Radar absorbing materials (RAM) are made from two basic ingredients: particles that convert radar energy into heat, and a binder that holds everything together. The energy-absorbing particles fall into two camps, magnetic and dielectric, and the choice between them depends on which radar frequencies need to be defeated and how much weight the application can tolerate.
Magnetic Absorbers: Iron and Ferrite
The most widely used magnetic absorber is carbonyl iron powder, a fine black powder with particles just 2 to 3 micrometers across. Each particle is a near-perfect sphere with a layered, onion-like internal structure and an iron purity above 97%. When radar waves pass through a coating loaded with carbonyl iron, the alternating electromagnetic field forces the magnetic domains inside each particle to flip back and forth rapidly. That constant realignment generates friction at the atomic level, converting the radar signal into a tiny amount of heat.
Ferrite compounds are the other major magnetic option. The most common are nickel-zinc and manganese-zinc ferrites, both members of a family of iron-oxide ceramics called spinels. These materials have tunable magnetic properties: their magnetic permeability (a measure of how strongly they interact with a magnetic field) can range from a few dozen up to 25,000, depending on the specific chemistry. In a typical radar-absorbing composite, nickel-zinc ferrite makes up 60 to 80% of the total weight and can reduce a reflected radar signal by 20 to 34 decibels in the 2 to 4 GHz range. Manganese-zinc ferrite performs well at slightly higher frequencies, achieving 20 to 44 decibels of absorption between 2 and 7 GHz, though it requires a coating thickness of 5 to 7 millimeters.
The drawback of magnetic absorbers is weight. Carbonyl iron has a density of 7.87 grams per cubic centimeter, nearly as dense as solid steel. Ferrite-loaded composites need high filler concentrations to work, which makes them heavy. This is manageable on ships or ground installations but becomes a serious constraint on aircraft.
Carbon-Based Absorbers: Lightweight Alternatives
Carbon materials absorb radar through a different mechanism. Instead of magnetic loss, they rely on dielectric loss: the radar wave’s electric field drives electrons and charges back and forth inside the carbon structure, and that movement dissipates energy as heat. Carbon black, carbon nanotubes, and graphene are the three most common forms.
Carbon black is the simplest and cheapest option. It’s loaded into honeycomb structures or composite panels by repeatedly dipping the base material in a carbon-black-and-resin mixture until reaching a target weight. The process is straightforward, and the loading level can be adjusted to tune performance across different frequency bands, particularly the X-band (8.2 to 12.4 GHz) used by many military and aviation radars.
Carbon nanotubes and graphene push performance further. Epoxy composites loaded with 20% metal-filled carbon nanotubes by weight can achieve reflection losses of around minus 22 decibels at a thickness of just 2.2 millimeters. Graphene-based composites perform even better in some configurations: a 2.5-millimeter-thick film containing 20% graphene by weight can cover an effective absorption bandwidth of 14.2 GHz. One of the most impressive recent results came from a carbon nanotube/MXene sandwich structure only 1.55 millimeters thick, which achieved a peak reflection loss of minus 52.9 decibels at 7.15 GHz. For context, minus 10 decibels means 90% of the radar energy is absorbed, and minus 50 decibels means 99.999% is absorbed.
The Binder Matrix
Absorbing particles need a host material to hold them in place, and the choice of binder shapes where and how the RAM can be used. Epoxy resins are the default for structural applications because they bond well to aircraft skins and fiberglass panels. Silicone (organosilicon) elastomers handle extreme heat, remaining functional up to 300 to 400°C, which makes them suitable for engine areas or surfaces exposed to aerodynamic heating and UV radiation. Beyond those two, the list of viable binders is long: polyester, polyurethane, polyimide, polycarbonate, polyethylene, polystyrene, and fluorocarbon resins all appear in various formulations, each chosen to match specific temperature, flexibility, or chemical resistance requirements.
The binder itself is designed to be as transparent to radar as possible. Its dielectric constant and loss tangent are kept low so that the electromagnetic wave passes through to the absorbing particles rather than bouncing off the surface of the coating.
Layered and Structural Designs
Not all RAM is a simple filled coating. Some designs use geometry and layering to cancel radar reflections.
The Salisbury screen is the simplest layered absorber. It has three layers: a thin resistive sheet on top, a dielectric spacer in the middle, and a metal backing on the bottom. The spacer thickness is set to exactly one quarter of the target radar wavelength. At that thickness, the wave reflecting off the metal backing travels half a wavelength further than the wave reflecting off the top sheet, so the two reflections arrive out of phase and cancel each other. The top sheet can be made from metal film, carbon nanotubes, or graphene, and it needs an electrical resistance matched to 377 ohms (the impedance of free space) for maximum cancellation. The spacer can be ordinary polymer film or tape.
Metamaterial absorbers take this idea further by replacing the plain top sheet with a patterned metallic surface. A typical design prints silver nanoparticle ink in precise geometric shapes (square patches surrounded by square rings, for instance) onto a thin flexible substrate like PET plastic, backed by a copper ground plane. The patterned surface creates resonant structures far smaller than the radar wavelength, allowing the absorber to be extremely thin: some designs use substrates only 0.27 millimeters thick. These metamaterial absorbers can be tuned to specific frequency bands by adjusting the pattern geometry.
Foam Pyramids for Test Chambers
The spiky foam pyramids lining the walls of anechoic chambers are another form of RAM, though built for a different purpose. These are open-cell polyurethane foam loaded with carbon-based dielectric material and coated in latex. The pyramid shape works by forcing incoming radar waves to bounce multiple times along the tapered surfaces, losing energy with each bounce. By the time the wave reaches the flat base, almost nothing is left to reflect back. These pyramids are not suitable for vehicles (they’re bulky and fragile) but they create the reflection-free environments needed to test antennas and radar systems.
How Material Choice Shapes Performance
Each material type has a natural frequency range where it works best. Magnetic materials like ferrite excel at lower frequencies in the megahertz to low gigahertz range, where their magnetic resonance is strongest. Carbon-based dielectric materials perform better at higher gigahertz frequencies. This is why many modern RAM designs combine both types: mixing carbonyl iron with graphene or carbon nanotubes in the same composite creates absorption across a wider frequency band than either material could cover alone. A composite of iron-cobalt nanocrystals on graphene sheets, for example, achieved a peak absorption of minus 40.2 decibels at 8.9 GHz with a thickness of only 2.5 millimeters, drawing on magnetic loss from the metal and dielectric loss from the carbon simultaneously.
Thickness is the other major variable. Thicker coatings absorb more and at lower frequencies, but they add weight and bulk. The engineering challenge is always the same: absorb as much radar energy as possible, across the widest frequency range, with the thinnest and lightest material. Carbon nanomaterials are pushing that boundary, achieving strong absorption at thicknesses under 2 millimeters, roughly the thickness of two credit cards stacked together.