Radar Wave Band Research Articles

Ultra-wideband, optically transparent, and flexible microwave metasurface absorber

In this work, an ultra-wideband microwave metasurface absorber with optically transparent and flexible properties is proposed. The metasurface is composed of a reflective backplane and a microwave absorption layer sandwiched between two dielectric substrates. The impedance matching curves of the microwave absorption layer are deduced based on the impedance matching theory, which is quite helpful and useful to improve the accuracy and efficiency of the broadband optimization design. Simulated results show that absorption higher than 90% can be achieved in the frequency band ranging from 5.8 GHz to 27.3 GHz, which covers the radar wavebands of C, X, Ku and K. The relative bandwidth reaches up to 130%, thus realizing ultra-wideband absorption while the thickness of the metasurface is only 0.12 times the upper-cutoff wavelength. For the TE (transverse electric) wave incidence, the metasurface maintains good performance when incident angle θ ≤ 50°, while for the TM (transverse magnetic) wave incidence, the absorption higher than 90% can be still achieved in a broad frequency band when θ ≤ 60°. It can be seen that using double-layer dielectric substrate in the metasurface not only greatly expands the microwave absorption bandwidth, but also improves the oblique incident properties. In addition, the metasurface is insensitive to polarization since its unit cell is symmetrical. Moreover, by rationally designing materials, the metasurface in this work is optically transparent and flexible, thus quite suitable for window radar stealth and equipment conformal stealth.

A transparent and flexible metasurface with both low infrared emission and broadband microwave absorption

Researches on radar and infrared stealth compatibility have drawn much attention in recent years. In this work, a flexible metasurface with low infrared emission, broadband microwave absorption as well as high optical transmission is simultaneously achieved. The whole structure is composed of an infrared shielding layer (IRSL), a radar absorption layer (RAL), a substrate and a backplane, and the total thickness is only 3.5 mm. Based on the impedance matching theory, the microwave absorption higher than 90% can be achieved in the radar waveband ranging from 7.3 to 18.8 GHz, corresponding to a relative bandwidth of 88.1%. By using a conductive patch array as the IRSL, a low infrared emissivity of 0.49 can be realized in the infrared region from 8 to 14 μm. Moreover, by rational designing both the structures and materials, the metasurface in this work cannot only achieve bi-stealth functions with low infrared emission and broad microwave absorption, but also shows optically transparent and flexible properties, thus quite suitable for practical applications. Both the simulated and experimental results suggest that the proposed metasurface is promising in the multispectral stealth fields.

Hybrid Metasurfaces for Infrared-Multiband Radar Stealth-Compatible Materials Applications

The compatible stealth functionality in the infrared (IR) and radar wave bands is the most important research topic in the field of stealth material technology. Here, a new hybrid metasurface (HMS) for infrared-multiband radar stealth-compatible materials was proposed and studied. Two specifically designed metasurface layers that can control the infrared emission and microwave absorption were combined to realize radar and IR bi-stealth. The simulated and experimental results show that the HMS has five strong absorption peaks at f 1 = 6.35, f 2 = 8.38, f 3 = 12.10, f 4 = 15.37 and f 5 = 18.05 GHz. In addition, the emissivity of the proposed HMS is less than 0.32 from 3 to 14 μm and shows low emissivity characteristics in the infrared band. These results demonstrate that the proposal has practical application to multispectral stealth technology.

Galvanic displacement synthesis of Al/Ni core–shell pigments and their low infrared emissivity application

We have successfully developed a magnetic Al/Ni core–shell pigment via a galvanic displacement reaction to obtain low infrared emissivity pigment with low lightness and visible light reflectance. Al/Ni core–shell particles were prepared via a simple one-step synthetic method where Ni was deposited onto the Al surface at the expense of Al atoms. The influence of pH and the amount of NH4F complexing agent on phase structure, surface morphology, optical and magnetic properties were studied systematically. The neutral condition and high concentration of NH4F forms smooth, flat, uniform and dense Ni shell on the surface of flake Al particles, which can significantly reduce the lightness and visible light reflectance but slightly increase the infrared emissivity. When the core–shell pigments are prepared in neutral pH solution at NH4F = 11.2 g/L, the lightness (L*) and visual light reflectivity can be reduced by 12.6 and 0.46, respectively versus uncoated flake Al pigments, but the infrared emissivity is only increased by 0.02. The color changes from brilliant silver to gray black and the saturation magnetization value is 6.59 emu/g. Therefore, these Al/Ni magnetic composite pigments can be used as a novel low infrared emissivity pigment to improve the multispectral stealth performance of low-E coatings in the visual, IR and Radar wavebands.

Design and realization of one-dimensional double hetero-structure photonic crystals for infrared-radar stealth-compatible materials applications

In this paper, a new type one-dimensional (1D) double hetero-structure composite photonic crystal (CPC) for infrared-radar stealth-compatible materials applications was proposed and studied numerically and experimentally. First, based on transfer matrix method of thin-film optical theory, the propagation characteristics of the proposed structure comprising a stack of different alternating micrometer-thick layers of germanium and zinc sulfide were investigated numerically. Calculation results exhibit that this 1D single hetero-structure PC could achieve a flat high reflectivity gradually with increasing the number of the alternating media layers in a single broadband range. Then, based on principles of distributed Bragg reflector micro-cavity, a 1D double hetero-structure CPC comprising four PCs with thickness of 0.797 μm, 0.592 μm, 1.480 μm, and 2.114 μm, respectively, was proposed. Calculation results exhibit that this CPC could achieve a high reflectance of greater than 0.99 in the wavelength ranges of 3–5 μm and 8–14 μm and agreed well with experiment. Further experiments exhibit that the infrared emissivity of the proposed CPC is as low as 0.073 and 0.042 in the wavelength ranges of 3–5 μm and 8–12 μm, respectively. In addition, the proposed CPC can be used to construct infrared-radar stealth-compatible materials due to its high transmittance in radar wave band.

Optical and magnetic properties of Al/Fe3O4 core–shell low infrared emissivity pigments

Abstract A core–shell Al/Fe3O4 magnetic composite pigment has been prepared by solvothermal method in order to obtain low infrared emissivity pigment with low lightness and visible reflectance. The influence of the shell layer thickness, surface roughness and radiation wavelength on spectral reflectivity of Al/Fe3O4 composite pigment is systematically researched by geometric optics theory and rough surface theory. The Al/Fe3O4 composite pigment with low visible reflectance and high infrared reflectance can be designed by thin thickness and high surface roughness of the shell layer. Then, Al/Fe3O4 composite pigments with different covering content of Fe3O4 are prepared by adjusting the molar ratio of Fe3+:Al. The phase structure, surface morphology, reflectance spectra and magnetic hysteresis loop of samples are characterized by XRD, FE-SEM, UV/VIS/NIR spectroscopy, Fourier transform infrared spectrometer and VSM. The result shows that the surface covering content rises with the increase of Fe3+:Al, which significantly reduces the VIS reflectance and lightness of flake Al pigment but only slightly increases the infrared emissivity at low molar ratio of Fe3+:Al. There are only a few of the surface coated with a thin Fe3O4 layer when the molar ratio of Fe3+:Al is 0.075:1, but the surface roughness of flake Al pigment significantly increases. The lightness L* and visual light reflectivity can be decreased by 12.8 and 0.34 as compared to the uncoated flake Al pigment, but the infrared emissivity is only increased by 0.04. The saturation magnetization value of the composite pigment is 11.9 emu/g, which could help improve the radar stealthy performance. Therefore, these Al/Fe3O4 magnetic composite pigments can be used as a novel low infrared emissivity pigment to improve the stealthy performance of the coating in the visual, IR and Radar wavebands.

The Turbulence Characteristics of Radar Backscatter Coefficient Caused by Wave and Weak Current Interaction on Ocean Surface

Based on radar backscattering turbulence model of ocean surface caused by wave-current interaction,For weak ocean current,it simplifies the model and calculates the radar backscattering coefficient's turbulence caused by weak ocean current.It analyzes the relationships between the results and current field,current amplitude,wind speed,wave-current angle,radar wave band and radar incidence angle.Then it gives the turbulence characteristics.Thus this paper will provide valuable help for studying the detecting capability of radar for ocean surface current.

雷达波段内磁性吸波颗粒光散射特性分析

雷达波段内磁性吸波颗粒光散射特性分析

Optically transparent band-pass frequency selective surface

Effects of the conductivity of transparent conductive film on the optically transparent band-pass frequency selective surface (FSS) are investigated by using Galerkin's method in the spectral domain. According to the results of the calculation and measurement, it is found that the conductivity of transparent conductive film mainly affects the transmission coefficient at the resonance frequency in radar wave band. The optically transparent band-pass frequency selective surface can transmit optical band and reflect radar wave band, however, it can also selectively transmit the radar wave.

A Basic Experimental Study of Earthquake Prediction in Terms of Radar Technology

To apply the radar technology in earthquake prediction, experiments of detecting the variation in the reflection radar wave intensity associated with loading of rock samples have been performed for 27 rock samples with different mechanical characters in the laboratory. The radar measuring instrument used was the special RSC system produced by HP Co., USA. Observation was made in four radar wavebands of 8mm, 2cm, 3cm and 5cm. The variation in the radar wave reflection intensity of the rock samples with the loading stress was measured automatically and continuously in the loading process. The experiment shows that there do exist variations in the reflection radar wave intensity which is closely related to the microwave reflection characteristics of the subject in the loading phase and unloading phase for all samples without exceptions, with only differences in the quantity and trends of the variations for different samples or at different wave band and different polarization modes. The relative variation of the reflection radar wave intensity for most of rock samples is a few percent to more than ten percent, and the maximum variation of the granite (No.113 sample) even reaches 20.2% and that of the sandstone (No.212 sample) reaches 27.9% as the biggest two in the experiments. The experimental results would provide an experimental evidence and physical basis for applying the radar technique in the earthquake prediction and in the monitoring and assessment of stability for rock masses engineering.

Spatial Variability of Soil Hydraulics and Remotely Sensed Soil Parameters

AbstractThe development of methods to correctly interpret remotely sensed information about soil moisture and soil temperature requires an understanding of water and energy flow in soil, because the signals originate from the surface, or from a shallow surface layer, but reflect processes in the entire profile. One formidable difficulty in this application of soil physics is the spatial heterogeneity of natural soils. Earlier work has suggested that the heterogeneity of soil hydraulic properties may be described by the frequency distribution of a single scale factor. The sensitivity of hydraulic and energetic processes to the variation of this scale factor is explored with a suitable numerical model. We believe that such an analysis can help in deciding how accurately and extensively basic physical properties of field soils need to be known in order to interpret thermal or radar waveband signals. It appears that the saturated hydraulic conductivity needs to be known only to its order of magnitude, and that the required accuracy of the soil water retention function is about 0.02 volume fraction. Furthermore, the results may be helpful in deciding how the total scene or view field, as perceived through a sensor, is composed from the actual mosaic of transient soil properties, such as surface temperature or surface soil moisture. However, the latter proposition presupposes a random distribution of permanent properties, a condition that may not be met in many instances, and no solution of the problem is apparent.