Radar absorbing materials and structures are designed to absorb radar waves and convert them to heat. Because these radar waves are not returned, radar absorbing materials (RAM) and radar absorbing structures (RAS) provide a reduced signature for detection. Typically, RAM and RAS are used in defense applications and in commercial communications activities that require the absorption of electromagnetic interference (EMI) and radio frequency interference (RFI). EMI refers to unwanted electromagnetic emissions that can interfere with or degrade the performance of electrical equipment. RFI is unwanted electrical energy within the frequency range used by RF transmissions.
Radar absorbing materials and structures differ in terms of electromagnetic absorption performance. They carry product specifications for frequency range, surface resistivity, operating temperature, form factor, and thickness. Typically, high-performance magnetic materials are used to produce radar-absorbent materials in the 1 GHz to 40 GHz range. To create EMI sheets that are suitable for use over a range of microwave frequencies, various substrates and filler materials are selected. These materials differ in terms of composition, specific gravity, and service temperature. Flexible sheets have the physical characteristics of elastomers. They usually attenuate radiated noise by suppressing magnetic fields at their source.
Materials and Structures
Radar absorbing materials and structures include iron ball paint, foam absorbers, and Jaumann absorbers. Iron ball paint consists of tiny iron particles that are coated with carbonyl iron or ferrite. These particles are similar to the ferrite grains and carbon black particles embedded in the neoprene polymer sheets used in some military applications. Foam absorbers also use carbon black, but in urethane foam. Pyramidal RAM is achieved via scattering and absorption. Jaumann absorbers or Jaumann layers are radar absorbing materials and structures that use wave-interfering techniques to cancel the reflected waves. Specialized and proprietary RAM and RAS products are also available.
Radar absorbing materials and structures are probably best-know for their use in stealth technology, such as on the surfaces of the U.S. Air Force’s F-117 Nighthawk. In addition to their use on stealth bombers, however, RAS and RAM are used in commercial microwave communications applications. Examples include antennas, car radios, mobile phones, and telecommunications base stations. Microwave absorbers that can be die-cut or machined from solid microwave materials are also available.