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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 absorbency at some frequencies than others; there is no one RAM that is suited to absorption of all radar frequencies.
A common misunderstanding 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.
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The earliest forms of RAM were the materials called Sumpf and Schornsteinfeger, a coating used in Germany 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.
Germany also pioneered the first aircraft to use RAM during World War II, in the form of the Horten Ho 229. It used a carbon-impregnated plywood that would have made it very stealthy to Britain's primitive radar of the time. It is unknown if the carbon was incorporated for stealth reasons or because of Germany's metal shortage.
Types of RAM 
Iron ball paint 
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.
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.(Invented by Wayne Gindrup of Conroe, Texas)
Foam absorber 
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 length from base to tip of the pyramid structure is chosen based on the lowest expected frequency and the amount of absorption required. For low frequency damping, this distance is often 24 inches, while high frequency panels are as short as 3-4 inches. Panels of RAM are installed with the tips pointing inward to the chamber. Pyramidal RAM attenuates signal by two effects: scattering and absorption. Scattering can occur both coherently, when reflected waves are in-phase but directed away from the receiver, or incoherently where waves are picked up by the receiver but are out of phase and thus have lower signal strength. This incoherent scattering also occurs within the foam structure, with the suspended carbon particles promoting destructive interference. Internal scattering can result in as much as 10dB of attenuation. Meanwhile, the pyramid shapes are cut at angles that maximize the number of bounces a wave makes within the structure. With each bounce, the wave loses energy to the foam material and thus exits with lower signal strength. Other foam absorbers are available in flat sheets, using an increasing gradient of carbon loadings in different layers.
Jaumann absorber 
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.
See also 
- Radar cross-section (RCS)
- Stealth technology
- Salisbury screen
- Stealth aircraft
- Radar jamming and deception
- Hepcke, Gerhard. The Radar War, 1930-1945 (PDF). Radar World.
- "The History of Radar". 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.
- E Knott, J Shaeffer, M Tulley, Radar Cross Section. pp 528-531. ISBN 0-89006-618-3