Radar-absorbent materials are 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; no one RAM 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.
History The earliest forms of stealth coating were radar absorbing paints developed by Major K. Mano of the Tama Technical Institute, and Dr. Shiba of the Tokyo Engineering College for the IJAAF. Multiple paint mixtures were tested with ferric oxide and liquid rubber, as well as ferric oxide, asphalt and airplane dope having the best results. Despite success in laboratory tests, the paints saw little practical application as they were heavy and would significantly impact the performance of any aircraft they were applied to. Conversely the IJN saw great potential in anti-radar materials and the Second Naval Technical Institute began research on layered materials to absorb radar waves rather than paint. Rubber and plastic with carbon powder with varying ratios were layered to absorb and disperse radar waves. The results were promising against 3 GHz (
S band) frequencies, but poor against 3 cm wave length (10 GHz,
X band) radar. Work on the program was halted due to allied bombing raids, but research was continued post war by the Americans to mild success. materials called
Sumpf and
Schornsteinfeger, coatings used by the German navy during
World War II for the
snorkels (or
periscopes) of
submarines, to lower their reflectivity in the 20 cm radar band (1.5 GHz,
L 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.
Types of radar-absorbent material (RAM) Iron ball paint absorber utilizes 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. 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
silicon dioxide as an insulator through a proprietary process. Then, during the panel fabrication process, while the paint is still liquid, a magnetic field is applied with a specific Gauss strength and at a specific distance to create magnetic field patterns in the carbonyl iron balls within the liquid paint
ferrofluid. The paint then hardens with the magnetic field holding the particles in 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 the paint can be applied consistently in specific layer thicknesses and densities. The plane is covered in tiles "glued" to the fuselage and the remaining gaps are filled with iron ball "glue." The
United States Air Force introduced a radar-absorbent paint made from both
ferrofluidic and nonmagnetic 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. 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 which is known to have displayed both stealth warships and planes to the public.
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 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 the tip of the pyramid has the lowest percentage 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 , while high-frequency panels are as short as . 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 around long and a base. That pyramid would set on a 5 cm x 5 cm cubical base that is high (total height of pyramid and base of about ). 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 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 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 10 dB 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. ==See also==