The absorber used in the microwave anechoic chamber is often made into a wedge shape (the shape of a tower of gold), which is mainly composed of polyurethane foam type, non-woven fabric flame-retardant type, and silicate plate metal film assembly type. As the frequency decreases (wavelength increases), the length of the absorber also increases greatly. The common wedge-shaped absorber has an approximate relationship of L/λ≈1, so at 100MHz, the wedge length reaches 3000mm, which is not only difficult to achieve in the process , And the effective available space of the microwave anechoic chamber is also greatly reduced.
Single-layer flat shape
The first absorber developed abroad is a single-layer flat plate, and the absorber made later is directly attached to the metal shielding layer, which is thin in thickness and light in weight, but has a narrow working frequency range.
Double or multilayer flat shape
This kind of absorber can work in a wide operating frequency range and can be made into any shape. For example, NEC Corporation of Japan disperses ferrite and short metal fibers evenly in a suitable organic polymer resin to make composite materials, and the working frequency band can be expanded by 40% to 50%. Its disadvantages are large thickness, complicated process and high cost.
Only coating-type absorbing materials can be used on the surface of the aircraft, which broaden the frequency band, and generally use composite coatings. For example, when the thickness of the lithium cadmium ferrite coating is 2.5mm～5mm, it can attenuate 8.5dB in the centimeter band; when the thickness of the spinel ferrite coating is 2.5mm, it can attenuate 24dB at 9GHz; the ferrite is chlorinated When the thickness of the butadiene rubber coating is 1.7mm～2.5mm, the attenuation is about 30dB at 5GHz～10GHz.
Incorporating absorbing materials into engineering plastics makes them have both absorbing characteristics and load-bearing capacity, which is a direction for the development of absorbing materials.
Nowadays, in order to further improve the performance of absorbent materials, several complex types of absorbent bodies with combinations of shapes have been developed abroad. For example, a microwave anechoic chamber made of this type of absorber in Japan has performances: 136MHz, 25dB; 300MHz, 30dB; 500MHz, 40dB; 1GHz-40GHz, 45dB.