Radar Absorbing Materials Information

Radar absorbing materialRadar absorbing materials and structures are designed to absorb radar waves and minimize or eliminate reflection. The absorbed energy is converted 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 communication 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.

Specifications

Radar absorbing materials and structures differ in 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.

Types

  • Foam absorbers are very lightweight conductive carbon-loaded sheet and shaped absorbers that provide high levels of loss at normal and off-normal angles of incidence. They are manufactured with a continuous gradient coating that produces broadband reflection loss performance.
  • Surface wave absorbers are elastomers that are heavily magnetically loaded. Surface wave absorbers are designed to exhibit the highest loss and are intended to be applied to a conductive or metal surface.
  • 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.
  • Low frequency absorbers exhibit high loss at sub-microwave frequencies. Low frequency absorbers are designed with shaped magnetic particles that exhibit high permeability.
  • Jaumann absorbers, or Jaumann layers, are radar absorbing materials and structures that use wave-interfering techniques to cancel the reflected waves. 
  • Tuned frequency absorbers, or resonant frequency absorbers, have pronounced reflection loss at specific frequencies.
  • Cavity resonance absorbers are specifically designed to have high loss within a microwave cavity.

Stealth bomberApplications

Radar absorbing materials and structures are probably best-known 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, RAS and RAM are used in commercial microwave communications applications. Recently, RAM has been used in civil and commercial environments to reduce radar scattering from large buildings at airports that can interfere with civil aviation radar systems. Consumer electronics usage examples include antennas, car radios, mobile phones, and telecommunications base stations.

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Image credit:

Mike Peel / CC BY-SA 4.0