International Defense Security & Technology Your trusted Source for News, Research and Analysis
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.
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. 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. 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.
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.
Researchers then turned to Metamaterials for designing efficient stealth materials. Metamaterials 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.
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. According to South China Morning Post, Professor Luo Xiangang and his colleagues at the Chengdu-based Institute of Optics and Electronics, Chinese Academy of Sciences, have created the world’s first-ever mathematical model which precisely describes the behavior of electromagnetic waves when they “strike a piece of metal engraved with microscopic patterns”. As per a test conducted, the Metamaterials are aimed at making the aircraft, stealthier, lighter and cheaper to build, cut the strength of a reflected radar signal (measured in decibels) by between 10 and nearly 30dB in a frequency range from 0.3 to 40 gigahertz.
Chinese metamaterial breakthroughs
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.
In 2015, it was reported thar a team of researchers from Huazhong University of Science and Technology in China led by Wenhua Xu, has developed an ultra-thin, tunable broadband microwave absorber for ultra-high frequency applications. In a paper published in the Journal of Applied Physics, the researchers presented this work.
Chinese researchers have developed an ultra-thin active frequency-selective surface absorber, consisting of arrays of patterned conductors loaded with two common types of circuit elements known as resistors and varactors. The unit patterned cell absorbs microwaves and can also be actively controlled by stretching to expand the tunable bandwidth.
“Our proposed absorber was fabricated with a stretching transformation pattern, which is both thin and can absorb a wide range of frequencies for near-meter microwave application,” said Wenhua Xu, the primary researcher in the team led by Jianjun Jiang, a professor of School of Optical and Electronic Information at the Huazhong University of Science and Technology, China. “Its absorption range covers a broad band from 0.7 to 1.9 gigahertz below -10 decibel, and the total thickness of the absorber is only 7.8 millimeters, which is one of the thinnest microwave absorbers reported.”
“Usually the thickness of conventional radar absorbers is a quarter the wavelength of the incident microwave. In the high frequency regime, take one gigahertz as an example, the thickness of the absorber would be around 7.5 centimeters, which is too thick and heavy to be used in aircrafts or warships. Our proposed absorber is almost ten times thinner than conventional ones,” Xu said.
In the experiment, Jiang’s team fabricated a broadband active frequency-selective surface with a stretching transformation pattern on a printed circuit board, and soldered the resistors and varactors between each of the two unit patterned cells. The fact that the surface could be stretched meant that the parameters of the unit patterned cell can be actively controlled by stretching.
By modeling the absorber using a transmission line, the researchers found that the varactor provides a variable capacitance at varying bias voltage, which produces the device’s tunability, while the lumped resistor with constant resistance reliably produces strong absorption at the resonance frequency. Besides the lumped impedances of the loaded elements, the researchers discovered that the parameters of the unit patterned cells contribute to the device’s absorption performance as well.
“As radar detection equipment continues to improve, our thin absorbers with broad bandwidth and working in the UHF band will be widely useful.” “Such ultra-thin absorber with broad bandwidth may be widely used in warship stealth, airplane cloaking and tunable, broadband antennae.” The researchers’ next step is to study the polarization and the oblique incidence performance for the proposed active frequency-selective surface absorber.
Chinese scientists hail ‘incredible’ stealth breakthrough that may blind military radar systems
In July 2019, Professor Luo Xiangang and colleagues at the Institute of Optics and Electronics, Chinese Academy of Sciences in Chengdu, Sichuan province, said they had created the world’s first mathematical model to precisely describe the behaviour of electromagnetic waves when they strike a piece of metal engraved with microscopic patterns. With their new model and breakthroughs in materials fabrication, they developed a membrane, known as a meta surface, which can absorb radar waves in the widest spectrum yet reported.
In one test, the new technology cut the strength of a reflected radar signal – measured in decibels – by between 10 and nearly 30dB in a frequency range from 0.3 to 40 gigahertz. A stealth technologist from Fudan University in Shanghai, who was not involved in the work, said a fighter jet or warship using the new technology could feasibly fool all military radar systems in operation today.
“This detection range is incredible,” the researcher said. “I have never heard of anyone even coming close to this performance. At present, absorbing technology with an effective range of between 4 and 18 GHz is considered very, very good.” “Materials with meta surface technology are already found on military hardware in China, although what they are and where they are used remains largely classified,” the Fudan researcher said.
According to Chinese media reports, the Metamaterial was designed and developed by a research team at the State Key Laboratory of Millimetre Waves in Southeast University in Nanjing, Jiangsu province. According to a researcher at the lab, the Metamaterials were being tested on aircraft at a major military aircraft production base in Shenyang, Liaoning province.
Chinese Scientists employ Metamaterials to make the J-11, J-15, J-20 an extra stealthy
While the researcher has refused to name the test site or the aircraft used for the Metamaterial, there are suggestions that it is indeed the Shenyang Aircraft Corporation, as the Aviation Industry Corporation of China subsidiary builds non-stealth fighter jets, like the J-11 and the J-15. The materials are not only looked at as enhancing the stealth properties of current fighters but also for making non-stealthy fighters stealthy. Beijing which has around 20 J-20 stealth fighters and roughly 1,500 other fighters in their ranks will drastically improve upon their aerial prowess in comparison to the United States and Russia if Metamaterials are successful in upgrading their existing non-stealth aircraft.
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.
While the materials hold the key to unlocking the true potential of stealth technology in the future, according to Han Yiping, Director of applied physics at Xidian University, the materials alone could not be able to hide the fighter jets from enemy radar. “Stealth aircraft relied on a range of tactics, including low-reflection aerodynamic design and cloaks of ionized particles, to fly undetected,” Han said. Metamaterials were also extremely difficult to mass-produce, and the technology would have to withstand the heat and shock of battle, meaning some performance would have to be sacrificed for reliability,” And at present, the technology was effective within only certain radio bandwidths.” Han said
References and resources also include:
