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Abstract
In this paper a novel antenna array-based metasurface design method for wide-angle and polarization-insensitive radar cross section (RCS) reduction has been proposed, which can be applied to a variety of RCS reduction scenarios. The proposed metasurface subarray design employs a dual-element antenna array in which the two ports of each element are connected through a Wilkinson power divider, and meanwhile, two power dividers are connected through a microstrip line with a lumped resistor. The use of dual-polarized wide-beam antennas enables the metasurface array to respond to arbitrarily polarized as well as wide-angle obliquely incident electromagnetic waves. A portion of the electromagnetic waves received will be absorbed by the lumped resistor and converted into heat, while the remaining portion will be canceled in the space, achieving the low RCS characteristic. The proof-of-concept experiments have been conducted in several application scenarios for RCS reduction, including a metasurface array integrated with a microstrip antenna, a densely distributed dual-element metasurface array, and a randomly distributed dual-element metasurface array. Simulated and measured results confirm that the proposed method opens up a new avenue for more flexible and versatile RCS reduction devices and systems.
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1. Introduction
With rapid development of the stealth technology, research on target detectability has caused tremendous interest. Radar cross section (RCS) reduction of the target can greatly improve survivability in safe communication scenarios, and thus a variety of RCS reduction technologies have been proposed. One of the commonest methods to reduce the scattering fields is to modify shape of the target [1,2]. However, the change in the shape adds more complexities to the shape design, and may result in the disruption of the aerodynamic structures. In addition to the adjustment of the shape, the use of the lossy materials can decrease the RCS of the object. In [3–5], by integrating the resistive strips, the lumped resistors, or other lossy dielectrics, the broadband 10 dB RCS reduction characteristic is achieved. However, these lossy materials lead to deterioration of the antenna’s radiation performance. In recent years, electromagnetic metamaterials/metasurfaces, with the advantages of their small size, low loss, easy integration, and effective manipulation of waves, have provided a new approach to achieving RCS reduction [6]. The metasurface absorbers (MMAs) are capable of absorbing the incident wave with the reflection as small as possible and converting the absorbed energy into heat [7–12]. As an alternative to the absorber, the RCS reduction can be achieved by redirecting the incident wave. Two kinds of the metasurface structures with the reflective phase difference of 180°, for instance, artificial magnetic conductor (AMC) structure and perfectly conducting structure, are arranged in a checkboard manner to reduce the backward RCS based on phase cancellation [13–18]. On the other hand, polarization rotation surface (PRS) can be used for the purpose of the RCS reduction. When a PRS is rotated 90° around the center axis, the resulting structure has a 180° reflective phase difference compared to the PRS. With the phase cancellation, the PRS and its rotated structure can be arranged to decrease the RCS [19–25]. The emergence of concept of the digitally coding metasurfaces greatly simplifies the metasurface design. By flexibly regulating the phase gradient, manipulation of the beam pointing and scattering characteristics of electromagnetic waves can be realized [26–28], providing a new research approach in the field of RCS reduction based on diffuse scattering [29–34]. Although the above metasurface based designs can be used to achieve low detectability characteristics, the resulting structures are generally of large sizes. More importantly, the metasurface topologies are specially tailored for a given application scenario. As the application scenario changes, such as introduction with the radiators [35–41], rapid deployment of large platforms [42,43], etc., the metasurface topologies need to be redesigned, thus greatly limiting the flexibility of the metasurface based designs.
In this paper, an antenna array-based metasurface method for multi-scenario wide-angle polarization-insensitive RCS reduction has been proposed. The metasurface unit cell is composed of two dual-polarized slot-coupled antenna arranged in a mirror symmetry manner, with two Wilkinson power dividers connected by a microstrip line loaded by a 50 Ω lumped resistor. This innovative design method allows for the absorption of a part of the received electromagnetic waves with arbitrary polarization, and the far-field cancellation of the remaining part of electromagnetic waves overflowing through the antenna ports. Three different application scenarios of RCS reduction have been presented, including an antenna RCS reduction array integrated with a microstrip antenna, a densely arranged RCS reduction metasurface array, and a randomly distributed RCS reduction metasurface array. Through the simulation and practical experiments of the dual-element antenna-based metasurface arrays, the proposed designs demonstrate polarization-insensitive and incidence-insensitive characteristics. The flexible arrangement of the dual-element antenna in either tightly packed or randomly packed manner ensures the proposed design good RCS reduction performance without the customized design, greatly extending versatile application scenarios.
2. Design and analysis of the antenna-based dual-element array
2.1 Design of the dual-element array
In order to achieve efficient reception of incident electromagnetic waves with arbitrary polarization and wide angle, a slot-coupled antenna with dual polarization, high isolation, and wide 3 dB beamwidth is designed, as illustrated in Fig. 1. The antenna element is composed of two dielectric substrates. The upper substrate is F4BM with a dielectric constant of 2.2 and a loss tangent of 0.001, while the lower substrate is TP-2 with a dielectric constant of 10.2 and a loss tangent of 0.001. The radiation patch is located on the upper surface of the upper substrate. The metal ground plane with two vertically placed I-shaped slots is located on the upper surface of the lower substrate. The T-shaped feeding stub is located on the lower surface of the lower dielectric substrate. The detailed parameters of the proposed antenna can be found in Table 1. The performance of the designed antenna is depicted in Fig. 2. In this paper, all simulations have been performed by using the commercial finite-element solver Ansys Electronics Suite. It can be seen that the two orthogonally polarized radiation patterns of the designed antenna element exhibit high consistency, with excellent matching and port isolation characteristics in the frequency range of 9.36∼9.93 GHz where the S11 and S22 are less than −10 dB, and the cross-polarization level is lower than −22 dB, as depicted in Fig. 2(a). Simultaneously, the realized gain in the frequency band is 6.4 ± 0.4 dBi when the port1 and port2 are excited, separately. The 3 dB beamwidths cover −41°∼50° in xoz plane and −40°∼40° in yoz plane at the frequency of 9.5 GHz, respectively, as illustrated in Fig. 2(b) and (c).
Fig. 2. The performance of the dual-polarized slot-coupled antenna element. (a) S parameters. (b) Realized gain. (c) Radiation patterns at 9.5 GHz in xoz and yoz planes.
Table 1. Dimensions of The Proposed Antenna Element (Unit: mm)
2.2 Analysis of the dual-element array
As depicted in Fig. 3(a) and (b), the dual-element antenna array consists of two same elements arranged in a mirror symmetry manner. Two ports of each element are connected with a Wilkinson power divider, and two Wilkinson power dividers are connected through a microstrip line loaded by a 50 Ω lumped resistor in the middle. The structure of the Wilkinson power divider is depicted in Fig. 3(c) and (d). The detailed parameters of the proposed dual-element array can be found in Table 2. The simulated S parameters of the Wilkinson power divider are presented in Fig. 3(e) and (f), and it can be seen that, in the working band of the dual-polarized slot-coupled antenna element, the transmission amplitudes of the two ports of the Wilkinson power divider are approximately equal and not less than −3.5 dB. The reflection coefficient S11 of port 1 is less than −14.5 dB, and the isolation between port 2 and port 3 is better than −15 dB. Meanwhile, ports 2 and 3 have a stable phase difference about 58°.
Fig. 3. The dual-element antenna array. (a) 3D view. (b) The top-view. (c) The feeding networks. (d) The Wilkinson power divider. (e) The S parameter of the Wilkinson power divider. (f) The transmission phases of the Wilkinson power divider.
Table 2. Dimensions of The Dual-Element Antenna Array (Unit: mm)
To elaborate the operating mechanism of the proposed dual-element antenna array, consider an x-polarized plane wave incident on it. As shown in Fig. 4(a) and (b), the x-polarized ports of the designed dual-element array receive the x-polarized incident wave. A portion of the received electromagnetic waves will be absorbed by the lumped resistor inserted into the microstrip line, a portion of the received electromagnetic waves will be re-radiated through the other two y-polarized ports of the dual-element array. Owing to a phase difference of 180° between two y-polarized ports, the re-radiated fields are cancelled, thus resulting in the low RCS characteristics of the dual-element array. It can be seen from Fig. 4(b) that the strong currents distribute on the microstrip line close to the lumped resistor and the y-polarized ports, with very weak current at the x-polarized ports. On the other hand, as illustrated in Fig. 4(c) and (d), when the y-polarized electromagnetic wave is received through the y-polarized ports of the dual-element array, apart from the absorbed portion, the remaining part is re-radiated through the x-polarized ports with the phase difference of 180° caused by the phase difference of 180° between two y-polarized ports. Finally, it also contributes to the low RCS characteristics of the dual-element array.
Fig. 4. The mechanism of the dual-element antenna array. (a) Working mechanism for x-polarized incident wave. (b) Surface currents on the microstrip line for the x-polarized incident wave at 9.6 GHz. (c) Working mechanism for y-polarized incident wave. (d) Surface currents on the microstrip line for the y-polarized incident wave at 9.6 GHz. (e) Simulation of the dual-element antenna array. (f) The AR with the vertically incident wave. (g) The reflection coefficient with the vertically incident wave. (h) The AR for the oblique incident wave with TM polarization. (i) The AR for the oblique incident wave with TE polarization.
Furthermore, the absorption and cancellation characteristics of the proposed dual-element antenna are quantitatively investigated. Among various analysis approaches, the equivalent circuit model methods [44] can provide the unit cell characteristics in a rapid and less accurate manner. As an alternative, the full-wave simulation of the unit cell gives an accurate response of the unit cell. As shown in Fig. 4(e), the periodic boundaries are used to enclose the dual-element antenna, and the Floquet’s ports are placed 10 mm (≥ 0.25λL, λL is the wavelength of the lowest working frequency in vacuum space) away from the upper and lower surface of the dual-element antenna, respectively, to implement the plane excitation. The absorption ratio (AR) for y-polarized incoming wave is defined as
in which |ryy| is the co-polarized reflection coefficient, |rxy| is the cross-polarized reflection coefficient, |txy| is the cross-polarized transmission coefficient, and |tyy| is the co-polarized transmission coefficient, respectively. It should be noted that for x-polarized incident electromagnetic waves, the similar method can be used to calculate the absorbing characteristics of the dual-element array.The AR obtained from the simulation is shown in Fig. 4(f), from which it can be seen that when the x-polarized or y-polarized electromagnetic wave is normally incident on the dual-element antenna, respectively, the AR can reach more than 0.8 in the frequency band from 9.3 GHz to 10 GHz. Furthermore, the AR of the proposed dual-element antenna under the oblique incident illumination is shown in Fig. 4(h) and (i). For the transverse magnetic (TM) polarized incident wave with the oblique angle of 0°∼-50° in the yoz plane, the band for the AR ≥ 0.8 is from 9.3 GHz to 10 GHz. For the transverse electric (TE) polarized incident wave, the AR ≥ 0.63 can be achieved in the frequency band from 9.3 GHz to 10 GHz.
On the other hand, in order to demonstrate the cancellation characteristic of fields radiated by the cross-polarized ports, Fig. 4(g) gives S-parameters of the dual-element antenna. Owing to the metallic plate, the resulting transmission coefficients are zero. The cross-polarized reflection coefficients, i.e., rxy and ryx, are about −60 dB in the band of 9∼10 GHz, meaning negligible fields re-radiated by the cross-polarized ports in the dual-element antenna.
3. RCS reduction metasurface array
3.1 Application of the antenna RCS reduction
Based on the above working principle, the designed dual-element array is first applied to the RCS reduction application scenario of the antenna. Specifically, a radiating antenna fed through the coaxial line called the reference antenna is placed at the center of a circular structure consisting of four dual-element arrays, as shown in Fig. 5. It should be noted that four dual-element arrays and the radiating element have a common metal ground which is located on the lower surface of the upper substrate. The proposed antenna has been simulated, fabricated, and measured to validate the proposed method. The prototypes of the reference antenna and the proposed antenna are depicted in Fig. 6(a), and their radiation characteristics have been measured in the anechoic chamber. In the measurement of the far-field radiation performance, the proposed antenna is placed on a rotating platform 2.7 m away from the receiving antenna. The excitation port of the reference/proposed antenna is connected to the transmitting port of the vector network analyzer (Agilent Technologies N5244A) through a coaxial cable, and the wide-band receiving horn (HD-10200DRHA10S) mounted on the scanning stand is connected to the receiving port of the vector network analyzer through a coaxial cable. The rotation of the rotary table enables the measurement of the far field of the proposed antenna, and the measurement schematic and environments are shown in Fig. 6(b) and (c). In addition, the S parameters of the reference/proposed antenna is also measured with the vector network analyzer (Keysight N9918A). The comparisons of the S parameter and the radiation characteristic between the reference antenna and the proposed antenna are illustrated in Fig. 7. It can be seen that the working bandwidths of the reference antenna without and with the dual-element arrays for S11≤-10 dB are 9.37∼9.91 GHz and 9.36∼9.98 GHz, consistent with the simulation results of 9.35∼9.98 GHz and 9.36∼10 GHz, as shown in Fig. 7(a). The simulated and measured gains of the reference antenna and the proposed antenna are shown in Fig. 7(b)∼(d). It can be observed that the measured gain of the proposed antenna in the working band is from 6.5 dBi to 7.46 dBi, which is slightly less than the simulation one of 6.86 ∼8.4 dBi. It is attributed to the loss of the transmission cables. However, it’s worthwhile noting that the gain of the proposed antenna increased by up to 1 dB compared with the gain of the reference antenna due to the use of the dual-element array. The improvement of the radiation characteristics is that the surface wave is suppressed when the radiating patch is enclosed by the dual-element arrays.
Fig. 5. The proposed antenna with four dual-element arrays. (a) 3D view. (b) The top-view. The parameter of the proposed antenna: L10 = 60, L11 = 5.55, W7 = 60, W8 = 3.45, R1 = 11.3 (unit: mm).
Fig. 6. The structure and the experiment environment of the reference antenna and the proposed antenna with four dual-element arrays. (a) The structure of the reference and the proposed antenna. (b) The experiment schematic for radiation characteristics (c) The experiment environment for radiation characteristics. (d) The experiment schematic for scattering characteristics. (e) The experiment environment for scattering characteristics.
Fig. 7. Performance comparison of the reference antenna and the proposed antenna between the simulation and the measurement. (a) S parameter. (b) The gain. (c) The radiation patterns at 9.6 GHz in xoz plane. (d) The radiation patterns at 9.6 GHz in yoz plane.
The scattering characteristics of the reference antenna and the proposed antenna are also measured, as shown in Fig. 6(d) and (e). In the measurement of the scattering performance, the reference/proposed antenna is placed on a foam rotating platform 2.7 m away from the transmitting antenna (HD-100SGAH15N) and receiving antenna (HD-100SGAH15N). The transmitting and receiving antennas are mounted on the scanning stand and connected to the transmitting and receiving ports of the vector network analyzer (Agilent Technologies N5244A) by coaxial cables, respectively. Furthermore, a piece of absorbing material is placed between the two antennas in order to reduce the coupling between them. When the φ- and θ-polarized electromagnetic waves are normally incident, the monostatic RCS comparisons between the reference antenna and the proposed antenna are illustrated in Fig. 8. It can be seen that for φ-polarized incoming waves, the measured −10 dB RCS reduction bandwidth covers 9.25∼9.95 GHz in xoz plane and 9.2∼10 GHz in yoz plane, which are consistent with the simulated ones of 9.3∼10 GHz in xoz plane and 9.1∼9.9 GHz in yoz plane. Similarly, for θ-polarized incident waves, the measured −10 dB RCS reduction bandwidth is 9.15∼10 GHz in xoz plane and 9.2∼10 GHz in yoz plane, in good agreement with the simulated ones of 9.05∼9.9 GHz in xoz plane and 9.2∼9.95 GHz in yoz plane. In order to verify the angular stability of the proposed method, the bistatic RCSs of the reference antenna and the proposed antenna in the xoz plane and yoz plane are compared in Fig. 9, when the θ-polarized electromagnetic wave is incident. It can be seen that the maximum value of the bistatic RCS of the proposed antenna is smaller than that of the reference antenna in both xoz and yoz planes, as the incident angle θ varies from 0° to −60°. For θ=-60°, the RCS reduction above 7 dB can be still obtained. Extensive simulations and experiments show that the proposed design method can reduce the scattering characteristics of the antenna while improving the radiation characteristics of the antenna, and has very good angle stabilization characteristics, which provides a new solution to improve the stealth performance of platforms with radiation characteristics. The 3D bistatic RCSs of the reference antenna and the proposed antenna with θ-polarized incident waves in xoz and yoz planes are shown in Fig. 10. It can be seen that when the electromagnetic wave is irradiated to the proposed antenna at different incident angles, the intensity of the scattering wave becomes very small due to the absorption and field cancellation caused by the proposed array, thus achieving low RCS antenna design.
Fig. 8. The monostatic RCSs of the reference antenna and the proposed antenna with φ- and θ-polarized incident waves in xoz and yoz planes. (a) φ-polarized incident waves in xoz plane. (b) φ-polarized incident waves in yoz plane. (c) θ-polarized incident waves in xoz plane. (d) θ-polarized incident waves in yoz plane.
Fig. 9. The bistatic RCSs of the reference antenna and the proposed antenna at 9.6 GHz with θ-polarized incident waves in xoz and yoz planes. (a) θ-polarized incident waves in xoz plane. (b) θ-polarized incident waves in yoz plane.
Fig. 10. The 3D bistatic RCSs of the reference antenna and the proposed antenna at 9.6 GHz with θ-polarized incident waves in xoz and yoz planes.
3.2 Application of the RCS reduction metasurface in a compact size
The proposed dual-element array is employed to achieve a densely distributed RCS reduction metasurface array. By rotating and translating the dual-element arrays and arranging them closely, the proposed design method achieves angle-stable RCS reduction characteristics. The simulation models of two metasurface arrays are depicted in Fig. 11. The metasurface array obtained by translating the dual-element array is called array 1, as shown in Fig. 11(a) and (b), and the metasurface array obtained by rotating and translating the dual-element array is called array 2, as shown in Fig. 11(c) and (d). Similarly, the proposed metasurface arrays are also fabricated and measured. Differently, in order to improve the flexibility of the use of the dual-element arrays and reduce the fabrication costs, the dual-element arrays are first fabricated, as shown in Fig. 12(a). Then, the proposed metasurface arrays are manually assembled using the puzzle-like approach, as depicted in Fig. 12(b) and (c).
Fig. 11. The structures of the metasurface arrays for RCS reduction. (a) 3D view of the array 1. (b) Top view of the array 1. (c) 3D view of the array 2. (d) Top view of the array 2. The parameter of the proposed metasurface arrays: L12 = L15 = 60, L13 = 15, L14 = 9.95, W9 = W11 = 60, W10 = 1.05 (unit: mm).
Fig. 12. The dual-element array and two metasurface arrays for the RCS reduction. (a) The dual-element array. (b) The array 1. (c) The array 2.
Fig. 13. The monostatic RCSs of the reference metal plate, array 1 and array 2. (a) φ-polarized incident waves in xoz plane. (b) φ-polarized incident waves in yoz plane. (c) θ-polarized incident waves in xoz plane. (d) θ-polarized incident waves in yoz plane.
The monostatic RCSs of the proposed metasurface array 1 and array 2 are measured in the anechoic chamber and compared with the metal plate with the same size as the metasurface array, as illustrated in Fig. 13. The summarized results of the RCS reduction characteristics between the simulation and the measurement are summarized in Table 3. It can be seen that the proposed dual-element metasurface arrays exhibit the excellent RCS reduction characteristics for the incoming waves with arbitrary polarization. Meanwhile, the bistatic RCSs of the proposed metasurfaces are also simulated, as illustrated in Fig. 14. When irradiated by the φ-polarized electromagnetic waves in xoz and yoz planes, the bistatic RCSs of the proposed metasurface array 1 are shown in Fig. 14(a) and (b), from which it can be seen that the maximum value of the bistatic RCS of the metasurface array 1 is smaller than that of the metal plate with the same size. When the incident angle θ reaches −50° in xoz plane, the above 7 dB RCS reduction can be achieved. For the incident angle θ = -50° in yoz plane, the above 10 dB RCS reduction can be obtained. When irradiated by the θ-polarized electromagnetic waves in xoz and yoz planes, the bistatic RCSs of the proposed metasurface array 1 are given in Fig. 14(c) and (d), from which it can be seen that the maximum value of the bistatic RCS of the metasurface array 1 is smaller than that of the metal plate with the same size. When the incident angle θ reaches −30° in the xoz plane, the above 6 dB RCS reduction can be achieved, while the above 10 dB RCS reduction can be obtained at the incident angles of −30° in the yoz plane.
Fig. 14. The bistatic RCSs of the proposed metasurface array 1 and array 2 with different polarized waves at the different incident angles in xoz and yoz planes. (a) Array 1 with φ-polarized waves in xoz plane. (b) Array 1 with φ-polarized waves in yoz plane. (c) Array 1 with θ-polarized waves in xoz plane. (d) Array 1 with θ-polarized waves in yoz plane. (e) Array 2 with φ-polarized waves in xoz plane. (f) Array 2 with φ-polarized waves in yoz plane. (g) Array 2 with θ-polarized waves in xoz plane. (h) Array 2 with θ-polarized waves in yoz plane.
Table 3. Simulated and Measured −10 dB Monostatic RCS Reduction Bandwidths of Array1, and Array 2
On the other hand, the bistatic RCSs of the proposed metasurface array 2 for φ-polarized waves in xoz and yoz planes are given in Fig. 14(e) and (f), from which it can be seen that the maximum value of the bistatic RCS of the metasurface array 2 is smaller than that of the metal plate with the same size. When the incident angle θ reaches −50° in both xoz planes and yoz planes, the above 8 dB RCS reduction can be achieved. When irradiated by the θ-polarized electromagnetic waves in xoz and yoz planes, the bistatic RCSs of the proposed metasurface array 2 are shown in Fig. 14(g) and (h), from which it can be seen that the maximum value of the bistatic RCS of the metasurface array 2 is smaller than that of the metal plate with the same size. When the incident angle θ reaches −50° in both xoz plane and yoz plane, the above 8 dB RCS reduction can be achieved. The 3D bistatic RCSs of the proposed metasurface array 1 and array 2 with φ-polarized waves at the different incident angles in xoz and yoz planes are shown in Fig. 15. It can be seen that by tightly arranging the dual-element array to construct metasurface arrays, the scattering waves are greatly reduced for the incident electromagnetic waves over a wide angle. As a result, the proposed dual-element metasurface array can be utilized to design tightly-packed RCS metasurface arrays with excellent dual-polarization characteristics and wide angular stability.
Fig. 15. The 3D bistatic RCSs of the proposed metasurface array 1 and array 2 with φ-polarized waves at the different incident angles in xoz and yoz planes.
It is worth mentioning that the dual-element array can also be arranged in other desired configuration to construct a tightly-packed metasurface array, while still maintaining the low RCS characteristics. Here we used the dual-element array to design two closely arranged RCS reduction metasurface arrays different from array 1 and array 2, which is called array 3 and array 4, as shown in Fig. 16. Figure 17 presents their RCS scattering characteristics. It can be seen that when the φ-polarized electromagnetic wave is incident along the xoz plane, the 10 dB RCS reduction bandwidths of array 3 and array 4 are 9.3∼9.9 GHz, and 9.25∼9.9 GHz, respectively. When the φ-polarized electromagnetic wave is incident along the yoz plane, the −10 dB RCS reduction bandwidths of array 3 and array 4 are 9.0∼9.8 GHz, and 9.0∼9.9 GHz, respectively. For the θ-polarized incident wave in the xoz plane, the −10 dB RCS reduction bandwidths of array 3 and array 4 are 9.0∼9.75 GHz and 9.0∼9.65 GHz, respectively. For the θ-polarized incoming wave in the yoz plane, the −10 dB RCS reduction bandwidths of array 3 and array 4 are 9.0∼9.85 GHz and 9.2∼9.7 GHz, respectively. It can be found that different tightly packed layouts can result in good RCS reduction performance.
Fig. 16. The structures of the metasurface array 3 and array 4 for RCS reduction. (a) 3D view of the array 3. (b) Top view of the array 3. (c) 3D view of the array 4. (d) Top view of the array 4.
Fig. 17. The simulated monostatic RCSs of the metasurface array 3 and array 4 for RCS reduction. (a) φ-polarized incident waves in xoz plane. (b) φ-polarized incident waves in yoz plane. (c) θ-polarized incident waves in xoz plane. (d) θ-polarized incident waves in yoz plane.
Fig. 18. Three randomly distributed metasuface arrays based on the dual-element antennas. (a) The array 5. (b) The array 6. (c) The array 7.
3.3 Application of the RCS reduction metasurface in a randomly distributed way
Finally, the proposed dual-element arrays are used to construct the metasurface arrays by distributing them randomly on metal-grounded thin dielectric substrate for the RCS reduction. This random arrangement method provides a new degree of freedom for the use of dual-element arrays, offering a richer range of applications for the proposed dual-element arrays.
To verify the RCS reduction characteristics of the proposed dual-element array-based randomly distributed metasurface arrays, the metasurfaces are composed of a certain number of randomly distributed dual-element arrays on a finite printed circuit board of 150 × 150 mm2. Three randomly placed metasurface arrays with different numbers of the dual-element antennas are considered. The metasurface array composed of 22 dual-element arrays is called array 5, as shown in Fig. 18(a), the metasurface array composed of 20 dual-element arrays is called array 6, as shown in Fig. 18(b), and the metasurface array composed of 18 dual-element arrays is called array 7, as shown in Fig. 18(c). Their RCS reduction characteristics are measured in the microwave anechoic chamber, and the corresponding experiment environment is as shown in Fig. 19. Figure 20 shows the monostatic RCSs of the array 5, the array 6, and the array 7 for the φ- and θ-polarized normally incident electromagnetic waves in xoz plane. It can be seen that, compared to the metal plate with the same size, the −10 dB RCS reduction bandwidths of the array 5 are 9.3∼9.95 GHz and 9.25∼9.8 GHz for the φ- and θ-polarized incident waves, respectively. And the −10 dB RCS reduction bandwidths of the array 6 are 9.2∼9.9 GHz and 9.2∼9.8 GHz for the φ- and θ-polarized incident waves, respectively. Although the array 5 has more dual-element antennas than the array 6, they have similar RCS performance. This is because the array 6 well distributes on the conducting plate in the random arrangement, and thus good RCS reduction has been achieved by the array 6. Introduction of more dual-element antennas cannot ensure the further improvement of the RCS reduction. Therefore, the effective random distribution of the dual-element antennas is essential for the RCS reduction. On the other hand, the RCS reduction performance of array 7 becomes worse than those of the array 5 and the array 6 due to with the least number of the dual-element antennas. Therefore, the number of dual-element arrays in the random arrangement is determined based on two criterions. One is to ensure the resulting randomly placed metasurface array to effectively cover the metallic platform for good RCS reduction. The second is to choose the number of dual-element antennas in the random arrangement less than those in the tightly packed array as small as possible for the simpler deployment. These experiment results verify it as a simple and efficient method to use the dual-element antennas to build the RCS reduction metasurface arrays. The design principle be potentially applied to the stealth target on a large scale or the application scenarios that need to be rapidly deployed.
Fig. 20. The monostatic RCS of the randomly distributed metasurface arrays in xoz plane. (a) φ-polarized incident waves. (b) θ-polarized incident waves.
Finally, the performance comparison between the proposed metasurface array designs and the newly reported designs is given in Table 4. Compared with the published designs, the proposed dual-element arrays can be applied to a variety of RCS reduction scenarios with very good angular stability and polarization insensitivity, fewer number of elements, and more flexibility in usage. Therefore, the proposed design method can further promote the application of electromagnetic metamaterials in the stealth field.
Table 4. Performance Comparison Between Proposed and Reported Designs
4. Conclusion
The paper presents a novel design method for reducing the RCS of the target in multiple application scenarios with the wide-angle and polarization insensitivity by using an antenna array-based metasurface. The design involves arranging dual-polarized slot-coupled antennas in a mirror configuration, connecting the antenna ports with Wilkinson power dividers, and integrating them through a microstrip line with a lumped resistor. This enables the absorption of incoming waves with arbitrary polarization and cancellation of overflowing waves from the antenna ports. Three application scenarios for RCS reduction have been conducted, including an antenna array integrated with a microstrip antenna, densely arranged metasurface arrays, and randomly distributed metasurface arrays. The proposed method has been verified through simulations and measurements. Compared to previous studies, this proposed design approach simplifies fabrication and provides flexible enough to achieve the low detectability in versatile scenarios.
Funding
National Natural Science Foundation of China (62371355); National Key Research and Development Program of China (2021YFA1401001).
Disclosures
The authors declare no conflicts of interest.
Data availability
The data that support the findings of this study are available on request from the corresponding author.
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