Stealth aircraft like the F35 fighter generally rely on high-tech absorption materials and unusual geometries to scatter, deflect, and sponge up incoming radar signals. These techniques are exceptionally good at masking a vehicle's shape and size, particularly when swept with side-scanning radar. However, with lower-frequency, directed anti-stealth radar-targeting systems being developed, these surfaces prove much less able to hide an object. To help address this, a team of Chinese scientists has developed a thin electronic material that sheaths an object and effectively absorbs radar signals over a wide range of frequencies.
Super High Frequency (SHF) is used by many military fire-control radars to guide missiles to targets and to detect and identify objects using synthetic aperture side-scanning radar. Current stealth materials and geometries are relatively effective at hiding from equipment operating at these frequencies. However longer wavelength radars operating in the Ultra High Frequency (UHF) range are much harder to hide from because the materials used to absorb them are too thick for practical use, and scattering geometries are not as effective at these wavelengths.
In a new system, dubbed "Active Frequency Selecting Surface" (AFSS), the material developed by scientists at the Huazhong University of Science and Technology is a much thinner absorber as it uses active electronic components to help adjust its effectiveness at the relevant frequencies, thereby relying on tuned circuitry rather than bulk to achieve its goal.
Made up of four distinct layers, for an overall thickness of just 7.8 mm (0.3 in), the new material consists of an a 0.8 mm layer of epoxy laminate support material, a 0.04 mm layer of copper sheeting, a 7 mm honeycomb dielectric that electrically separates the material from the craft, and a final layer of metal with which to attach to the vehicle. This gives an overall thickness to the material much thinner than comparable radar absorbers operating at the same frequencies.
Essentially a variable resonance circuit made up of the copper sheet acting as an antenna and tuned using varactor diodes (in concert with inductors and resistors), the team was able to tune the AFSS to absorb radio frequencies in the range of 0.7 to 1.9 GHz, with a corresponding 10 to 40 dB reduction in reflectivity. In other words, the AFSS effectively absorbed incoming radar signals in the bandwidth used by modern targeting radar.
Interestingly, despite the constant one-upmanship of constant developments in the fields of radar detection and corresponding radar avoidance systems, along with the military saber-rattling between the west and east, the scientists at Huazhong University of Science and Technology have elected to release their research publicly.
The paper outlining this work was recently published in the Journal of Applied Physics
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