Stealth technology has proven to be one of the effective approaches to enhance the survivability of Aircrafts. Aircraft/helicopter designers are making them stealthier by reducing their signatures; viz. visual, aural, infrared (IR), and RADAR cross section. Advancements in stealth technologies, as demonstrated by the very low RCS of stealth aircraft such as F-117, B-2 and F-22, make such targets extremely difficult to detect. At present, stealth aircraft mainly rely on special geometry – their body shape – to deflect radar signals, but those designs can affect aerodynamic performance.
Therefore militaries employ Microwave absorbers, a kind of material that can effectively absorb incident microwave energy to effectively reduce the radar cross sections and radar dectability and hence commonly used in aircrafts and warships for stealth missions. In 2006 researchers demonstrated it was possible to absorb or direct electromagnetic waves around an object through a coating and make it “invisible”. However, it only worked on microwaves and in two dimensions.
However, conventional absorbers for the ultra-high regime are usually thick, heavy or have narrow absorption bandwidth, making them unsuitable for stealth missions. For example, the most basic Jaumann absorber, which works on the principle of using interference to cancel reflected waves, required a minimum thickness of half the wavelength and the required thickness of RAM becomes impractical at low frequencies. Therefore the cloaking efforts required materials as much as 10 times thicker than the wavelength being dodged. Missile guidance and marine radar wavelengths measure roughly 3 centimeters; that would require about a foot of coating. Conventional Radar-absorbent materials (RAM) and Radar absorbent structures (RAS) are only effective in a narrow frequency band and they are limited in their ability to absorb low frequency incoming electromagnetic energy.
Researchers have now turned to Metamaterials to solve these challenges. Matamaterials are artificially structured materials designed to control and manipulate physical phenomena such as light and other electromagnetic waves, sound waves and seismic waves in unconventional ways, resulting in exotic behavior that’s not found in nature. Researchers have shown that metamaterials formed by resonant metallic structure printed on a dielectric substrate acts as a strong resonant absorber, and such a metamaterial absorber is significantly thinner than the wavelengths absorbed. Research on active frequency selective surfaces (AFSSs) shows that a frequency selective surface (FSS) loaded with lumped elements, such as varactors and PIN diodes, can exhibit a tunable absorption bandwidth.
These studies show that the resistance of the PIN diode and the capacitance of the varactor contribute to the input impedance of the absorber. At different bias voltages, the absorber impedance matches with free space at different frequencies (to minimize reflection of incident waves), and thus the envelope of reflectivity curves measured at various bias voltages covers a broad absorption bandwidth. This result suggests that AFSS absorbers are practical candidates for broadband applications.
2015: Ultra-thin Dielectric metasurface cloak of University of California-San Diego
Boubacar Kante, a professor at the University of California-San Diego and his colleagues, have developed the first effective “dielectric metasurface cloak” based on a new material consisting of a layer of Teflon substrate with tiny ceramic cylinders embedded into it. Both teflon and ceramic is abundant, making it easy and inexpensive to produce. The combination of this very thin material, its non-metallic properties, and its cheap manufacturing costs set the UCSD design apart from competing cloaking technologies.
Kante said his material requires thickness of only 1/10 of the wavelength. Hiding from that same 3 cm wavelength would thus only require about a 3 mm coat. Different thicknesses (thinner) could be used for electromagnetic waves as small as those of visible light (which ranges from about 400 to 700 nanometers.) The cloak would be useful for Unmanned Aerial Vehicles and other planes, ships and anything else interested in dodging radar could have a use for it.
The military is interested in the breakthrough because it is “basically what the military’s looking for” in a cloaking solution, says Kayla Matola, research analyst from Homeland Defense & Security Information Analysis Center. It’s lightweight, cheap, and easily adaptable to new situations. Initial plans would call for small scale uses such as camouflaging unmanned aerial vehicles, while future uses may include larger endeavors like hiding ships, aircraft, and convoy vehicles from enemy radar.
In 2016, IOWA Engineers reported to have developed flexible skin that traps radar waves, cloaks objects
Iowa State University engineers have developed a new flexible, stretchable and tunable “metaskin” that uses rows of small, liquidmetal devices to cloak an object from radar. By stretching and flexing the polymer metaskin, it can be tuned to reduce the reflection of a wide range of radar frequencies. “It is believed that the present meta-skin technology will find many applications in electromagnetic frequency tuning, shielding and scattering suppression,” the lead authors Liang Dong, associate professor; and Jiming Song, professor wrote in journal Scientific Reports.
Metaskin is composed of rows of split ring resonators embedded inside layers of silicone sheets. The electric resonators are filled with galinstan, a metal alloy that’s liquid at room temperature and less toxic than other liquid metals such as mercury. Those resonators are small rings with an outer radius of 2.5 millimeters and a thickness of half a millimeter. They have a 1 millimeter gap, essentially creating a small, curved segment of liquid wire.
The rings create electric inductors and the gaps create electric capacitors. Together they create a resonator that can trap and suppress radar waves at a certain frequency. Stretching the meta-skin changes the size of the liquid metal rings inside and changes the frequency the devices suppress. Tests showed radar suppression was about 75 percent in the frequency range of 8 to 10 gigahertz, according to the paper. When objects are wrapped in the meta-skin, the radar waves are suppressed in all incident directions and observation angles.
“The long-term goal is to shrink the size of these devices,” Dong said. “Then hopefully we can do this with higher-frequency electromagnetic waves such as visible or infrared light. While that would require advanced nanomanufacturing technologies and appropriate structural modifications, we think this study proves the concept of frequency tuning and broadening, and multidirectional wave suppression with skin-type metamaterials.”
In 2018 Chinese Researchers claimed Metamaterials make the J-20 an extra stealthy
Chinese scientists have achieved a series of breakthroughs in stealth materials technology that they claim can make fighter jets and other weaponry lighter, cheaper to build and less vulnerable to radar detection. Chinese state media says the country is also mass-producing metamaterials for the aircraft, which could make it an electromagnetic force.
The metamaterials on the J-20 are likely to be used for as antennas and absorbers, given that the facility making them specializes in electromagnetic tech. Metamaterial antennas can increase radiated power, resulting in longer-range and more precise radar, as well as powerful jammers and datalinks. In turn, by fine-tuning their structures, metamaterial absorbers can be engineered to absorb specific wavelength ranges, such as those from the radars of enemy fighters and missiles. Such absorbers would likely be put on areas likely to reflect radar waves, such as the edges of canards, weapon bay doors, and engine nozzles.
Additionally, metamaterials optimized for infrared radiation can improve the sensitivity of the J-20’s infrared sensors for tracking missiles and aircraft. Or, in large enough quantities, metamaterials could reduce the fighter’s own infrared signature.
The next generation of Chinese aviation metamaterials could further increase stealth, improve communications, sensors and jamming, and even lighten airframe weight. Yang Wei, the J-20’s chief designer, told the People’s Daily that China is already laying down the requirements for a sixth-generation follow-up to the J-20. Much as the USAF’s envisioned futuristic Penetrating Counter Air platform, too, will likely include much more metamaterials.
In 2018 Breakthrough Invisibility Cloak and Absorber Technologies Receive Patents
Fractal Antenna Systems, Inc. ‘s newest patents, enable the next generation of stealth-like invisibility, and absorptive shielding, with commercial applications as well as defense options.
Recently issued patent 10,027,033 is a continuation of that state of the art innovation. It discloses a novel means of turning invisibility cloaks on and off, by changing the characteristics of a boundary layer. Notes inventor Nathan Cohen: “The person or sensor inside the cloak is thus no longer blind.” Cohen asserts that not being able to sense the outside has previously been the number one impediment to the use of invisibility cloaks.
The newest patent, 10,030,917, describes related technology where electromagnetic energy is absorbed by fractal-based metamaterials. Called ‘fractal absorbers’, the innovation uses evanescent waves to divert such impinging energy off to the sides, where it is absorbed in a resistive layer. Previously, absorbers relied on the thickness, not the width, of materials to accomplish this. Now these very thin fractal absorbers accomplish the same result with dramatic thickness and weight reductions. Cohen sees a variety of commercial applications for fractal absorbers, whose broad bandwidths and ultra-thinness are especially sought.
Fractal absorbers have been known and recognized as important for many years. Explains Cohen: “It is outrageous and bizarre to see teams from PRC (China) claiming invention of fractal absorbers. They have received unusual attention for their alleged invention, under the premise of so-called ‘supermaterials’. Fractal absorbers were discovered many years ago, at this firm, and the new patent conclusively establishes fractal absorbers as an American invention that pre-dates others’ alleged invention: we didn’t give it to them. We held it under wraps, waiting for this patent issuance. The patent application was withheld from publication. Ironic for them, the Chinese have unwittingly established credibility for our invention and its American uses. And, in my opinion, they are now very far behind in the game.”
In 2020, Russian-Italian Scientists create stealth technology that hides metal masts and antennas completely
Physicists from NUST MISIS and institutional collaborators have developed a technology that conceals elongated objects such as antennas and sensors, aircraft landing gear, ship masts and airport towers. The invention is based on an innovative metamaterial that eliminates an object’s electric type scattering. The result has been published in Scientific Reports.
Earlier the sceintists had reported, “The experimental part of our research was the creation of a one-of-a-kind metamaterial consisting of a small flat grid of the so-called meta-molecules cut out from a solid piece of ordinary steel,” the project’s director Alexei Basharin was quoted as saying by the NUST MISIS press service. The NUST MISIS team worked closely with colleagues from the University of Crete, Greece Basharin said that thanks to the special shape and configuration of these cells the scientists managed to obtain metamaterial with absolutely unique properties. This metamaterial can be used to make supersensitive sensors to detect explosives and chemical weapons.
“We have already deduced the theory for super-toroidal configurations earlier. Now we want to show it experimentally. Thus, we will come closer to solving the problem of complete invisibility. Although according to the optical theorem, it is impossible to create perfect invisibility, we can take a big step towards this,” concluded Basharin.
Elongated metal objects like antennas or cell towers have an electric response—a signal that appears in response to an impact. To hide such an object from radar, the object must scatter light, like an object with a magnetic response, which is very weak. This was accomplished by scientists from the Russian-Italian scientific collaboration, in the framework of the “ANASTASIA” project (Advanced Non-radiating Architectures Scattering Tenuously And Sustaining Invisible Anapoles), named after the Grand Duchess of the Russian Empire Anastasia Romanova.
“We came up with a special coating based on an ideal magnetic dipole scatterer that turns an elongated metal object with an electric response into an object with a magnetic response,” said one of the researchers, associate professor at the NUST MISIS Superconducting Metamaterials Laboratory, Alexey Basharin. “This has become possible due to the anapole state of the coating material, which lowers the electric type scattering to the level of the magnetic one and even lower. As a result, the object becomes invisible.”
The first of the possible applications of the new coating will be Stealth technology for military and civilian purposes—to hide various elongated objects, such as aircraft landing gear, antennas and various sensors, ship masts and airport towers. The developers emphasize, that if the task of hiding these objects from enemy radars is trivial, the task of electromagnetic compatibility of antennas on satellites is vital: antennas must not affect each other. And this will be possible only if they are invisible.
The method will help hide the structures of airports and operator towers so that they do not interfere with radar and communications with pilots. The development will find application in so-called “magnetic light” tasks, where it is necessary to enhance various magnetic phenomena: in nano-antennas, nano-lasers, etc. “Another idea discussed in this work is that we were able to develop a coating that makes the impedance of the cylinder equal to the impedance of the surrounding space due to the special form of the sinusoidal metamaterial. It gives the following effect: the incident electromagnetic wave completely does not notice the cylinder object and passes through it without hindrance. Important progress of our work is that we have applied a flat coating, and not bulk heavy structures,” added Alexey Basharin.
The research of the team is a theoretical work and demonstrates new methods and open effects. The next stage of the project and the immediate goal, according to the researchers, is to learn how to reduce the magnetic response of elongated metal structures
“An addition of a nonlinear semiconductor will turn the metamaterial into an adjustable screen for stealth technologies, which make fighting vehicles less visible in radio, infrared and other bands,” the NUST MISIS press service said in a statement. The newly obtained metamaterial can also become a vital element of the latest types of lasers and serve the basis for quantum computers.
While metamaterials may not yet be making objects invisible to the eye; they could be used to redirect other kinds of waves, including mechanical waves such as sound and ocean waves. Ong points to the possibility of using what has been learnt in reconfiguring the geometry of materials to divert tsunamis from strategic buildings. French researchers earlier this year, for example, diverted seismic waves around specially placed holes in the ground, reflecting the waves backward.
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