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Abstract
In this paper, a polarization-insensitive high transmittance bandpass filter with low radar cross section (RCS) in both S- and X-band is proposed. This is the first study to use the partition layout loading approach for conformal structures with transmissive windows, reducing the operating band RCS. Curved structures have stronger radiation at a smaller angle to the incident wave, and that is how their scattering differs from uniform scattering from flat structures. The structure is divided by analyzing the radiative contribution of different regions. The surface was discussed in regions according to surface angles, and a new partition layout loading method was used to suppress the side currents and decreased backward scattering, achieving a backward RCS reduction of more than 10 dB at 4-8 GHz (66.7%). The bandpass layer operating at 6.9 GHz is designed through equivalent circuit theory. In combination with the lossy layer, absorption above 0.8 at 3.7-5.6 GHz and 9.1-12.5 GHz was achieved. Further, the structure was fashioned into a curved surface with varying curvature, demonstrating its effective absorption and transmission properties across different curvatures. A 15 × 15 cell structure was designed and fabricated, and there was good agreement between the test results and simulation results. The proposed structure has important applications in radomes, conformal structures, and electromagnetic shielding.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
In a free space, frequency-selective surfaces (FSS) [1–4] act as spatial filters or reflectors, which are the most effective means of removing interference. Over the past few decades, FSSs have been extensively utilized as hybrid radomes to protect antennas from physical environment and RCS. However, with conventional radomes [5–7]., in-band signals remain highly transmissive, while out-of-band signals are highly reflected, making them easy to detect. Contemporary common flat FSS can be achieved by etching technology, with the advantages of easy production and low cost [8]. The FSS-based curved radar radome [9,10] has important practical applications in various domains such as aerospace and ships.
In 2019 [11], Wang designed an ultra-wideband frequency selective absorber to reduce reflections between the passband and the absorption band, with a broadband of less than 10 dB (126.4%). Chen achieved miniaturisation of a frequency selective absorber by introducing a large capacitance in the bandpass layer that exhibits an infinite impedance [12]. In [13], Fan proposed a passband tunable frequency selective absorber that combines resistive devices to achieve wideband absorption. Some researchers have also used the equivalent circuit principle to design the bandpass and lossy layers separately and combine them to form a broadband frequency selective absorber. In the Ref. [14], the problem of impedance matching between the absorber and free space at oblique incidence was solved using a compensation layer to achieve wave absorption at different curvatures at 4.3-11.1 GHz.
Compared to flat surfaces, curved structures [15–19] have more extensive applications in engineering applications. For flat structures, within a certain angle, scattering from the front side plays a major role. Wave absorption [20,21] and phase cancellation [22–25] are two common methods of reducing RCS. Research on curved surfaces is typically used in the field of radar cross section reduction [26–28]. In 2022, Jiang et al. researched the scattering characteristics of surfaces at different frequency points and proposed the concept of partition layout loading to further reduce the RCS of conformal structures [29]. This included a maximum reduction of 17 dB at 1-1.5 GHz. Partition layout loading implies dividing a structure into different regions according to the intensity of radiation. In this study, strong and weak radiation were defined as regions A and B at positive incidence. For the same kind of cell structure, different regions had different angles of incidence, which led to different absorption performance. Thus, by adjusting the structure, its performance in different regions could be optimized. This presented an important design idea for curved surfaces in terms of electromagnetic (EM) stealth.
Research on transmissive conformal curved surfaces is crucial for signal transmission in communication systems. In 2020, a miniaturized dual-band frequency-selective surface was designed using an equivalent circuit [30,31]. It provided stable resonant frequencies, and stable performance was verified even at different curvatures [32]. However, it lacked out-of-band RCS reduction and cannot be applied to EM stealth. In [33], a new bandpass filter was designed using an equivalent circuit that reduced the insertion loss in the passband and increased the absorption bandwidth. It also showed good transmission characteristics in curved structures, but its absorption band was affected by the curved surface and the stealth performance becomes worse. In Ref. [34], wide frequency absorption was achieved using graphene instead of resistive absorption. The FSR using metal-graphene hybrid metamaterial not only avoids the welding process but also facilitates flexibility. But this increases the fabrication cost. The stealth problem for curved structures with transmissive windows [35,36], such as airplanes and ships, is a difficult research challenge. Contemporary studies addressing the RCS reduction problem of transmissive conformal structures are unable to achieve the balance between the large curvature of the conformal structure and the high performance of metasurfaces. Therefore, for conformal structures, it is an urgent problem to realize both broadband RCS reduction and high transmittance.
In this paper, a curved frequency selective rasorber (CFSR) with low RCS and high absorption was proposed for S-, C- and X-band applications. The medium frequency transmission band maintained excellent transmittance when the EM wave was at an incidence angle of 60°, indicating that the structural band has high stability. The surface was defined as different regions A and B according to the scattering intensities, and different cells are loaded into the partitioned A and B regions. Different angular impedance matching was achieved, which improved the absorption rate and further reduced the RCS of the curved surface. The innovation of this study is that adjusting the structure can improve its absorption performance at oblique incidence and further reduce the RCS of the curved surface without affecting the transmission window. Comparing the results before and after partition layout loading highlighted the advantages of partition layout loading in the proposed surface design. The CFSR has important applications in the fields of radar stealth, satellite communication, and aircraft communication.
This paper is organized as follows. In Section II, we present the theory of partition layout loading and explain the reasons for using partition layout loading. Section III provides a detailed description of designs of cells A and B as well as the results of the optimized structure. The test results of the unpartitioned planar structure are also presented. Section IV presents the simulated S-parameters for different curvatures as well as the test results for a curvature of 120 mm after partition layout loading. The effect of RCS reduction before and after partition layout loading was compared. Finally, the conclusion is drawn in Section V.
2. Curved surface scattering characteristics analysis
The schematic diagram of the proposed Absorption-Transmission-Absorption (ATA)-type surface structure is shown in Fig. 1. When the EM wave was frontally incidence, it operated in the absorbing state at 3.7-5.6 GHz and 9.1-12.5 GHz and in the transmitting state at 6.4-7.6 GHz. The operating band was at 3.7-12.5 GHz, including S-, C- and X-band, which achieved the backward RCS reduction of the EM wave in a wide band. It had high efficiency and wide band characteristics.
Planar metallic structures exhibit almost the same radiation intensities in different regions at positively incidence EM waves. The radiation intensity in different regions of a curved metal structure is affected by different angles (the angle Phi of the incident wave to the surface normal line). To reduce the RCS of a curved structure, it is necessary to reduce the scattering in regions with stronger radiation intensities.
For the columnar surface structure, we analyzed its RCS calculation equation [28]:
Where a and b are the radius and the height of the cylinder, respectively. λ is the free-space wavelength of the incident microwave. The height of the cylinder has a higher influence on the RCS than its radius. These characteristics determine that RCS reduction in the vertical plane is essential for cylindrical stealth technology. Therefore, this study focused on the effect of TE-polarized wave incidence on the overall RCS.First, we examined the scattering characteristics of metal surfaces under the same conditions. Figure 2 shows an analytical comparison of the radiation direction diagram for a metal cylinder, and the radiation characteristics at different frequencies were analyzed using CST Microwave Studio. This design is typically used for curved surfaces of S-, C- and X- band, so we analyzed the far-field radiation plots of metallic curved surface. Figure 2 shows the radiation direction diagram when the Y-polarized EM waves were incident. At low frequencies, radiation intensity is slightly higher from the front than from the sides. At high frequencies, radiation intensity is almost the same from the front and the sides. The radiation intensity remains constant when Phi changes from -90° to +90°. For curved structures, both frontal and lateral influences on backward radiation are significant. There is a common problem in the stealth design of curved structures. Front metasurfaces typically exhibit good stealth performance for a fixed direction of incident EM waves. For the side area, when the incidence angle ($\varphi $) is larger, the absorption effect worsens, resulting in greater radiation intensity and affecting the overall stealth effect. Therefore, how to reduce the influence of side scattering on stealth is the focus of our next discussion.
Fig. 2. (a) Scattering intensity of metal cylinder at different frequencies and angles (b) schematic diagram of partition layout loading by region.
Based on the above analysis, we designed the curved structure in regions. Using 45° as the division boundary, the surface was divided into regions A and B. In Fig. 2(b), different colors represent different region. When the EM wave was incident along the + Z direction, region A was forward incidence and region B was oblique incidence. Usually, a metasurface is better for positive incidence and progressively worsens for oblique incidence. It is essential to enhance the absorption in region B, thereby reducing the overall scattering. Since the oblique incidence properties of most metasurfaces deteriorate gradually, we discuss the surfaces by dividing them into regions A and B. In Section III, we discuss how to optimize the structure of region B for enhancing its absorption properties.
3. Unit structure design and principle analysis
3.1 Design of planar bandpass filter cell structure at positive incidence (region A)
In this study, a conformal radome with transmission window and low reflection was designed, and a planar structure was designed first. To design a common ATA-type bandpass filter, we designed a cell structure with two resistive bands and a passband adjacent to each other, and its transmission window had low insertion loss. Figure 3 shows the designed absorption-transmission integrated frequency selective absorber consisting of a lossy layer, a transmission layer and an air layer. The equivalent circuit is shown in Fig. 4 and consists of two parts (Lossy layer and Bandpass Layer). Based on the equivalent circuit theory, series LC and parallel LC loops were designed to achieve signal transmission at frequencies up to 7 GHz, as shown in Fig. 5(b). The metal block and the metal ring can be equated to the LC circuit. The transmission layer consists of two series LC circuits that are generally capacitive. The lossy layer consists of a cross frequency selective structure and a metallic ring. The bent long dipole of the inner metal ring can be equated to a LC circuit, denoted by ${L_2}{C_2}$. Similarly, the outer metal ring loading resistance and the short metal arm of the cross structure can be represented by series RLC circuits. ${R_1}{L_1}{C_1}$ are the metal outer ring structure and resistance respectively. At the same time, we use KL for the bent long dipole, which operates mainly in the low frequency band, and KH for the combined structure, which controls mainly high frequency transmission. ${Z_1}$ is the cascaded transmission line equivalent impedance between KL and KH, which is the dielectric impedance between KL and KH. ${Z_1}$ can be calculated using equation of ${Z_0}/\sqrt {{\varepsilon _r}}$. Similarly, the equivalent impedance of the cascaded transmission line, denoted by ${Z_2}$, is the impedance of the air layer between the lossy layer and the bandpass layer. ${Z_0}$ denotes the free space impedance above and below the frequency selective rasorber. ${Z_0}$ and ${Z_2}$ are numerically equal and both are 377 $\Omega $. In addition, the parallel circuit ${L_3}{C_3}{L_4}{C_4}$ is the lower layer of the bandpass filter and has the same resonant frequency with the ${L_2}{C_2}$. The CST Microwave Studio simulation results are in good agreement with ADS and the exact values have been given in the Fig. 5. We analyzed the surface current distribution of the structure and verified that different resonant frequency points are controlled by different square loops. The outer and inner rings controlled the front and rear resonant frequencies, which is consistent with the design concept.
Fig. 3. (a) Schematic diagram of the overall structure, (b) front view of the lossy layer, (c) front view of bandpass layer. The specific parameters are as follows: ${L_1}$= 14.3 mm, ${K_1}$=0.77 mm, ${L_2}$=7.28 mm, ${K_2}$=0.5 mm, ${L_3}$=2.6 mm, ${L_4}$= 6.5 mm, ${K_4}$=0.7 mm, ${L_5}$=2.2 mm, ${K_5}$=0.2 mm, ${L_6}$=8 mm, ${K_6}$=0.3 mm, ${L_7}$=2.7 mm, ${K_7}$=0.3 mm, ${L_8}$=2.7 mm, ${K_8}$=0.4 mm.
Fig. 4. Equivalent circuit diagram of the CFSR. The parameters obtained by ADS calculations are as follows: ${L_1}$=1.900 nH, ${L_2}$ = 3.615 nH, ${L_3}$ = 10.542 nH, ${L_4}$ = 6.092 nH, ${C_1}$ = 0.130 pF, ${C_2}$ = 0.157 pF, ${C_3}$ = 0.172 pF, ${C_4}$ = 0.038pF, ${R_1}$ = 395.210 $\Omega $, ${Z_0}$ = 377 $\Omega $, ${Z_1}$ = 217.661 $\Omega $, ${Z_2}$ = 377 $\Omega $.
Fig. 5. (a) S-parameters of CFSR fitted by ADS and CST, (b) S-parameters of the bandpass layer fitted by ADS and CST.
In conventional radar radomes, stealth is typically achieved through wave absorption and phase cancellation. In this study, the focus was on the former approach. The proposed method achieved RCS reduction by utilizing absorption based on bandpass FSS, as depicted in Fig. 3(b). To achieve this, a combination of cross-selected structure, square ring and resistive elements were employed to absorb reflected waves. In addition, a lossy layer was combined with a bandpass filter, as shown in Fig. 3(a), and the thickness of the air layer was 5.5 mm. The top and bottom layers consist of F4B (${\varepsilon _r}$ = 3.0, and $\tan d$ = 0.001) with the thickness of 1 mm and 2 mm. Impedance matching was achieved by adjusting the thickness of the air layer, and EM wave absorption was achieved while keeping the transmission window unchanged. Surface current distribution of the lossy layer was simulated and analyzed, with results shown in Fig. 6(a)-(c) for 4 GHz, 6.9 GHz, and 11 GHz, respectively. At 4 GHz and 11 GHz, resonance was generated by the metal outer ring and cross structure. Current was mainly gathered on the outer ring and the cross, and the current at this moment generated absorption when it passed through the resistor. At 6.9 GHz, surface currents were mainly distributed on the inner metal ring, and transmission was enhanced by rectangular ring resonance. The proposed method yielded an absorption rate of above 0.8 in both low and high frequency bands, with small insertion loss at 7 GHz, which was consistent with simulation results. Resonance points generated at 4 GHz and 10.6 GHz remained unchanged, and the reflected EM waves were absorbed as they passed through the lossy layer. The resonance was mainly generated by the middle rectangular block at 7 GHz to enhance the transmission in the center band and reduce the insertion loss.
Figure 7 shows the simulated and measured S-parameters and absorption ratio, and the simulation results were in agreement with the experimental results. The transmittance was above 0.8 at 6.5-7.5 GHz, and the lowest insertion loss was only -0.4 dB at 6.9 GHz. The absorption rate of the structure was above 0.8 at 3.7-5.6 GHz and 9.1-12.5 GHz by the Formula (2) [38]. The design of the ATA-type bandpass filter was realized, and the specific structural dimensions are shown in Fig. 5. The small thickness of the lossy layer provided the necessary conditions for the conformal design as well. We analyzed the S-parameters at different incidence angles as shown in Fig. 7(c). In this paper, the demarcation limit for regions A and B is set at 45° due to significant performance degradation at this angle. As the incident angle increased, the impedance of the structure did not match well with the free space, leading to an upward shift of the ${S_{11}}$ curve and a decrease in the absorption ratio. The transmission band narrowed, and its transmission center frequency point did not change. The transmission window had angular stability.
Fig. 7. (a) The simulated and experimental S-parameters plots of the combined structure, (b) the simulated and experimental absorption results, (c) the simulated incident angle and (d) pictures of the fabricated FSS.
3.2 Optimisation of planar bandpass filter cell structure at positive incidence (region B)
In Section II, we analyzed the radiation field of a curved surface that exhibited minimal differences at intensities between ±90° and 0°. Therefore, to improve the effectiveness of electromagnetic stealth, it is necessary to consider the oblique incidence of EM waves. The impedance of the planar structure changed at oblique incidence, resulting in a mismatch in impedance with free space, and consequently leading to a decrease in the effectiveness of absorption. First, the effect of resistor resistance on the overall absorption effect was comparatively analyzed as shown in Fig. 8(a). When the angle of incidence was 45°, the S-parameters gradually decreased to below -10 dB as the resistance value increased. The high-frequency S-parameters were unstable and increased when the resistance value exceeds 175 Ω. Ultimately the resistance was determined to be 175 Ω. In the next step we adapted the dimensional structure of the metasurface. Adjusting ${L_4}$ considerably influenced the S-parameters. The length of ${L_4}$ determined the capacitance generated between neighboring cells and also affects the frequency of absorption. The absorption effect at oblique incidence was further improved when ${L_4}$ was eventually increased to 7 mm. Figure 8(b) shows the impedance diagram of the cell at an incidence angle of 45°, with imaginary part of 0.4 at 4-6 GHz and real part of greater than 1 at 8-11 GHz. At incidence angles greater than 45°, the absorbing performance of the structure significantly decreased. This was because the impedance of the structure cannot match that of the free space, resulting in a lower absorption rate. Figure 8(b) shows the real and imaginary values of the relative impedance of region B units. In the range of 4-10.5 GHz, the real part of the relative impedance was close to 1 and the imaginary part was approximately 0. The absorber matched well with the impedance of free space, and most of the energy is effectively absorbed in the absorber. Finally, by adjusting ${L_4}$ (7 mm) with the resistor (175 Ω), the real part of the impedance was reduced to 1 and the imaginary part was close to 0. In the case of oblique incidence, impedance matching was achieved by adjustment, which improved the oblique incidence performance of the structure. Good preparation was done to increase the absorption in region B of the curved structure.
Fig. 8. (a) S-parameters at different angles of incidence and resistor resistances (Ω), (b) impedance parameter diagram of A (R = 125Ω, ${L_4}$=6.5 mm) and B (R = 175Ω, ${L_4}$=7 mm). The blue colour in the diagram represents 125 Ω and the red colour represents 175 Ω.
4. Analysis and discussion of experimental and simulation results
4.1 Different curvature analysis
To cater to different sizes of aircraft or ships, frequency-selective absorbers with curved surfaces of varying curvatures are essential. Hence, we conducted a comparative analysis of the structural properties at different curvatures. Figure (9) represents a schematic diagram of the conformal structure with R1 = 80 mm, R2 = 100 mm and R3 = 120 mm. The S-parameters of the incidence of TM-polarized and TE-polarized waves are shown in Fig. 10(a) and (b). At TE-polarized wave incidence, the low-frequency absorption band became less effective as the radius of curvature increased. When the radius of curvature was 120 mm, the angle of oblique incidence was smaller and the wave absorption effect was better. The transmission window in the middle frequency remains almost unchanged, but the lower absorption band became slightly narrower. The overall characteristics remained good, with good RCS reduction in the S-, C-, X-band. On the other hand, when the TM-polarized wave was incident, the transmission window and absorption band were almost independent of the radius of curvature and showed good stability.
Fig. 9. Schematic diagram of convex surface with radius of curvature of (a) 80 mm, (b) 100 mm, (c) 120 mm.
Fig. 10. S-parameter of the convex surface when (a) TM-polarized wave and (b) TE-polarized wave incident with different radius of curvature.
We further analyzed the S-parameters of the concave surface at different curvatures, as shown in Fig. 11 and 12. Good absorption and transmission properties were maintained at both TE-polarized and TM-polarized wave incidence. For complex concave and convex surface structures, the proposed structure has significant applications.
Fig. 11. Schematic diagram of concave surface with radius of curvature of (a) 80 mm, (b) 100 mm, (c) 120 mm.
Fig. 12. S-parameters of the convex surface when (a) TM-polarized wave and (b) TE-polarized wave incident with different radius of curvature.
4.2 Partition layout loading
When an EM wave is positively incident, different regions on the surface exhibit different performance because of different incidence angles. To address this issue, we proposed a partitioned loading method to further reduce the backward RCS of the surface structure. The significance of layout loading was that the weakening of the side currents of curved structures can be achieved without affecting the performance of the conformal structure. The combined structure can reduce the amount of lateral current contribution to the overall backward radiation. The RCS reduction was enhanced by further optimizing the structure and improving the absorption performance in the region where the backscattering from the surface was relatively large.
To verify the experimental results, we tested the reflection and transmission coefficients at a curvature of 120 mm, as shown in Fig. 13. Good characteristics were maintained at both TE-polarized and TM-polarized wave incidence, and the test results matched the simulation results. Figure 13(c) and (d) show the front and side views, respectively, of the test sample with a curvature of 120 mm.
Fig. 13. (a) S-parameters of TE-polarized wave incidence, s-parameters of (b) TE-polarized wave incidence, (c) front view of the experimental sample and (d) side view of the experimental sample.
We simulated the RCS reduction of the loaded surface when the curvature radius was 120 mm. Figure 14(a) shows the simulation results of the RCS reduction before and after partition. Figure 14(b) shows the test environment. The backward RCS after partition reduced by about 10 dB at 5-6.5 GHz and 10-11.5 GHz.
Fig. 14. (a) Simulation results of RCS reduction before and after partition, (b) Photo of the experimental environment.
Table 1. Performance comparison of frequency selection rabsorbersa
Finally, we analyzed and compared our findings with the findings of recently published articles related to frequency selection absorbers, as shown in Table 1. Compared to planar structure, our proposed structure has a very low insertion loss, as low as 0.4 dB. Compared with the curved surface structure, the proposed structure has wider transmittance bands and ultra-thin properties, with a thickness of only 0.17 wavelength. It is obvious that the proposed structure has the characteristics of wide band, low loss and ultra-thin. From a stealth perspective, partition layout loading was used for the first time to reduce RCS for ultra-wideband conformal structures. A design with stable transmission window and broadband stealth was realized, providing an important idea for curved stealth design.
5. Conclusion
In this paper, a conformal surface with low RCS was proposed. By using partition layout loading, the backward RCS of the metasurface is further reduced while having high transmittance in the communication band. This design optimises the problem of poor performance of transmissive windows under flexible structures and reduces the amount of backward scattering. The S-parameters were analyzed at different curvatures, and the results indicate that the structure maintains good transmittance at different radius of curvature. Finally, the experimental and simulation analysis of RCS reduction when R is 120 mm is carried out. At present, the structure has great application value in electromagnetic shielding and conformal structure.
Funding
Central Universities in China (No. CCNU22JC018).
Disclosures
The authors declare no conflicts of interest.
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.
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