Bistatic Radar Cross Section Reduction Research Articles

A Dynamically Phase Tunable Metasurface for a Broad Bandwidth Ultra-Low Radar Cross Section

An electrically tunable metasurface is designed for radar cross-section (RCS) reduction application in this paper. It consists of 24 × 24 cells individually loaded with variable capacitors so that their phase profiles can be independently controlled by the bias voltage. Therefore, based on the diffusive scattering principle and reflect-array theory, we arrange the adaptive phase differences between adjacent basic elements and reconfigure the appropriate phase distribution determined by a genetic algorithm (GA) corresponding to the operation frequency. Thus, stable monostatic and bistatic RCS reduction with normal incidence can be realized over a wide band by independently tuning the capacitance of each loaded varactor. Simulation results verify that the bistatic RCS reduction can reach at least 10 dBSm within both 4.2-7.3 GHz and 9.3-11.3 GHz. Simultaneously, the monostatic RCS reduction reaches at least 17.5 dBSm and 14 dBSm within the above two bands, respectively.

Multiple Diffuse Coding Metasurface of Independent Polarization for RCS Reduction

In this article, a multiple diffuse coding metasurface (MDCM) of independent polarization is designed to control the propagation direction of diffuse reflections under different polarizations and to improve the monostatic and bistatic RCS (radar cross section) reduction effect. First, a method for controlling the distribution range and propagation direction of the diffuse field is studied, and the diffuse field distribution of the random phase metasurface is optimized by a genetic algorithm to improve the uniformity of the diffuse scattering distribution. Then, the random phase distribution is superimposed on the periodic gradient phase distributions of the linear and hedge types in the orthogonal direction so that the main propagation direction of the diffuse metasurface deviates from the specular reflection region under different polarizations, showing single and two diffuse beams. Finally, the anisotropic unit cell with a rectangle inside and an improved Jerusalem cross on the outside is employed as the basic coding element of the MDCM due to its independent polarization phase response. The numerical and experimental results show that the MDCM features multiple diffuse scattering, independent polarization and angle insensitivity and can efficiently improve the monostatic and bistatic RCS reduction effect simultaneously. Because the scattered energies are redirected away from the specular reflection direction, the specular scattering reduction effect is better than the isotropic diffuse metasurface. The proposed method increases the difficulty of detection by single or netted radar and has the potential for the applications of stealth techniques.

Ultra‐wideband Low‐Detectable Coding Metasurface

A coding metasurface is designed based on hybrid Array pattern synthesis (APS) and Particle swarm optimization (PSO) method for ultra-wideband low-detectable application. The metasurface is composed of Electromagnetic band-gap (EBG) structures of two Minkowski fractal elements with reflection phase difference of 180° (±37°) over a wide frequency range. Two different types of EBG unit cell printed on a thin grounded dielectric substrate produce reflection phase difference of about 180 degrees over a wide frequency range. Ultra-wideband Radar cross section (RCS) reduction results from the phase cancellation between two local waves produced by these two unit cells. The diffuse scattering of EM waves is caused by the optimization of phase distribution, leading to a low monostatic and bistatic RCS simultaneously. The proposed metasurface can achieve 10 dB monostatic and bistatic RCS reduction in a wide frequency band from 5.8 to 18.0 GHz with a ratio bandwidth (fH/fL) of 3.10:1 under normal incidence for both polarizations. The theoretical analysis, simulation and experiment results are in good agreement and validate the proposed metasurface can achieve ultra-wideband RCS reduction and diffuse scattering.

Ultrawideband Monostatic and Bistatic RCS Reductions for Both Copolarization and Cross Polarization Based on Polarization Conversion and Destructive Interference

Based on polarization conversion and destructive interference, a novel monolayered metasurface is proposed for ultrawideband monostatic and bistatic radar cross section (RCS) reductions for both copolarization and cross polarization. Unit cells share a simple asymmetric double arrow shape, but with variable geometry parameters, correspondingly with different polarization conversion capabilities and different reflection coefficients. The particle swarm optimization (PSO) algorithm and array theory are combined to optimize and select appropriate parameters for unit cells. The optimized unit cells (O-cells) simultaneously realize the polarization conversion and destructive interference to reduce copolarized RCS. In addition, mirrors of these O-cells (M-cells) are adopted to achieve the complete destructive interference to reduce cross-polarized RCS. Therefore, the total monostatic RCS can be greatly reduced. To improve the bistatic RCS reduction, random distribution of unit cells is carefully designed to realize diffusion patterns. In both the simulation and measurement results, a 10 dB monostatic RCS reduction at the normal incidence is achieved from 7.5 to 22.5 GHz (100% bandwidth). Furthermore, good specular and bistatic RCS reduction performances under oblique incident waves are also obtained. The proposed methodology opens up a new route for ultrawideband RCS reduction.

Scatterer Surface Design for Wave Scattering Application

In this paper, radio wave scattering is considered and an optimal scatterer surface is realized. For this purpose, an ultrawideband and high-efficiency polarization converter unit cell is proposed. Here, the proposed polarization converter unit cell and its mirror are combined to provide 180° phase difference for scattering purposes. The desired combination of unit cells which produces uniform scattering pattern and maximum wave attenuation achieved employing group search optimization (GSO) algorithm. For the problem at hand, the GSO optimization algorithm is presented in bilevel format including master and slave levels. Simulation and experimental results showed that the 10 dB bandwidth of normalized radar cross section (RCS) reduction is achieved in ultrawideband ranging from 5.5 to 20.51 GHz with a fractional bandwidth of 115%, assuring a favorable performance of the proposed method. In addition, a wide-angle bistatic RCS reduction over a wide frequency band is attained.

A Novel Checkerboard Metasurface Based on Optimized Multielement Phase Cancellation for Superwideband RCS Reduction

In this paper, a checkerboard metasurface based on a novel physical mechanism, optimized multielement phase cancellation, is proposed for greatly expanding the bandwidth of radar cross section (RCS) reduction. More basic metaparticles and, in particular, the variable phase difference between them, greatly increase the ability to control electromagnetic waves. Interactions between multiple local waves produced by the basic metaparticles at multiple frequencies sampled in a superwide frequency band are manipulated and optimized simultaneously to achieve phase cancellation. The proposed metasurface can achieve a 10 dB RCS reduction in a superwide frequency band from 5.5 to 32.3 GHz with a ratio bandwidth ( ${f}_{H}/{f}_{L}$ ) of 5.87:1 under normal incidence for both polarizations. Furthermore, the RCS reduction is larger than 8 dB from 5.4 to 40 GHz with a ratio bandwidth of 7.4:1. The metasurface also has a good performance under wide-angle oblique incidences. The optimal metaparticle distribution is found to obtain the superwideband bistatic RCS reduction. The theoretical analysis, simulation, and experimental results are in good agreement and verify the ability and capability of the proposed mechanism.

Fast coding method of metasurfaces based on 1D coding in orthogonal directions

Fast coding method of metasurfaces, based on 1D coding in orthogonal directions, is proposed in this paper. For the previous designed coding metasurfaces with M × N coding sequence, all the M × N codes need to be optimized as a whole, so that the optimized efficiency and flexibility are limited. In this regard, the coding metasurfaces are achieved via only designing the 1D coding sequences in orthogonal directions, and the coding sequences in the 2D plane can be derived by a binary addition algorithm of the two coding sequences. That means, for the M × N coding sequence, the number of coding elements that need to be optimized will be reduced from M × N to M + N. Thus, a faster optimizing method for coding metasurface design will be realized. To validate the method, 16 × 16 coding metasurfaces were designed by optimizing only two 1 × 16 random coding sequences. Both the simulation and experimental results of the designed coding metasurface demonstrated the coding metasurface design method and their excellent performance on mono-static and bi-static radar cross section reduction, which provides a more efficient coding method that allows more flexible manipulation of EM waves.

An Optimized Checkerboard Structure for Cross-Section Reduction: Producing a Coating Surface for Bistatic Radar Using the Equivalent Electric Circuit Model

In this article, checkerboard metallic surfaces are analytically studied for nonabsorptive radar cross-section (RCS) reduction. The patch periodic elements are modeled with the equivalent electric circuit. The circuit model is exploited to achieve an improved reflection phase from the unit cell. By employing the phase difference from the edges of two frequencyselective surfaces (FSSs), a destructive interference takes place. In the wavescattering scenario, a fitness function is derived for RCS reduction (RCSR). The optimum reflection phase for an incident wave frequency interval is obtained with the help of circuit model analysis. A genetic algorithm (GA) is used for reflection phase optimization. An optimum checkerboard is designed based on the analytical and optimization methods. The checkerboard is constructed with two different patch widths as the optimum board to realize a phase difference condition in two dimensions. The designed board is numerically studied for both polarizations, bistatic RCS, and an incident angle span. After evaluating the numerical solution, the designed checkerboard surface is fabricated. Experimental observation validates the analytical results. The studied board uses a wellknown periodic surface and has a concrete analytical basis, optimum design procedure, bistatic RCSR, and far-field investigation.

Circular configuration of perforated dielectrics for ultra‐broadband, wide‐angle, and polarisation‐insensitive monostatic/bistatic RCS reduction

Here, a new strategy of ultra-broadband monostatic and bistatic radar cross-section reduction (RCSR) was theoretically investigated. A circular configuration of perforated dielectrics, as the coating layer, was designed to have full control on the bistatic RCS signatures of flat metallic objects. Using the proposed phase cancellation method, a customised level of bistatic RCSR (BRR) as well as operating frequency bandwidth were interpretively achieved. The presented study offered a robust and straightforward RCSR technique which does not rely on conventional blind optimisations. The coating layer was capable of reducing the monostatic and BRR of >10 dB in a broad frequency range from 8.3 to 23.1 GHz (BW = 94%). The results were significantly improved compared to those of preceding researches. Besides, unlike the preceding researches, the performance of the designed coating layer was analytically justified under TE and TM polarisations of oblique incidences. A good agreement was observed between numerical simulations and theoretical predictions, confirming the polarisation-independent feature of the designed stealth coating layer for incident angles up to 60 ∘ . These superior performances and the flexibility of design guarantee the applicability of the proposed RCSR approach for various stealth applications.

FDTD simulation of radar cross section reduction by a collisional inhomogeneous magnetized plasma

The recursive convolution finite difference time domain method is addressed in the scattered field formulation and employed to investigate the bistatic radar cross-section (RCS) of a square conductive plate covered by a collisional inhomogeneous magnetized plasma. The RCS is calculated for two different configurations of the magnetic field, i.e., parallel and perpendicular to the plate. The results of numerical simulations show that, for a perpendicularly applied magnetic field, the backscattered RCS is significantly reduced when the magnetic field intensity coincides with the value corresponding to the electron cyclotron resonance. By increasing the collision frequency, the resonant absorption is suppressed, but due to enhanced wave penetration and bending, the reduction in the bistatic RCS is improved. At very high collision frequencies, the external magnetic field has no significant impact on the bistatic RCS reduction. Application of a parallel magnetic field has an adverse effect near the electron cyclotron resonance and results in a large and asymmetric RCS profile. But, the problem is resolved by increasing the magnetic field and/or the collision frequency. By choosing proper values of the collision frequency and the magnetic field intensity, a perpendicular magnetic field can be effectively used to reduce the bistatic RCS of a conductive plate.

Absorptive coding metasurface for further radar cross section reduction

Lossless coding metasurfaces and metamaterial absorbers have been widely used for radar cross section (RCS) reduction and stealth applications, which merely depend on redirecting electromagnetic wave energy into various oblique angles or absorbing electromagnetic energy, respectively. Here, an absorptive coding metasurface capable of both the flexible manipulation of backward scattering and further wideband bistatic RCS reduction is proposed. The original idea is carried out by utilizing absorptive elements, such as metamaterial absorbers, to establish a coding metasurface. We establish an analytical connection between an arbitrary absorptive coding metasurface arrangement of both the amplitude and phase and its far-field pattern. Then, as an example, an absorptive coding metasurface is demonstrated as a nonperiodic metamaterial absorber, which indicates an expected better performance of RCS reduction than the traditional lossless coding metasurface and periodic metamaterial-absorber. Both theoretical analysis and full-wave simulation results show good accordance with the experiment.

A Wideband and Polarization-Independent Metasurface Based on Phase Optimization for Monostatic and Bistatic Radar Cross Section Reduction

A broadband and polarization-independent metasurface is analyzed and designed for both monostatic and bistatic radar cross section (RCS) reduction in this paper. Metasurfaces are composed of two types of electromagnetic band-gap (EBG) lattice, which is a subarray with “0” or “π” phase responses, arranged in periodic and aperiodic fashions. A new mechanism is proposed for manipulating electromagnetic (EM) scattering and realizing the best reduction of monostatic and bistatic RCS by redirecting EM energy to more directions through controlling the wavefront of EM wave reflected from the metasurface. Scattering characteristics of two kinds of metasurfaces, periodic arrangement and optimized phase layout, are studied in detail. Optimizing phase layout through particle swarm optimization (PSO) together with far field pattern prediction can produce a lot of scattering lobes, leading to a great reduction of bistatic RCS. For the designed metasurface based on optimal phase layout, a bandwidth of more than 80% is achieved at the normal incidence for the −9.5 dB RCS reduction for both monostatic and bistatic. Bistatic RCS reduction at frequency points with exactly 180° phase difference reaches 17.6 dB. Both TE and TM polarizations for oblique incidence are considered. The measured results are in good agreement with the corresponding simulations.

Wideband, Wide Angle, Polarization Independent RCS Reduction Using Nonabsorptive Miniaturized-Element Frequency Selective Surfaces

A thin, wideband, nonabsorptive radar cross section (RCS) reducer (NARR) layer capable of operation over a wide range of incident angles and for any arbitrary incident wave polarization is presented. The design is based on implementation of two different types of subwavelength miniaturized-element frequency selective surfaces (MEFSS) backed by a thin grounded dielectric substrate. Two MEFSS structures are designed in such a way to produce reflection phase values with a difference of about 180° (±30°) over a wide frequency range. Segments of these surfaces are arranged in periodic and aperiodic fashions to distribute the scattered energy and minimize the maximum bistatic RCS of the underlying metallic surface over a wide range of incident angles at both polarizations. It is shown that the aperiodic NARR structure has 3-dB lower maximum bistatic RCS compared to the periodic one. The -6 dB bistatic RCS reduction bandwidth of 66% is achieved for both periodic and aperiodic NARR surfaces at the normal incidence. The monostatic RCS reduction level at normal incidence is the same for both structures, but the aperiodic NARR layer is shown to provide 13% higher -10 dB bandwidth. In addition, the performance of both structures for oblique angle of incidence up to 50 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> is examined and is shown that the aperiodic NARR layer shows better performance in suppressing the RCS sidelobes. As a partial experimental validation, two prototypes of periodic and aperiodic NARR layers are fabricated and their monostatic RCS reduction are characterized experimentally. The measured results are in good agreement with the corresponding simulations.

A broad-band gain improvement and wide-band, wide-angle low radar cross section microstrip antenna

A novel high-gain and low radar cross section (RCS) microstrip antenna is designed and fabricated. The proposed antenna obtained broad-band 3 dB gain bandwidth and wide-band, wide-angle low RCS properties after applying the frequency selective surface (FSS) as a superstrate of original microstrip antenna. The FSS cell is composed of two metallic layers separated by a dielectric substrate. A metallic square loop with four resistors mounted on each side of the loop is enched on the top layer and a metallic plane with a central cross slot and four fringe slots is enched on the bottom layer. The four resistors of top layer are mainly used to absorb radar incoming wave and reduce antenna RCS. The patch of bottom layer can constructe a Fabry-Perot resonance cavity with ground plane and improve the antenna gain. The reflection coefficient S22 and transmission coefficient S12 of top layer are all below -10 dB at 5.75-11.37 GHz. The reflection phase gradient of bottom layer is positive and the reflection magnitude value is above 0.86 from 11.21 GHz to 11.54 GHz. Measurement results show that the antenna gain is enhanced by about 3.4 dB at 11.73 GHz, and the half-power beam width of E-plane and H-plane is reduced 16° and 50° respectively. The 3 dB gain bandwidth is about 2.4 GHz which from 10.0 GHz to 12.4 GHz and well cover the impedance bandwidth. The proposed antenan achieved an RCS reduction of more than 3 dB in the normal direction at 4.10-11.30 GHz, the largest reduction reached 23.08 dB in comparison with the original antenna. The monostatic and bistatic RCS reduction are over 3 dB from -20° to 20° and -37° to 37° respectively at 4.95 GHz. The results proved the FSS superstrate can be applied to improve the radiation and scattering performance simultaneously.

Design of low-radar cross section microstrip antenna based on metamaterial absorber

A metamaterial absorber with high absorptivity, wide incident angle and no surface ullage layer is designed and applied to microstrip antenna to reduce its radar cross section (RCS). The results show that the absorber can exhibit an absorption of 99.9% with a thickness of 0.3 mm. Compared with the conventional microstrip antenna, the proposed antenna has an RCS reduction of more than 3 dB in the boresight direction in the working frequency band, and the largest reduction can reach 16.7 dB, the monostatic and bistatic RCS reduction are over 3 dB from -30° to +30° and -90° to +90° respectively, while the radiation performance is kept, which proves that the absorber has an excellent absorptivity and could be applied to microstrip antennas to achieve in-band stealth.