Radar Absorbent Material may come in various forms, and properties differ remarkably.
In fact, RAM is a misnomer: included in the term are radar Transparent materials as well, although if used correctly this is not a problem.
RAM is made up of a mixture of both absorbent dielectrics (carbons) and conductors (ferrites). These are mixed together into the end product.
Carbon-impregnated foams, honeycomb matrixes and several materials qualify as RAM. Used on their own they can "dispose" of narrow-band wavelengths/frequencies; used together, they eliminate a vast area of the ElectroMagnetic Spectrum.
Carbon fibre ("graphite") is the main, nonmetallic, nonconductive composite material used for areas of high structural load. It is, as the name suggests, made up of carbon chains, bonded in epoxy and bismaleimide resins, with EM characteristics falling between Absorbent and Transparent. Reinforced Carbon Carbon is an EM Absorbent material of high strength property. It also has a high EM Radiation coverage, being used for heat absorption on the Space Shuttles. The carbon family is also being supplemented by thermoplastics and other composites, for example the Transparent Glass-Reinforced Plastic ("fibreglass") and aramid-fibre Kevlar, again using epoxy and bismaleimide resins.
Polyelectrolytes -conducting polymer chains like polyaniline and polyphenylene vinyls - when crosslinked with beads of polystyrene, have good rheological properties. These produce a flow of heat when a current is passed through them, collect static, and are, as all polymers, lightweight and uniform. A matrix of polyaniline impregnated with minute cyanate whiskers has effectively demonstarted breaking up of EMR, converting them into heat which dissipates through the material and atmosphere.
"Circuit-analogue RAM" comprises many hundreds of thousands of repeated dipole, crossed-dipole and intersecting dipoles (resembling I,+ and x in shape) of conductive, ductile metals and semiconductors (ieg copper and silicon): these are placed between two layers of dielectric/ lossy RAM, conducting EMR deeper into the materials. They are semi-advantageous in reducing damage after Electro-Magnetic Pulses, too.
Conductivity of electrical/ thermal current needs to be maintained if RAM is to work effectively: transparencies can be covered with layers of indium-tin oxides, allowing 95% of light to pass through, whilst 5% is conducting EMR.
Obviously, the success of a stealth design depends on careful matching of properties as well as use of materials: incorrect placing and matching of panels may result in severely degraded performance of the RAM, which - if RAM is being used for stealth - will defeat its purpose.
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Lately, however, technology has advanced to the point where organometallic matrix can be applied, which change colours when oxidised or reduced, whilst having RAM qualities. It is obvious that once the RCS has been significantly reduced, the optical and thermal signatures will need to be addressed. Here is where colour-changing "active" camouflage could be applied; to this end, embedded fibre optics are being considered for use on tanks and aircraft.
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