A Co-Polarization Broadband Radar Absorber for RCS Reduction

Ultra-Wideband Electromagnetic Composite Absorber Based on Pixelated Metasurface with Optimization Algorithm

An ultra-wideband electromagnetic (EM) absorber is proposed. The proposed absorber consists of two thin metasurfaces, four dielectric layers, a glass fiber reinforced polymer (GFRP), and a carbon fiber reinforced polymer (CFRP) which works as a conductive reflector. The thin metasurfaces are accomplished with 1-bit pixelated patterns and optimized by a genetic algorithm. Composite materials of GFRP and CFRP are incorporated to improve the durability of the proposed absorber. From the full-wave simulation, more than 90% absorption rate bandwidth is computed from 2.2 to 18 GHz such that the fractional bandwidth is about 156% for the incidence angles from 0° to 30°. Absorptivity is measured using the Naval Research Laboratory (NRL) arch method in an EM anechoic environment. It was shown that the measured results correlated with the simulated results. In addition, the proposed absorber underwent high temperature and humidity tests under military environment test conditions in order to investigate its durability.

Low-frequency magnetic microwave absorber using reactive ground for extended bandwidth

A design method to extend the operating bandwidth of a low-frequency magnetic microwave absorber is presented. According to the mapped impedance distribution from the design objective, a synthesized reactive ground consisting of a triple-ring array, instead of the perfect electric conductor (PEC), is conveniently placed at the back of a commercially available magnetic sheet to realize impedance matching. Compared with the conventional PEC, the reactive ground extends the operating bandwidth of reflection coefficient being less than −10 dB by introducing four consecutive absorption peaks so that the optimized structure achieves broadband absorption from 2.3 to 4.3 GHz with a thickness of 0.027 times the free-space wavelength at the lowest operating frequency and a 60.6% fractional bandwidth which is nearly two times that of the conventional structure. The physical mechanism is further investigated through the distributions of current flow and power loss. Finally, a prototype is fabricated and measured. The good agreement between the simulated and measured results verifies the effectiveness of the presented design method.

Additively manufactured metastructure design for broadband radar absorption

Background

Recent advances in material science and electronics led to the rapid development of communication devices and radar detection techniques resulting in an ever-increasing demand for improved stealth performance of air vehicles during scouts. Absorber design employing metastructure concept has recently become a popular approach to improving radar stealth performance. Metastructure permits the realization of desired absorption characteristics by careful design of geometrical structures and material compositions. In this study, a metastructure designed based on graphite SLS composite for radar absorption has been demonstrated. The unit cell of the proposed structure is simulated by COMSOL Multiphysics to determine the frequency-dependent absorption characteristic of the structure. It is fabricated by using a low-cost selective laser sintering technique of additive manufacturing technology.

Results

The prototype, while measured, shows effective absorption bandwidth of 1.04 GHz that is in reasonable agreement with the simulated response of 2.08 GHz. The optimized structure exhibits ≤ − 10 dB reflectivity within a broad frequency range extending from 7.60 GHz to 18.00 GHz under normal incidence in both TE and TM polarizations. Furthermore, the absorption performance under different polarizations and incident angles has been investigated. Results indicate that the absorber displays polarization indifference and exhibits a wide-angle of incidence tolerance of up to 45° in TE polarization and 30° in TM polarizations.

Conclusion

In this paper, the feasibility of using graphite SLS material to design and 3D print a metastructure design for radar absorbing has been established as confirmed by the simulation and the measurement results. The advantages of low cost, ultra-broad operating band, wide-angle of incidence feature, and polarization insensitivity qualifies the proposed absorber for stealth and electromagnetic shielding applications.

Alignment-insensitive bilayer THz metasurface absorbers exceeding 100% bandwidth

Metamaterial absorbers have been a topic of considerable interest in recent years, with a particular focus on Terahertz (THz) frequencies due to many natural materials having a weak interaction with THz light. Great efforts have aimed to expand such THz absorbers to cover a wide bandwidth whilst also being highly efficient. However, many of these require cascaded or stacked multilayer resonant elements, where even a small deviation in the alignment between layers is extremely detrimental to the performance. Here, we propose a bilayer metasurface absorber (thickness ∼ λ/6) that is immune to such layer misalignments capable of exceeding a fractional bandwidth (FWHM) of 100% of the central frequency. The design works due to a novel absorption mechanism based on Salisbury Screen and anti-reflection absorption mechanisms, using fractal cross absorbers to expand the bandwidth. Our work is of particular benefit to developing devices which require ultra-wide bandwidth, such as bolometric sensing and planar blackbody absorbers, with the extremely robust absorption responses being unaffected by any misalignments between layers - a limiting factor of previous absorbers.

Multi-polarization, polarization-independent, wide-angle RCS reduction metasurface based on random phase gradients

A novel metasurface based on random phase gradients is proposed for radar cross-section (RCS) reduction. In this work, wideband, polarization-independent, wide-angle RCS reduction is realized for both circularly polarized (CP) waves and linearly polarized (LP) waves, respectively. Thus, true polarization-independent RCS reduction is realized by the proposed metasurface. This proposed metasurface is composed of different types of units, and these units do not have certain periods. Under both CP incidence and LP incidence, random phase gradients can be formed on the proposed metasurface. The incidence can be diffused because of these random phase gradients, resulting in multi-polarization, polarization-independent, wide-angle RCS reduction. The 10 dB RCS reduction ranges from 12.6 GHz to 17.0 GHz and 18 GHz to 22 GHz for right-hand circularly polarized incident waves, and from 12.4 GHz to 17.0 GHz and 18.0 GHz to 21.8 GHz for left-hand circularly polarized incident waves. Meanwhile, the 10 dB RCS reduction ranges from 12.0 GHz to 17.0 GHz for x-polarized incident waves and from 13.0 GHz to 17.0 GHz and 17.6 to 21.8 GHz for y-polarized incident waves. Both the simulation and experimental results verify the value of this proposed metasurface in stealth technology.