Radar absorbent material
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 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. A particularly disastrous example occurred during the Kosovo war
, in which moisture on the surface of an F-117 Nighthawk
allowed long-wavelength radar to track and shoot it down. RAM is only a part of achieving stealth.
for the snorkel
s (or periscopes) of submarine
s, 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. However it is unknown if the carbon was incorporated for stealth reasons or because of Germany's metal shortage.
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 ferrofluid
ic and non-magnetic substances. By reducing the reflection of electromagnetic waves, this material helps to reduce the visibility of RAM painted aircraft on radar.
The Israeli firm Nanoflight has also made a radar-absorbing paint that uses nanoparticles.
s 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.
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.
Stealth technology
Stealth technology also termed LO technology is a sub-discipline of military tactics and passive electronic countermeasures, which cover a range of techniques used with personnel, aircraft, ships, submarines, and missiles, to make them less visible to radar, infrared, sonar and other detection...
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. A particularly disastrous example occurred during the Kosovo war
Kosovo War
The term Kosovo War or Kosovo conflict was two sequential, and at times parallel, armed conflicts in Kosovo province, then part of FR Yugoslav Republic of Serbia; from early 1998 to 1999, there was an armed conflict initiated by the ethnic Albanian "Kosovo Liberation Army" , who sought independence...
, in which moisture on the surface of an F-117 Nighthawk
F-117 Nighthawk
The Lockheed F-117 Nighthawk was a single-seat, twin-engine stealth ground-attack aircraft formerly operated by the United States Air Force . The F-117A's first flight was in 1981, and it achieved initial operating capability status in October 1983...
allowed long-wavelength radar to track and shoot it down. RAM is only a part of achieving stealth.
History
The earliest forms of RAM were the materials called Sumpf and Schornsteinfeger, a coating used in Germany during the World War IIWorld War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
for the snorkel
Submarine snorkel
A submarine snorkel is a device which allows a submarine to operate submerged while still taking in air from above the surface. Navy personnel often refer to it as the snort.-History:...
s (or periscopes) of submarine
Submarine
A submarine is a watercraft capable of independent operation below the surface of the water. It differs from a submersible, which has more limited underwater capability...
s, to lower their reflectivity in the 20-centimeter radar band the Allies used. The material had a layered structure and was based on graphite
Graphite
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...
particles and other semiconductive materials embedded in a rubber
Rubber
Natural rubber, also called India rubber or caoutchouc, is an elastomer that was originally derived from latex, a milky colloid produced by some plants. The plants would be ‘tapped’, that is, an incision made into the bark of the tree and the sticky, milk colored latex sap collected and refined...
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
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
, in the form of the Horten Ho 229
Horten Ho 229
The Horten H.IX, RLM designation Ho 229 was a late–World War II prototype fighter/bomber designed by Reimar and Walter Horten and built by Gothaer Waggonfabrik...
. It used a carbon-impregnated plywood that would have made it very stealthy to Britain's primitive radar of the time. However it is unknown if the carbon was incorporated for stealth reasons or because of Germany's metal shortage.
Iron ball paint
One of the most commonly known types of RAM is iron ball paint. It contains tiny spheres coated with carbonyl ironCarbonyl iron
Carbonyl iron is a highly pure iron, prepared by chemical decomposition of purified iron pentacarbonyl. It usually has the appearance of grey powder, composed of spherical microparticles. Most of the impurities are carbon, oxygen, and nitrogen.BASF invented carbonyl iron powder in 1925, and claims...
or ferrite
Ferrite (magnet)
Ferrites are chemical compounds consisting of ceramic materials with iron oxide as their principal component. Many of them are magnetic materials and they are used to make permanent magnets, ferrite cores for transformers, and in various other applications.Many ferrites are spinels with the...
. Radar
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
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
Iron pentacarbonyl
Iron pentacarbonyl, also known as iron carbonyl, is the compound with formula 5. Under standard conditions Fe5 is a free-flowing, straw-colored liquid with a pungent odour. This compound is a common precursor to diverse iron compounds, including many that are useful in organic synthesis. Fe5 is...
and may contain traces of carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
, oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
and nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
.
A related type of RAM consists of neoprene
Neoprene
Neoprene or polychloroprene is a family of synthetic rubbers that are produced by polymerization of chloroprene. Neoprene in general has good chemical stability, and maintains flexibility over a wide temperature range...
polymer sheets with ferrite grains or carbon black
Carbon black
Carbon black is a material produced by the incomplete combustion of heavy petroleum products such as FCC tar, coal tar, ethylene cracking tar, and a small amount from vegetable oil. Carbon black is a form of amorphous carbon that has a high surface-area-to-volume ratio, although its...
particles (containing about 30% of crystalline graphite
Graphite
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...
) 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
United States Air Force
The United States Air Force is the aerial warfare service branch of the United States Armed Forces and one of the American uniformed services. Initially part of the United States Army, the USAF was formed as a separate branch of the military on September 18, 1947 under the National Security Act of...
introduced a radar absorbent paint made from both ferrofluid
Ferrofluid
A ferrofluid is a liquid which becomes strongly magnetized in the presence of a magnetic field.Ferrofluids are colloidal liquids made of nanoscale ferromagnetic, or ferrimagnetic, particles suspended in a carrier fluid . Each tiny particle is thoroughly coated with a surfactant to inhibit clumping...
ic and non-magnetic substances. By reducing the reflection of electromagnetic waves, this material helps to reduce the visibility of RAM painted aircraft on radar.
The Israeli firm Nanoflight has also made a radar-absorbing paint that uses nanoparticles.
Foam absorber
Foam absorber is used as lining of anechoic chamberAnechoic chamber
An anechoic chamber is a room designed to stop reflections of either sound or electromagnetic waves.They are also insulated from exterior sources of noise...
s 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 screenSalisbury screen
The Salisbury screen, invented by American engineer Winfield Salisbury in 1952, was one of the first concepts in radar absorbent material, later known as "stealth technology", used to prevent enemy radar detection of military vehicles. It was first applied to ship radar cross section reduction...
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
Dielectric
A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing 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 technologyStealth technologyStealth technology also termed LO technology is a sub-discipline of military tactics and passive electronic countermeasures, which cover a range of techniques used with personnel, aircraft, ships, submarines, and missiles, to make them less visible to radar, infrared, sonar and other detection...
- Salisbury screenSalisbury screenThe Salisbury screen, invented by American engineer Winfield Salisbury in 1952, was one of the first concepts in radar absorbent material, later known as "stealth technology", used to prevent enemy radar detection of military vehicles. It was first applied to ship radar cross section reduction...
- Stealth aircraftStealth aircraftStealth aircraft are aircraft that use stealth technology to avoid detection by employing a combination of features to interfere with radar as well as reduce visibility in the infrared, visual, audio, and radio frequency spectrum. Development of stealth technology likely began in Germany during...
- Ferrofluid#Defense
- Radar jamming and deceptionRadar jamming and deceptionRadar jamming and deception is the intentional emission of radio frequency signals to interfere with the operation of a radar by saturating its receiver with noise or false information...