Radiation-absorbent material

In materials science, radiation-absorbent material (RAM) is a material which has been specially designed and shaped to absorb incident radio frequency (RF) radiation (also known as non-ionising radiation), as effectively as possible, from as many incident directions as possible. The more effective the RAM, the lower the resulting level of reflected RF radiation. Many measurements in electromagnetic compatibility (EMC) and antenna radiation patterns require that spurious signals arising from the test setup, including reflections, are negligible to avoid the risk of causing measurement errors and ambiguities. RAM are widely used in RF anechoic chambers to achieve an "echo-free" environment.

One of the most effective types of RAM comprises arrays of pyramid-shaped pieces, each of which is constructed from a suitably lossy material. To work effectively, all internal surfaces of the anechoic chamber must be entirely covered with RAM. Sections of RAM may be temporarily removed to install equipment but they must be replaced before performing any tests. To be sufficiently lossy, RAM can be neither a good electrical conductor nor a good electrical insulator as neither type actually absorbs any power. Typically pyramidal RAM will comprise a rubberized foam material impregnated with controlled mixtures of carbon and iron. 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 60 cm (24 in), while high-frequency panels are as short as 7.5 to 10 cm (3 to 4 in). Panels of RAM are typically installed on the walls of an EMC test chamber 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 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.

An alternative type of RAM comprises flat plates of ferrite material, in the form of flat tiles fixed to all interior surfaces of the chamber. This type has a smaller effective frequency range than the pyramidal RAM and is designed to be fixed to good conductive surfaces. It is generally easier to fit and more durable than the pyramidal type RAM but is less effective at higher frequencies. Its performance might however be quite adequate if tests are limited to lower frequencies (ferrite plates have a damping curve that makes them most effective between 30–1000 MHz). There is also a hybrid type, a ferrite in pyramidal shape. Containing the advantages of both technologies, the frequency range can be maximized while the pyramid remains small, about 10 cm (3.9 in).

For physically-realizable radiation-absorbent materials, there is a trade-off between thickness and bandwidth: optimal thickness to bandwidth ratio of a radiation-absorbent material is given by the Rozanov limit.

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.

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.

In September of 1944, 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.