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Abstract
Smart structures with tunable electromagnetic characteristics are required for camouflaging high-value targets, such as land warfare moving equipment, in continuously changing complex electromagnetic environments. The targets must control their radar cross section (RCS) to avoid detection and tracking. Frequency selective surfaces (FSSs) are the next-generation smart structures in which active RCS control is realized via impedance loading. In this paper, a multistate transformable FSS absorber/reflector that operates in the 3.9–11.0 GHz band is introduced and analyzed. The reflectivity amplitude of this absorber/reflector could be smoothly changed from 0 to -10 dB in 6.0-8.0 GHz. Each unit cell of the FSS structure consisted of four symmetrical diamond-shaped patterns, and the adjacent units were connected by PIN diodes. The absorption intensity of the FSS could be changed from 0 to -10 dB by adjusting the PIN bias voltage, which was applied via a simple bias network. The multistate switching characteristic of the FSS was verified by simulations and measurements. The results showed that adjustable absorbing intensity and switchable working states are the desirable characteristics that allow high-value targets to adapt to changing electromagnetic environments. Our work will bridge the gap between the available stealth strategies and practical applications, such as moving stealth vehicles.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The radar cross section (RCS) of a target plays a predominant role in its detection by radar detectors [1,2]. Controlling the electromagnetic signature of a body in the form of enhancement or reduction in the RCS of the body is important for numerous radar applications. Several studies have been conducted to identify and develop methods and procedures for reducing the RCS of a target using metamaterials and other novel structures. These studies primarily focused on enhancing the absorption and expanding the absorptive bandwidth of the protected object [3–5]. In recent years, absorbers with smart structures [6–9], such as active frequency selective surfaces (AFSS) [10–12], switchable FSS [13–17], and other tunable novel structures [18–20], have attracted considerable attention owing to their tunable properties. These structures can meet the various practical requirements and demands of smart applications. A switchable absorber/reflector is one such smart material that has stealth characteristics [21,22]. It can improve the survivability of a target in its absorption state and allow the detection of the target by friendly parties in its reflection state. This switching feature is beneficial for military reconnaissance. Most of the previous studies on absorber/reflector materials were focused on evaluating their bandwidth and polarization sensitivities, and little attention was paid to the dynamic regulation of their reflectivity amplitudes. However, in reality, the electromagnetic background is constantly changing, which is a significant challenge for stealth applications in complex electromagnetic environments. An ideal stealth weapon should be able to rapidly adjust its state according to the electromagnetic environment to remain invisible at all times as if it has been endowed with a chameleon-like ability. Figure 1 shows a moving vehicle that has been loaded with a specially designed AFSS structure. To realize real-time stealth, the state of the FSS structure is adjusted such that the electromagnetic characteristics of the loaded structure, detected by the radar, are similar to those of the environment in which the vehicle is located. Therefore, it is imperative to design an absorber/reflector structure, whose RCS can be desirably controlled, for application in smart stealth systems.
In this study, a broadband multistate continuously adjustable AFSS absorber/reflector was proposed, designed, and analyzed for stealth applications. The absorption intensity of this absorber/reflector could be changed between 0 and -10 dB by varying the voltage applied on the PIN diodes. The impedance and working principle of this proposed structure were evaluated using an equivalent circuit. Considering the practical application scenarios, we simultaneously investigated the radar cross section reduction (RCSR) and the possibility of disturbing the induced current imaging of a target using the scattering intensity at selected points on the target. Furthermore, important characteristics of some fabricated structures are also presented in this paper to confirm the relative results. The proposed design showed attractive characteristics, which may find potential applications in electromagnetic camouflaging. The paper has been organized in the following manner: In section 2, a primary structure of the proposed AFSS absorber/reflector is analyzed via simulations, and the relevant principles are discussed using the equivalent circuit method. Next, we discuss the stability of the structure under oblique incidence, and finally, describe the RCS characteristics and RCSR imaging of the structure. Section 3 provides details on the experimental verification of the absorber/reflector performance. Section 4 outlines the major conclusions and implications of the study.
2. Structure design, simulated results, and RCSR analysis
2.1 Structure design and full-wave simulation analysis
A multistate AFSS absorber/reflector should be capable of switching between absorption and nearly perfect reflection in the operating frequency band. Further, the absorption intensity of the structure should be adjustable between -10 dB and 0 dB to adapt to different electromagnetic environments. In practice, a diamond-shaped unit cell pattern is used to achieve the required absorptive and switching characteristics.
The absorber/reflector, designed in this study, consisted of a dielectric substrate, an AFSS layer, isolation layer (honeycomb), and a ground plane, as shown in Fig. 2(a). FR-4, with a permittivity of 4.4, loss tangent of 0.02, and thickness of 0.15 mm, was used as the substrate material. The FSS units consisted of four symmetrical diamond-shaped metallic patterns, and the adjacent FSS units were connected using PIN diodes. The unit cell geometry is shown in Fig. 2(b). The optimized geometrical parameters of the AFSS element were: p = 20 mm, a = 0.8 mm, b = 3.4 mm, d = 7.0 mm, g = 1.4 mm (these variables are described in Fig. 2(b)). A honeycomb support layer was used to isolate the metallic patterns from the ground plane, owing to its high strength and low density. To achieve strong absorption performance, the total thickness of the structure should preferably be greater than 1/4 λ of the central frequency. In this study, the total thickness of the structure was limited to 6.15 mm because of the application requirement, which was far less than 1/4 λ of the central frequency (10 mm at 7.5 GHz).
Fig. 2. (a) Primary structure of the absorber/reflector. (b) Unit cell of the AFSS. (c) Simulated reflectivity curves of the proposed structure for different states under normal incidence.
The absorptive and switching performance of the proposed absorber/reflector structure was assessed using full-wave high-frequency structure simulations (HFSS). The full-wave HFSS software, based on the finite element method, was used to simulate the structural response under normal incidence. In the simulations, periodic boundary conditions were applied on both the x- and y-axes. When a plane electromagnetic wave is incident on the proposed structure along the z-axis, reflection occurs at the structure surface. The reflection characteristics of the structure under different cases are illustrated in Fig. 2(c). In the OFF state, the reflectivity was almost 0 dB. In the ON state, as the resistance was varied from 20 to 60 ohm, the reflectivity gradually decreased from -3 dB to -10 dB, and dropped even lower in some bands. In general, the working bandwidth of the proposed structure was 3.9–11.0 GHz. The results shown in Fig. 2(c) imply that in the frequency range of 6.0–8.0 GHz, the reflectivity amplitude can be smoothly changed from 0 to -10 dB by adjusting the PIN diodes. Moreover, the amplitude can even reach -15 dB in the frequency range of 4.3–5.1 and 9.1–10.4 GHz.
Herein, the equivalent circuit method is employed to elucidate the working principle of the proposed absorber/reflector structure. The metallic patch part in the AFSS unit pattern is considered equivalent to an inductor Le, and the gaps between the adjacent metallic patches can be modeled as an equivalent capacitor Ce [23]. Therefore, the AFSS layer of the structure can be considered to form an equivalent circuit with an impedance—ZPIN—in series with Le and Ce, as shown in Fig. 3. In the OFF state, i.e., under reverse bias, the PIN diode has a capacitance COFF. In contrast, in the ON state, i.e., under forward bias, the PIN diode acts as an equivalent series circuit—LON RON [21]. The top FR-4 layer has a negligible influence on the overall impedance of the AFSS layer, and hence, can be ignored.
The reflectivity of the absorber/reflector can be derived using the equivalent circuit model shown in Fig. 3. The input impedance of the whole structure can be expressed as:
Finally, the reflectivity (Γ) of the whole structure can be written as:
Equation (4) indicates that the resistance RON of the PIN diode influences both the real and imaginary parts of the input impedance of the whole structure. Figure 4 shows the curves of the real and imaginary parts of the input impedance relative to RON, i.e., a visual representation of how RON influences the input impedance. Evidently, in the ON state across the 6.0–8.0 GHz frequency range, the imaginary part of the input impedance of the whole structure is almost zero, while the real part gradually increases from 50 to 200 ohm with increasing RON. Moreover, the increasing RON gradually approaches the characteristic impedance of vacuum. Therefore, the reflectivity amplitude can be smoothly changed from 0 to -10 dB in the frequency range of 6.0–8.0 GHz. Conversely, in the OFF state, impedance mismatch occurs between the input impedance and the characteristic impedance of vacuum; consequently, the structure exhibits a strong reflectivity of 0 dB in this case.
Fig. 4. (a) Real part of the input impedance at different PIN resistances. (b) Imaginary part of the input impedance at different PIN resistances.
Fig. 5. Simulated reflectivity of the proposed structure under oblique incidence of electromagnetic waves at different angles and a resistance of 60 ohm: (a) TE polarization, (b) TM polarization.
The stability of the proposed structure under the oblique incidence of electromagnetic waves is critical for its practical applications. Figure 5 shows the simulated reflectivity of the proposed structure under obliquely incident electromagnetic waves at different angles for both transverse electric (TE) and transverse magnetic (TM) polarizations. These curves were simulated by considering a resistance of 60 ohm. Notably, when the incident angle θ is increased from 0° to 45°, the absorption bandwidth becomes narrow, and the amplitude decreases. The difference between the curves obtained at TE and TM polarizations is mainly due to the difference in impedance matching under the oblique incidence conditions. However, the structure shows absorption characteristics in the 6.0–8.0 GHz range for obliquely incident TE and TM polarized waves at 45°. These results indicate that the proposed structure has good oblique incidence stability in the 6.0–8.0 GHz frequency range.
2.2 RCSR of the structure
The scattering property of an object is quantitatively determined from its RCS. Thus, it is essential to investigate the RCS scattering characteristics of a structure. In this study, the full-wave electromagnetic simulation tool, CST Studio Suite, was used to model and simulate the proposed structure. The plane structure, containing 15 × 15 elements, was 300 mm × 300 mm in size, as shown in Fig. 6(a).
Fig. 6. (a) RCSR simulation model of the structure. (b) RCSR results of the structure, obtained under different states and normal incidence.
The simulated RCSR results of the structure, obtained under different states and normal incidence, are illustrated in Fig. 6(b), which provides a visual representation of the RCSR under different states. Evidently, as the resistance varies from 20 to 60 ohm, the amplitude of the RCSR peaks gradually decrease to -10 dB at ON state. However, at the OFF state, the RCSR is almost 0 dB. These simulated RCSR results are consistent with those of the HFSS-simulated reflectivities and show that the proposed absorber/reflector structure has good RCSR performance.
2.3 Electromagnetic scattering control and RCSR imaging
In reality, the background environments are constantly changing, which is a considerable challenge to the stealth of high-value targets. However, the proposed AFSS structure can effectively solve this problem. To examine the applicability of the proposed AFSS structure in stealth technology, we studied the RCS of a truck in different environments. The CST-modeled metal truck is shown in Fig. 7(a). The truck model is approximately 2.4 m in length, 0.5 m in width, and 0.7 m in height. The AFSS structure is loaded on the top of the truck, as shown in Fig. 7(a), to control the intensity of the electromagnetic radiation incident on the truck. The current distribution of the truck, loaded with AFSS, under the normal incidence of 6.0–8.0 GHz electromagnetic waves was simulated using CST Studio Suite. The current induced by the incident electromagnetic wave solely determines the radiated electric field, which in turn determines the monostatic RCS. In the simulation, first, the three-dimensional current complex vector is calculated by the CST simulator. Next, the induced electric field is obtained by integrating the calculated current in a certain region, using Green's function in free space [24]. Finally, the RCS of the truck is obtained from the integrated current. Under the normal incidence condition, the RCS amplitude is proportional to the area of the object irradiated by the electromagnetic radiation. However, the RCSR value remains same irrespective of the object area irradiated by the electromagnetic radiation. Consequently, RCSR is used to characterize the reduction in RCS of a target under different ground environments. According to a previous report [25], the RCSR of forest land, cement, and desert at 8.0 GHz is -10.0, -5.9, and -4.6 dB, respectively. In our simulations, different RCSR were obtained when the truck model was illuminated by light of different colors (i.e., frequencies); the final results obtained at 8.0 GHz are shown in Fig. 7(b)–(d). Thus, we found that adjusting the resistance loaded in the AFSS structure allows us to control the RCSR of the entire truck. Furthermore, at a certain resistance value, the electromagnetic characteristics of the truck can become the same as those of the surrounding environment.
Fig. 7. (a) CST-simulated model of the AFSS-loaded truck. RCSR images of the truck and surrounding environment at 8.0 GHz, under normal incidence, in different states: (b) R = 60 ohm; (c) R = 30 ohm; (d) R = 20 ohm (R is resistance loaded on the AFSS structure).
3. Experiment results
To ascertain the performance of the proposed absorber/reflector, a sample (300 mm × 300 mm), consisting of 15 × 15 unit cells, was fabricated on a double-sided copper-clad flexible FR-4 substrate with a thickness of 0.15 mm, using the printed circuit board technology. The fabricated sample is shown in Fig. 8. Transmission lines were fabricated on both sides of the sample to provide voltage bias to the PIN diodes. The PIN diodes (BAP7003, whose operational characteristics can be easily extracted from their datasheet [26]) were welded in their designated positions and connected with inductors, which formed the baseline, as shown in Fig. 8(b) and (c). An anechoic chamber that simulates free space was used for the measurements, as shown in Fig. 8(a). A horn antenna (2–18 GHz), connected to a vector network analyzer (Keysight Technologies, N5227A), was used as both the transmitter and receiver to measure the reflectivity of the fabricated sample. During the measurements, a DC voltage source was used to apply bias voltages to the PIN diodes. Under forward bias, the PIN resistance and bias voltage are proportional to each other. Therefore, for convenience, we used different voltages to represent different resistance states during the measurement.
Fig. 8. (a) Measurement setup. (b) Partial view of the manufactured sample. (c). Complete view of the manufactured sample.
Figure 9 presents the simulation and experimental results, obtained under normal incidence, at different bias voltages. Evidently, when the PIN diode is in the OFF state, that is, the bias voltage is 0 V, the fabricated structure exhibits strong reflection characteristics in a wide frequency band (Fig. 9(b)). The best absorption performance of the sample, i.e., the reflectivity is below -10 dB in the 3.9–11.0 GHz range, is observed at a bias voltage of 18 V. Furthermore, as the bias voltage gradually increases from 18 V to 22 V, the resistance of the PIN diode decreases, resulting in a gradual reduction in the reflectivity of the structure in the 6.0–8.0 GHz frequency range. A comparison between the simulation and measurement results showed that the overall trend and amplitude adjustment range of the simulated and measured reflectivity curves were in good agreement in 6.0–8.0 GHz under different bias voltages. The differences between the simulation and experimental results, such as different positions of the absorption peaks and nonproportionality between the measurement voltages and simulated resistances, were possibly caused by the parasitic capacitance of the PIN diodes. However, the simulated and measured reflectivity curves in the operation bandwidth of the proposed absorber/reflector are similar. In general, the measurement results prove that the reflectivity amplitude of the proposed structure can be effectively adjusted or controlled in a wide frequency band. This feature will be beneficial for engineering the structure parameters such that it exhibits the best possible performance according to the application requirement.
4. Conclusion
In this study, a multistate AFSS absorber/reflector that can operate in the 3.9–11.0 GHz band was designed and analyzed using computer simulations and experiments. We observed that the absorption intensity of the AFSS structure could be smoothly adjusted between 0 and -10 dB in 6.0–8.0 GHz under normal incidence by varying the bias applied to the PIN diodes. In addition, the structure exhibited polarization independence because of its symmetric design. To demonstrate the effectiveness of the proposed design, a test sample was fabricated and experimentally analyzed. The results confirmed the multistate amplitude adjustable performance of the proposed absorber/reflector structure over a wide frequency band ranging from 6.0 to 8.0 GHz under normal incidence. Thus, high-value targets can be made to adapt to changing complex electromagnetic environments by widening the working frequency band, adjusting the absorbing intensity, and switching the working state of the absorber/reflector. The strategies proposed in this study, once implemented practically, will bring the available stealth strategies closer to a wide range of practical applications, especially moving stealth vehicles.
Funding
National Natural Science Foundation of China (62071196).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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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