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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 by the German navy during 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 absorber
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.
One technique used in the F-117A Nighthawk and other such stealth aircraft is to use electrically isolated carbonyl iron balls of specific dimensions suspended in a two part epoxy paint. Each of these microscopic spheres is coated in quartz (silicon dioxide) as an insulator through a proprietary process. Then during the fabrication panel process, while the paint is still liquid, a magnetic field is applied with a specific Gauss strength and at a specific distance to create a magnetic field patterns in the carbonyl iron balls within the liquid paint ferrofluid. The paint then cures [hardens] whilst the magnetic field holds the particles in suspension locking the balls into their magnetic pattern. Some experimentation has been done applying opposing North-South magnetic fields to opposing sides of the painted panels causing the carbonyl iron particles to align (standing up on end so they are three-dimensionally parallel to the magnetic field). The carbonyl iron ball paint is most effective when the balls are evenly dispersed, electrically isolated, and present a gradient of progressively greater density to the incoming radar waves.
A related type of RAM consists of neoprene polymer sheets with ferrite grains or conductive carbon black particles (containing about 0.30% of crystalline graphite by cured weight) 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 so that the paint can be applied consistently in specific layer thicknesses and densities. The plane covered in tiles "glued" to the fuselage and the remaining gaps filled with iron ball "glue."
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)
The Republic of China (Taiwan)'s military has also successfully developed radar-absorbing paint which is currently used on Taiwanese stealth warships and the Taiwanese built stealth jet fighter which is currently in development in response to the development of stealth technology by their rival the mainland People's Republic of China (PRC) which is known to have displayed both stealth warships and planes to the public.  
Foam absorber is used as lining of anechoic chambers for electromagnetic radiation measurements. This material typically consists of a fireproofed urethane foam loaded with conductive carbon black [carbonyl iron spherical particles, and/or crystalline graphite particles] in mixtures between 0.05% and 0.1% (by weight in finished product), and cut into square pyramids with dimensions set specific to the wavelengths of interest. Further improvements can be made when the conductive particulates are layered in a density gradient so that the tip of the pyramid has the lowest % of particles and the base contains the highest density of particles. This presents a "soft" impedance change to incoming radar waves and further reduces reflection (echo). The length from base to tip, and width of the base of the pyramid structure is chosen based on the lowest expected frequency when a wide-band absorber is sought. For low frequency damping in military applications, this distance is often 24 inches, while high frequency panels are as short as 3-4 inches. An example of a high-frequency application would be the Police radar (speed measuring radar K and Ka band) the pyramids would have a dimension of ~4" length and a 2"x2" base. That pyramid would set on and 2"x 2" cubical base that is 1" high (total height of pyramid and base of ~5"). The four edges of the pyramid are softly sweeping arcs giving the pyramid a slightly "bloated" look. This arc provides some additional scatter and prevents any sharp edge from creating a coherent reflection.
Panels of RAM are installed with the tips of the pyramids pointing toward the radar source. These pyramids may also be hidden behind an outer nearly radar-transparent shell where aerodynamics are required. 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 may be reflected back to the receiver but are out of phase and thus have lower signal strength. A good example of coherent reflection is in the faceted shape of the F-117A Nighthawk stealth aircraft which presents angles to the radar source such that coherent waves are reflected away from the point of origin (usually the detection source). Incoherent scattering also occurs within the foam structure, with the suspended conductive 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. Absorption within the foam material occurs when radar energy is converted to heat in the conductive particle. Therefore, in applications where high radar energies are involved, cooling fans are used to exhaust the heat that is generated.
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.
Split ring resonator absorber
Split ring resonators (SRR) in various test configurations have been shown to be extremely effective as radar absorbers. SRR technology can be used in conjunction with the technologies above to provide a cumulative absorption effect. SRR technology is particularly effective when used on faceted shapes that have perfectly flat surfaces that present no direct reflections back to the radar source (such as the F-117A Nighthawk Stealth aircraft). This technology uses photographic process to create a resist layer on a thin (~0.007") copper foil on a dielectric backing (thin circuit board material) that is etched into a tuned resonator arrays. Each individual resonator being in a "C" shape (or other shape -- such as a square). Each SRR is electrically isolated and all dimensions are carefully specified to optimize absorption at a specific radar wavelength. Not being a closed loop "O", the opening in the "C" presents a gap of specific dimension which acts as a capacitor. At 35 GHz the diameter of the "C" is ~5mm. The resonator can be tuned to specific wavelengths and multiple SSRs can be stacked with insulating layers of specific thicknesses between then to provide a wide-band absorption of radar energy. When stacked the smaller SSR (high-frequency) in the range faces the radar source first (like a stack of donuts that get progressively larger as you move away from the radar source) stacks of 3 have been shown to be effective in providing wide-band attenuation. SRR technology acts very much in the same way that anti-reflective (AR) coatings operate at optical wavelengths. SRR technology provides the most effective radar attenuation of any technologies known previously and brings us one step closer to reaching the "holy grail" known as complete invisibility (total stealth, "cloaking"). Work is also progressing in visual wavelengths as well as infrared wavelengths (LIDAR-absorbing materials).
- Lidar speed gun
- Radar cross-section (RCS)
- Stealth technology
- Salisbury screen
- Speed limit enforcement
- Split Ring Resonator
- 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