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.
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