Radar wave absorbing material is one of the most important stealth materials. It can absorb radar waves, weaken the reflected waves or even not reflect the radar waves, so as to achieve the purpose of stealth. For example, a broadband high-efficiency absorbing coating composed of an electrical impedance conversion layer and a low-impedance resonance layer developed in Japan. The conversion layer is composed of a mixture of ferrite and resin, and the resonance layer is composed of ferrite conductive short fibers and resin. The absorption rate is above 20 decibels on the radar band of 1-20 GHz.
1. Basic concepts
Radar absorbing stealth materials refer to materials that can reduce the target radar cross section through its own absorption, making it difficult to recover the echoes that are satisfactory for radar detection, and thus play a stealth effect. The basic principle is that when the radar wave radiates to the surface of the stealth material and penetrates, the stealth material itself can convert the radar wave energy into other forms of energy (such as mechanical energy, electrical energy or heat), and absorb it, thereby consuming part of the radar wave energy. , Make the echo incomplete and incomplete, thus greatly destroying the detection probability of radar. If the stealth material's own structural design and impedance matching are properly designed, coupled with the material selection and other formulas and the reasonable molding process, the stealth material can almost completely attenuate and absorb the incident radar wave energy to achieve the goal of safe stealth.
At present, radar absorbing materials are mainly composed of absorbing agents and polymer materials (such as resin, rubber and their modified materials). The key to determining the quality of the absorbing performance is the type and content of the selected absorbing agent. According to the different absorption principles of wave absorbing agents, they can usually be divided into two categories: electrical loss type and magnetic loss type. According to the structure of the absorbing agent, it can be divided into two types: the particulate intrinsic absorbing agent used for coating stealth materials and the fiber or whisker absorbing agent used for structural stealth materials.
Third, the mechanism of radar absorbing stealth
Current radar systems generally work in the frequency range of 1~18GHz, but new radar systems continue to develop, and the effective working bandwidth of the absorber will be expanded.
When the electromagnetic wave propagates in the air and meets the medium, because the impedance of the medium does not match the impedance of the free space, the electromagnetic wave is reflected at the interface between the air and the medium. The transmitted wave enters the medium, propagates and interacts with the medium, and is converted into other forms of energy such as mechanical energy, electrical energy, and thermal energy for consumption. The key to the absorption of electromagnetic waves by materials is whether the impedance of the absorber and the air medium match. Absorbing materials must meet two basic conditions to absorb electromagnetic waves
① When the electromagnetic wave is incident on the surface of the material, the electromagnetic wave can enter the material to the maximum extent (matching characteristics);
②The electromagnetic waves entering the material can be attenuated almost completely quickly (attenuation characteristics).
From the perspective of the absorbing mechanism of radar absorbing stealth materials, there are three phenomena that may occur when the absorbing material interacts with radar waves: ① Conductivity loss, high-frequency dielectric loss, hysteresis loss may occur or be transformed into Thermal energy attenuates the electromagnetic energy; ②After being affected by the absorbing material, the electromagnetic energy will be converted from energy in a certain direction to electromagnetic energy dispersed in all possible directions, so that its intensity is sharply reduced and the amount of echo is reduced; ③It acts on The first electromagnetic reflection wave on the surface of the material will superimpose the second electromagnetic reflection wave entering the material body, causing them to interfere with each other and cancel each other. Stealth material is an important part of stealth technology. Under the premise that the shape of the equipment cannot be changed, stealth material is the material basis for the realization of stealth technology. The use of stealth materials in the weapon system can reduce the detection rate, improve its own survival rate, increase offensiveness, and obtain the most direct military benefits. Therefore, the development of stealth materials and their application in aircraft, main battle tanks, ships, and rockets will become an important part of high-tech national defense.