Radar absorbing materials mainly have the following three application types: ① absorbing type, including dielectric absorbing type and magnetic absorbing type; ② resonance or interference type; ③ attenuation type.
1. Wave absorbing type
(1) Dielectric absorbing type
The dielectric absorbing material is composed of a absorbing agent and a matrix material. The conductive plastic is made by adding a lossy absorbing agent to the matrix resin. Commonly used absorbing agents are carbon fiber or graphite fiber, metal particles or fibers, etc., relying on resistance to The incident energy is lost, and the incident electromagnetic wave energy is converted into heat and dissipated. In the design and manufacture of absorbing materials, the distribution of absorbing agents with different electrical properties can be changed to achieve the purpose of changing their dielectric properties with their thickness and depth. The absorber has a good surface impedance matching the free space, and its military surface is less reflective, and most of the radar waves entering the body of the absorber will be dissipated or absorbed in it.
(2) Magnetic absorption type
Magnetic absorbers are mainly made of ferrite and rare earth elements; while the matrix polymer material is composed of synthetic rubber, polyurethane or other resin matrix, such as polyisoprene, polychloroprene rubber, nitrile rubber, silicon Resin, fluororesin and other thermoplastic or thermosetting resins, etc. Usually made of magnetic plastic or magnetic composite materials. During preparation, through effective control of magnetism and material thickness and reasonable design, the absorbing material has a higher magnetic permeability. When electromagnetic waves act on magnetic absorbing materials, they can spin their electrons, generate ferromagnetic resonance at a specific frequency, and forcefully absorb electromagnetic energy. A well-designed magnetic absorbing stealth material can attenuate the incident electromagnetic wave by 20~25dB at one or two frequency points, that is to say, it can absorb electromagnetic energy as high as 99%~99.7%; and at the peak between the two frequencies It has a greater ability to absorb electromagnetic energy, which can attenuate electromagnetic energy by 10-15dB, that is, absorb 90%-97% of electromagnetic energy. Typical broadband absorbing materials can attenuate the electromagnetic energy by 12dB, that is, absorb 95% of the electromagnetic energy.
2. Resonant type
Resonant type, also known as interference type absorbing material, realizes echo reduction through the principle of interference cancellation of electromagnetic waves. When the radar wave is incident on the surface of the absorbing material, part of the electromagnetic wave is directly reflected from the surface, and the other part is reflected from the bottom through the absorbing material. When the incident wave and the reflected wave have opposite phases and the same amplitude, they interfere with each other and cancel out, so that the radar echo energy is attenuated.
3. Attenuation type
The structure of the material is to sandwich the honeycomb structure of the absorbing material in the middle of the non-metallic wave-transmitting plate, which not only attenuates the electromagnetic wave and causes it to scatter, but also can bear a certain load. In the polyurethane foam honeycomb structure, absorbing agents such as graphite, carbon and carbonyl iron powder are usually added, so that part of the incident electromagnetic energy can be absorbed, and part of the honeycomb core material will experience multiple reflection interference. Attenuation, and finally achieve the purpose of offsetting each other. The above three forms are basically conductive polymer material systems. The effect of electromagnetic waves is basically composed of electric and magnetic fields, which emit electromagnetic waves in mutually perpendicular areas. Electromagnetic waves are emitted in a vacuum at a speed of approximately 3×10m/s and pass through non-conductive materials at the same speed. When encountering conductive polymer materials, they are partially reflected and partially absorbed. The energy of electromagnetic waves in the absorbing material becomes eddy currents, which can attenuate the electromagnetic waves. Conductive polymer materials can reflect 80% of electromagnetic waves and 20% of electromagnetic waves can be absorbed, while conductive metal materials can reflect all electromagnetic waves. This is the reason why resin or rubber matrix should be used as the absorbing material.