Chinese researchers have developed a material called an "active frequency selecting surface" (AFSS)—a thin layer of material covered with a substance used in printed circuit boards that can be "tuned" to absorb microwaves at a range of frequencies. The material could be used to reduce the radar cross-section of whatever it covers, including stealth aircraft, making it possible to reactively change the absorption of radar signals of different types. And the science behind their work is now public, published in the Journal of Applied Physics.

Stealth aircraft use a combination of geometry and exotic materials to reduce their radar cross section—bouncing radar signals off at angles that prevent search radars from receiving most of the return and absorbing the energy of the radio waves to further reduce what might get bounced back, making the aircraft nearly invisible. However, newer radar technologies use ultra-high frequencies to search for aircraft with much higher resolution and potentially defeat stealth materials' range of absorption.

Current stealth technology is effective against radars in the Super High Frequency (SHF) range—the S, C, X, and Ku bands used by the US' Aegis AN/SPY-1 radar and most military fire-control radars used to steer missiles at targets. But they aren’t very effective against longer wavelength radars operating in the UHF range, at least when it comes to avoiding detection entirely. And fire control radars are being developed that will operate at those lower frequencies, meaning stealth aircraft may soon lose the ability to hide from targeting systems.

However, the AFSS developed by the researchers—Wenhua Xu, Yun He, Peng Kong, Jialin Li, Haibing Xu, Ling Miao, Shaowei Bie, and Jianjun Jiang of the Huazhong University of Science and Technology—would make it possible for the surface of stealth aircraft to shift the frequency of their absorption to catch those longer-length waves. It consists of a material "loaded with lumped elements" that can be used to tune its frequency range of radar absorption: varactor diodes (a semiconductor capacitor often used in electronic tuners) and PIN diodes (semiconductors that act as variable resistors to electromagnetic radiation at RF and microwave frequencies).

Starting from other published research in the field of absorbers, the researchers fabricated an "ultra-thin" material measuring only 7.8mm in thickness: 0.8mm of FR4, a material used in printed circuit boards; a 0.04mm layer of the copper-and-semiconductor AFSS beneath that; and a 7mm honeycomb material that supports the upper layers and provides separation from the metal it is applied to.

In testing, the researchers found that their prototype AFSS was able to tune itself to absorb radio frequencies ranging from 0.7 to 1.9GHz, reducing reflectivity by anywhere from 10 to 40dB. "As radar detection equipment continues to improve, our thin absorbers with broad bandwidth and working in the UHF band will be widely useful," the researchers concluded.

Given that stealth has been a strategic advantage for the US military, the fact that Chinese researchers operating on a government foundation grant have developed this technology will be reason for concern among military planners. Of course, the fact that the research is out in the open may help accelerate development of US countermeasures to lower-frequency targeting radars as well.

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