Ferrite linings work by “magnetic permeability” and operate well at lower frequencies, where the wavelength is too long for pyramidal absorbers to work effectively. In some forms the tiles are supplied in panels ready assembled with a tuned dielectric backing layer. Since they are of about the same density as steel, the chamber support frame and roof structure need to be suitably substantial and they have to be attached to a backing layer.
They are made from sintered iron/nickel material, which is ground to a precise shape so that large wall sections can be precisely assembled without intertile gaps. 2.įerrite tiles, which are usually made as 100 mm×100 mm×6 mm thick pieces with a central fixing hole. The absorbing performance is directly related to the wavelength of the energy, and the figure quoted (in dB) relates to waves with a normal angle of incidence.
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The principle of this type of absorber is to mimic free space by “progressive impedance” that is, absorbing the radio or microwave by internal reflection through a resistive structure that has a progressively larger shape, during which the wave energy is being converted into heat.
RAM can be made from two distinctly different types of material, although both types are commonly met in different areas of the same chamber: 1.Ĭlosed-cell polyurethane or polystyrene foam molded into a steep pyramidal shape and “dosed” (impregnated) with carbon, then painted with a fire-resistant paint. The composition and morphology of the material is carefully tailored to absorb radar waves over a specific frequency band. RAM cannot absorb radar at all frequencies. The electromagnetic energy used to form these corrugations weakens or eliminates the energy of the reflected radar signal. When the fluid is subjected to a sufficiently strong electromagnetic field the polarisation causes corrugations to form on the surface. Ferrofluids are superparamagnetic, which means they are strongly polarised by electromagnetic radiation. Ferrofluids are colloidal mixtures composed of nano-sized ferromagnetic particles (under 10 nm) suspended in a carrier medium. The USAF has introduced radar-absorbent paints made from ferrofluidic and nonmagnetic materials to some of their stealth aircraft. This material, which was used on early versions of the F-117A Nighthawk, works on the same principle as iron ball paint by converting the radar waves to heat. The small amount of heat generated by the oscillations is conducted into the airframe where it dissipates.Īnother type of RAM consists of neoprene sheet containing ferrite or carbon black particles. The molecular oscillations then decay with the release of heat, and this is an effective mechanism of damping electromagnetic waves. When electromagnetic radiation enters iron ball paint it is absorbed by the ferrite or carbonyl iron molecules which causes them to oscillate. The spheres are coated with ferrite or carbonyl iron. One of the most common RAMs is called iron ball paint, which contains tiny metal-coated spheres suspended in an epoxy-based paint. Most RAMs consist of ferromagnetic particles embedded in a polymer matrix having a high dielectric constant. Information about the composition of RAMs is guarded by the military. Examples of stealth aircraft that use radar-absorbing materials: (a) F-22 Raptor and (b) F-35 Lightning II.