Since the current method of detecting targets in various countries is mainly microwave radar, it uses the principle of electromagnetic waves to induce electromagnetic flow at the interface when the medium changes during the propagation process, and radiates electromagnetic energy to the surroundings, intercepted (or induced) by analyzing the radar receiving antenna The radiated electromagnetic energy can judge the distance, position, size, type, etc. of the target. The purpose of stealth is to prevent the receiving antenna from intercepting this radiated energy (magnetic barrier). The first thing to avoid rabbits is to generate induced current, which is mainly realized by material design, and secondly, to avoid the antenna receiving electromagnetic energy radiation, which is mainly realized by shape design. Assuming that the transmitted power of the radar is P and the received radiation power is P, then there is a relationship: Pr=PtG2λσ/(4π)3R4 where G is the antenna gain (the ratio of the power in the maximum radiation direction to the average value), A Is the electromagnetic wave wavelength; R is the target distance; a is the radar cross section.
The only thing that depends on the characteristics of the target here is the radar cross section a, which is related to the size of the target, the electromagnetic characteristic parameters (related to shape and wavelength) and the reflection coefficient, and the reflection coefficient depends on the electrical properties of the interface material and the wavelength of the radar wave. Angle and incident polarization (the relationship between the electric field and the incident surface). For a plane interface, when the incident angle is perpendicular to the interface, the reflection coefficients of vertical polarization and parallel polarization are equal, that is, R=(Z2-21)/(Z2+21) where Z1, Z: are two types The intrinsic impedance of the medium is determined by the dielectric constant ε and the magnetic permeability of the medium μ, that is, Z1=√μ1/ε1, Z2=√μ2/ε2, and the non-reflective condition is Z1 from equation (2-5) =Z2 or μ1/ε1=μ2/ε2.
It can be seen that the way to reduce the radar cross section from the target structure material selection is to avoid the drastic changes in the impedance of the two media and ensure the impedance gradient or matching. It can be achieved through the special design of the material, that is, the material is designed to have a surface impedance close to the free space impedance. , As the thickness increases, the impedance decreases. There are two methods: one is to use a laminate structure with the above electrical characteristics; the other is to add material particles with opposite electrical characteristics to the main material-conductors and insulating particles such as ceramics, and insulators to add metal particles, and the thickness varies with the thickness. Different, the density of the particles is different. In addition, from the perspective of energy conservation, the reflection of electromagnetic waves decreases, and the refraction must increase. If it is not lost, it will be reflected when it encounters other interfaces (such as the internal skin or internal structure). The method of loss is to convert it into other (such as heat) energy, which also has to be realized by the special design of special materials. At present, there are three commonly used means of dissipating electromagnetic energy: one is dielectric or particle type. With the help of dielectric or particle-oriented molecules, they tend to move under electromagnetic action, and they will be limited by the limited conductivity to convert electromagnetic energy. It is the heat loss; the second is the magnet or particle type, which uses the internal dipole to move under the action of magnetic energy, and at the same time is affected by the limited permeability, the electromagnetic energy is converted into thermal energy for loss; the third is the anti-phase interference type, which uses a certain amount of The structure form makes the incident wave opposite to the reflected wave to attenuate the electromagnetic energy. At present, people are still exploring other ways, such as using the plasma generated by the opposite isotope to absorb electromagnetic waves to obtain high efficiency.