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Radar absorbent material
From Wikipedia, the free encyclopedia
Radar absorbent material, or RAM, is a class of materials used in stealth technology to disguise a vehicle or structure from radar detection. A material's absorbency at a given frequency of radar wave depends upon its composition. RAM cannot perfectly absorb radar at any frequency, but any given composition does have greater absorbancy at some frequencies than others. Different radar absorbent materials will have different frequency ranges at which they function best, so there is no one RAM that is suited to absorption of all radar frequencies.
A common misconception is that RAM makes an object invisible to radar. A radar absorbent material can significantly reduce an object's radar cross section in specific radar frequencies, but it does not result in "invisibility" on any frequency. Bad weather may contribute to deficiencies in stealth capability; a particularly disastrous example occurred during the Kosovo war, in which moisture on the surface of a F-117 Nighthawk allowed long-wavelength radar to track and shoot it down. RAM is only a part of achieving stealth.
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The earliest forms of RAM were the materials called Sumpf and Schornsteinfeger, a coating used by Germans during the World War II for the snorkels (or Periscopes) of submarines, to lower their reflectivity in the 20-centimeter radar band the Allies used. The material had a layered structure and was based on graphite particles and other semiconductive materials embedded in a rubber matrix. The material's efficiency was partially reduced by the action of sea water.[1][2]
Germany also pioneered the first aircraft to use RAM, in the form of the Horten Ho 229. It used a carbon-impregnated plywood that would have made it extremely stealthy to Britain's crude radar of the time. However it is unknown if the carbon was incorporated for stealth reasons or because of Germany's metal shortage.[3]
One of the most commonly known types of RAM is iron ball paint. It contains tiny spheres coated with carbonyl iron or ferrite. Radar waves induce molecular oscillations from the alternating magnetic field in this paint, which leads to conversion of the radar energy into heat. The heat is then transferred to the aircraft and dissipated.
The iron particles in the paint are obtained by decomposition of iron pentacarbonyl and may contain traces of carbon, oxygen and nitrogen. Carbonyl iron is also used as a catalyst and in medicine as an iron supplement, even though it is toxic in higher doses.
A related type of RAM consists of neoprene polymer sheets with ferrite grains or carbon black particles (containing about 30% of crystalline graphite) embedded in the polymer matrix. The tiles were used on early versions of the F-117A Nighthawk, although more recent models use painted RAM. The painting of the F-117 is done by industrial robots with the plane covered in tiles glued to the fuselage and the remaining gaps filled with iron ball paint.
The United States Air Force introduced a radar absorbent paint made from both ferrofluidic and non-magnetic substances. By reducing the reflection of electromagnetic waves, this material helps to reduce the visibility of RAM painted aircraft on radar.
Foam absorber is used as lining of anechoic chambers for electromagnetic radiation measurements. This material typically consists of a fireproofed urethane foam loaded with carbon black, and cut into long pyramids. The absorber is applied to the chamber walls with the tips of the pyramids pointing inward or toward the radar. As a radar wave strikes a pyramid, it experiences a gradual transition from free space at the tip of the pyramid to absorbing foam at the base. Other foam absorbers are available in flat sheets, using an increasing gradient of carbon loadings in different layers.
A Jaumann absorber or Jaumann layer is a radar absorbent device. When first introduced in 1943, the Jaumann layer consisted of two equally-spaced reflective surfaces and a conductive ground plane. One can think of it as a generalized, multi-layered Salisbury screen as the principles are similar.
Being a resonant absorber (i.e. it uses wave interfering to cancel the reflected wave), the Jaumann layer is dependent upon the λ/4 spacing between the first reflective surface and the ground plane and between the two reflective surfaces (a total of λ/4 + λ/4 ).
Because the wave can resonate at two frequencies, the Jaumann layer produces two absorption maxima across a band of wavelengths (if using the two layers configuration). These absorbers must have all of the layers parallel to each other and the ground plane that they conceal.
More elaborate Jaumann absorbers use series of dielectric surfaces that separate conductive sheets. The conductivity of those sheets increases with proximity to the ground plane.
- ^ Hepcke, Gerhard. "The Radar War, 1930-1945" (PDF). Radar World.
- ^ The History of Radar (HTML) (English). BBC (2003-07-14).
- ^ Shepelev, Andrei and Ottens, Huib. Ho 229 The Spirit of Thuringia: The Horten All-wing jet Fighter. London: Classic Publications, 2007. ISBN 1-903223-66-0.