2 GHz under TE polarization and 7.8 GHz to 13.4 GHz under TM polarization, wherein the RCS reduction is greater than 10 dB and exceeds -20 dB at multiple resonant frequencies. The simulated and measured results are in good agreement over the entire frequency range. Additionally, the RCS reduction properties of the cavity loaded with the PCM are also evaluated at various azimuth angles. The reduction of the ultra-wide-angle RCS from -180° to 180° is acquired, according to numerical and experimental data. The approach employed here provides a strategy for the design of the cavity’s RCS reduction.
2. Modeling, Simulation, and Discussion
Typically, a construction with an open end and a terminal metal plate can be used to represent EOC. Due to the existence of many corner reflectors in the cavity, the incident wave generates a strong backward echo after multiple reflections inside the cavity and a single reflection outside the cavity, resulting in a strong backward RCS of the cavity. To realize the backward RCS reduction of the cavity, it is necessary to design from the inside and outside of the cavity at the same time to destroy the corner reflector structure, which prevents the incident wave from forming severe echo scattering. The strong echo scattering of the corner reflector can be effectively eliminated by redirecting the electromagnetic wave away from the backscattering. The development of polarization conversion metasurfaces offers powerful support for putting this strategy into practice. The unit cell of PCM is reasonably arranged using the orthogonality principle to effectively regulate the scattering of electromagnetic waves. Then, the backward echo can be dispersed to nonthreatening areas by loading PCM on the cavity’s inner and outer walls.
As previously explained, due to the existence of corner reflectors in the cavity, the plane wave generates a strong backward echo after multiple reflections in the cavity and a single reflection outside the cavity. To reduce the RCS of the cavity, it is necessary to disrupt the corner reflector structure to prevent the incident plane wave from causing severe echo scattering. This can be attained by applying destructive interference to cancel the scattering, according to the array theory [21]. The typical method is to employ two PCMs with a 180° phase difference. Figure 1 shows the working principle diagram of the low-RCS cavity. As shown in Figure 1, the phase difference between PCM1 and PCM2 is 180 degrees. Then, according to the principles of orthogonality theory, the interior and exterior of the cavity are designed according to the following method [33]. The schematic diagram of the cavity’s internal and external working principles is shown in Figures 1(a) and 1(c), respectively. Figure 1(b) shows the schematic diagram of the whole cavity. To prevent the emergence of grating lobes in the region parallel to the plane of incidence, the array axis is designed to be perpendicular to the incident plane so as to regulate the lobes to the nonthreat area [33], that is, where and stand for the array axes, is the incidence wave vector, and represents the incident plane’s normal vector.
For the interior of the cavity, as shown in Figure 1(a) [33],
For the external of the cavity, as seen in Figure 1(c) [33], where and are the periods of the supercell and the value of is 1,2,3...
As previously stated, the unit cell of the PCM with a 180° phase difference is necessary to reduce the cavity’s RCS. Considering the performance of wideband RCS reduction, the polarizing reflector as the unit cell of the PCM has many benefits. The main advantages of using the polarizing reflector to construct a PCM are as follows: Firstly, it reduces the number of units that make up the PCM, typically requiring only half of the quantity. Secondly, a stable 180° phase difference can be created. At first, a square ring is chosen as the pattern of the unit cell, as seen in Figure 2(a). The unit cell is a classical sandwich structure. The pattern on the top is silver. The ground plane is made of aluminum. The dielectric separator is air with a thickness of 5.5 mm. Using CST Microwave Studio with the frequency domain solver, the boundary condition is set as unit cell boundary in both - and - directions and open (add space) boundary in the direction; the parameters of the element are optimized. The geometrical parameters are mm, mm, and mm. Only copolarized components dominate because the structure is not anisotropic and the cross-polarized magnitude is nearly 0, which is fairly apparent as can be seen in Figure 2(c). To introduce anisotropy and produce higher cross-polarized components Rxy, the C-shaped split-ring pattern is formed by truncating the corners of the square rings, as shown in Figure 2(b) [28]. The optimized geometrical parameters are mm, mm, mm, and mm. As can be observed from Figure 2(c), the cross-polarized components Rxy are significantly improved as a result of the introduced asymmetry in the pattern of the unit cell. The split-ring structure has the advantages of high anisotropy, flexible design, and simple manufacture [34]. Therefore, we select the C-shaped split-ring pattern as the polarized reflector, and the working principle of the polarization converter to realize polarization conversion is shown in Figure 3(a) [28, 31]. For clearer analysis, we created the - coordinate system and the - coordinate system, which are formed by turning the - coordinate system 45 degrees anticlockwise. Suppose that the electromagnetic wave is impacting the unit along the -polarization, we express the electric field as Eyi. The electromagnetic wave can be decomposed into two orthogonal components, Eui and Evi, in accordance with the vector decomposition theory. The element displays perfect magnetic conductor properties in the direction, which causes the electric field components Eur and Eui of the reflected wave to be in phase. However, since the element displays perfect magnetic conductor properties in the direction, the reflected wave electric field components Evr and Evi in this direction are opposite. Therefore, in accordance with the vector synthesis principle, the reflected wave electric field is along the axis, which is Exr. Namely, after the electric field direction of the incident wave passes through the polarization converter from the direction, it is reflected to the direction orthogonal to it, thus realizing the polarization conversion function.
For convenience of discussion, we refer to the unit of the PCM stated before as the “0” unit, as shown in Figure 3(a) [28]. According to the principle of the Pancharatnam-Berry (PB) geometric phase, the reflection phase of is obtained when the metal pattern is rotated at [35]. When the “0” unit is rotated clockwise by 90 degrees, we can obtain the “1” unit, as shown in Figure 3(b) [28], which has the same conversion characteristics as the “0” unit. As a result, the “0” unit is rotated 90 degrees clockwise to produce the “1” unit, which will have a 180° phase difference with the “0” unit, as illustrated in Figure 3(b) [28]. To further support the aforementioned point of view, we simulated the two units, respectively, and obtained their cross-polarization reflection coefficient and reflection phase. It can be seen from Figure 4(a) that the cross-polarization reflection coefficients of the “0” unit and the “1” unit are equal over the whole frequency band. The two structures can maintain a steady phase difference of 180 degrees throughout the whole frequency band, as can be seen in Figure 4(b). Namely, the reflection phase difference between the “0” and “1” units can be 180 degrees, and both units have the same conversion characteristics. Therefore, it can be further used for the design of scattering control.
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The detailed design process of the low-RCS EOC is given after obtaining the “0” unit, which can be periodically arranged to form PCM0, and the “1” unit, which can be periodically arranged to form PCM1, combined with the principle of orthogonality. As shown in Figure 5(c), which is the overall schematic diagram of the cavity loaded with the PCM, the inner and outer sides of the cavity are both loaded on three sides. These planes are perpendicular to the incident surface. For the arrangement of the inner side of the cavity, by formula (2), we take the case of , that is, . As seen in Figure 5(a), the PCM loaded at the inner side of the cavity’s two sidewalls is parallel to the plane. It is made up of , a supercell made up of “0” elements, and , a supercell made up of “1” elements, placed alternately. In Figure 5(b), the PCM loaded at the bottom of the inner side of the cavity is parallel to the plane. It consists of , which is a supercell composed of “0” elements, and arranged, which is a supercell composed of “1” elements, alternately. In terms of the configuration of the cavity’s exterior, according to formula (3), we take the case of , namely, . In Figure 5(d), the PCM loaded at the two sidewalls of the cavity’s outer side is parallel to the plane. It is assembled from , an arrangement of a supercell made up of “0” units, and , a supercell made up of “1” units. Similarly, the PCM loaded at the bottom of the cavity’s outer wall is parallel to the plane in Figure 5(e). It is formed by , a supercell generated of “0” cells, and , a supercell composed of “1” cells, placed alternately. To prevent the pattern of the basic unit from being obscured, the thickness of the air medium in the middle of the metasurface is considered. Therefore, the size of the designed metal cavity as a control group is . The cavity’s dimensions become when loaded with the metasurface.
The simulation results by Feko software with the multilevel fast multipole method (MLFMM) method are shown in Figure 6 when ° and ° under the incidence of TM polarized and TE polarized waves. We can see from Figures 6(a) and 6(b) that the cavity loaded with PCM has the effect of reducing RCS from 6 GHz to 16 GHz under TE and TM polarization. Under TE and TM polarization, the RCS reduction value after the cavity has been loaded with PCM is further shown in Figure 7. Under TE polarization, it achieves a -3 dB RCS reduction between 6 GHz and 16 GHz and a 10 dB decrease between 6.3 GHz and 13.2 GHz with fractional bandwidths of 70.77%, as shown in Figure 7(a). When TM polarization occurs, a -3 dB RCS reduction is attained between 6 GHz and 16 GHz, notably when a 10 dB reduction is attained between 7.8 GHz and 13.4 GHz with fractional bandwidths of 52.83%, as demonstrated in Figure 7(b). The results mentioned above demonstrate that the proposed low-RCS cavity structure loaded with PCM has broadband RCS reduction performance.
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To further explore the ultra-wide-angle performance, the cavity is simulated before and after loading PCM at 10 GHz. When is 90° and changes from -180° to 180°, the simulation results for TE polarization and TM polarization are shown in Figure 8. Under TE and TM polarization, the low-RCS cavity has a very excellent RCS reduction between -180° and 180° after loading the PCM, as Figures 8(a) and 8(b) show. The aforementioned findings demonstrate the ultra-wide-angle RCS reduction performance of the proposed low-RCS cavity structure loaded with PCM. In conclusion, the low-RCS cavity structure loaded with PCM suggested in this paper exhibits excellent performance for both broadband and ultra-wide-angle RCS reduction under TE and TM polarization.
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To further evaluate the performance of the RCSR, the 3D bistatic scattering pattern of the cavity with and without PCM is simulated. When the operation frequency is 10 GHz and the incident angle is 0°, 45°, 90°, 135°, and 180° under TE polarization and TM polarization, the result is shown in Figures 9 and 10, respectively. The scattered energy of the cavity without PCM is focused in the main lobe, as shown in Figures 9(a)–9(e). In contrast, when PCM is loaded, the main lobe separates and its strength decreases significant strength, as shown in Figures 9(f)–9(j). The main lobe of the cavity with PCM is successfully reduced in accordance with energy conservation theory. The simulation results in Figure 10 are similar to those in Figure 9. It is once again demonstrated that the designed low-RCS cavity with PCM has the characteristic of RCS reduction.
3. Experimental Results
To validate the accuracy and validity of the simulation design, we prepared experimental samples and tested them in the anechoic chamber. The experimental setup diagram is shown in Figure 11. The monostatic RCS test system includes a plane reflector, a vector network analyzer, a transceiver antenna, and a foam turntable. The reflector expands the electromagnetic waves’ propagation distance. The electromagnetic wave that the measured object was irradiated with is nearly a plane wave, which satisfies the RCS test’s requirements for far-field conditions. The experimental samples are placed on the turntable. By rotating the turntable, the omnidirectional monostatic RCS test may be carried out because the transceiver antenna’s position is fixed. To obtain the metal pattern, we employ screen printing technology to print PCM on aramid paper with silver paste. The air medium layer in the middle is replaced by ROHACELL HF 51 foam ( and ). Aluminum foil is pasted on the back of the foam as a metal ground plate. Our experimental sample diagram is displayed in Figure 12. The experimental sample is in total.
As depicted in Figure 13, the simulation and measurement results of the low-RCS cavity under TE and TM polarization are presented. The outcomes of TE polarization and TM polarization are shown in Figures 13(a) and 13(b), respectively. As can be observed from Figure 13, the measurement results and simulation results are mostly in accord, and the 10 dB reduction bandwidth range is also in good agreement. This indicates that the PCM can be utilized to design the cavity to accomplish broadband RCS reduction. The simulation and experimental findings of the low-RCS cavity at 10 GHz, at various azimuth angles, are depicted in Figure 14. Figures 14(a) and 14(b) display the results of TE and TM polarization, respectively. From Figure 14, it is obvious that the experiment findings and simulation results, from -180° to 180°, are essentially in agreement. This shows that the RCS reduction of the ultrawide angle can be accomplished by employing the PCM to design the cavity. From the comparison results of simulation and experiment, it can be seen that there are some differences between the two. These differences can primarily be attributed to the following factors: There is a certain error in the cutting process due to the large foam particles, and the relative dielectric constant of foam is close to but not equal to 1 within the tested frequency range; the pattern uses silver paste, which is easily oxidized and causes the performance to change. The ideal plane wave is used in the simulation, while the spherical wave generated by the horn antenna radiation is used in the experiment, which can be approximated as a plane wave in the far field. The experiment results demonstrate once again the viability and effectiveness of employing PCM to achieve broadband and ultra-wide-angle RCS reduction of the cavity.
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The performance of our design is compared with the reported literature works, and the comparison is shown in Table 1. It can be seen from Table 1 that our work offers wideband and ultra-wide-angle RCS reduction compared to others reported work.
4. Conclusion
In conclusion, we have demonstrated a broadband and ultra-wide-angle RCS reduction approach for the EOC loaded with the PCM. The PCM construct, composed of a collection of polarizing reflectors, is loaded onto the inner and outer walls of the cavity in accordance with the array theory and the orthogonality principle to produce a 10 dB RCS reduction over 6.3 GHz to 13.2 GHz with fractional bandwidths of 70.77% for TE polarization and 7.8 GHz to 13.4 GHz with fractional bandwidths of 52.83% for TM polarization under normal incidence. The RCS performances under wide-angle incidence are also considered, and the ultra-wide-angle RCS reduction from -180° to 180° is achieved. The simulated and measured results are good consistent over the entire frequency range. The orthogonality principle is effective for controlling the scattering of electromagnetic waves from complex targets such as EOC, dihedral angle, and trihedral angle. To the best of our knowledge, this is the first time the PCM, which is composed of a number of polarizing reflectors, has been employed to reduce the RCS of the EOC in both wideband and ultrawide angle. This approach, which is straightforward, efficient, and inexpensive, has a wide range of potential applications in the cavity’s RCS reduction design. Overall, our research provides valuable insights and opens up new possibilities for reducing detectability in radar systems.
Data Availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Conflicts of Interest
The authors declare no conflicts of interest.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (no. 52021001 and 51972046) and “111” Center (no. B13042) and partly supported by the Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT).