Polyurethane foam-based absorbing materials are mainly used to construct non-echo anechoic chambers. In the military, in order to create an ideal, non-reflective free space environment for radar countermeasures in-field radiation simulation experiments, and to prevent electromagnetic interference from the surrounding environment from entering and leakage of experimental information, one of the best ways is to Experiments in a non-echo anechoic chamber, such as in a non-echo anechoic chamber constructed of rigid polyurethane foam plastic-based absorbing materials, the product's EMC (electromagnetic compatibility), EMS (electromagnetic shielding), EMI (electromagnetic interference), antenna can be measured Scattering performance and some calibration work. In terms of civilian use, in order to smoothly carry out non-interference tests and eliminate electromagnetic interference from radio, television, motor vehicles and other radio systems in cities, especially large cities, some large automobile and home appliance companies have also used this material to build. Anechoic chamber without echo.
Polyurethane foam-based absorbing materials can also be used on stealth aircraft. For example, a kind of absorbing structure called "foam LA-1" developed by Plessey Microwave Materials Company in the United Kingdom is composed of lightweight polyurethane foam. The ~18GHz wide frequency band has good absorption performance and has been used on the fuselage and wings of stealth aircraft. In addition, polyurethane foam-based absorbing materials can also be used to build echoless boxes to cover reflective objects in the test environment, such as radar antenna cabins, antenna brackets, turntables, experimental racks, etc., to improve measurement reliability and accuracy. A domestic research institute used a wedge-shaped rigid polyurethane foam-based wave absorbing material with a thickness of 50mm to apply the inner wall surface of the ventilating waveguide and achieved an ideal effect. If the absorbing material is covered on the ground between a pair of transmitting antennas and receiving antennas, the emitted electromagnetic waves and the received electromagnetic waves can be prevented from interfering with each other. Polyurethane foam-based absorbing materials can also be used to build microwave absorbing walls to eliminate microwave pollution Wait.
In recent years, composite absorbing materials have developed rapidly. It is a combination of polyurethane foam-based absorbing materials, transition layers and ferrite plates or ferrite grid materials, as shown in Figure 3-16.
The ferrite board or ferrite grid material has particularly good absorbing performance in the low frequency range, which makes up for the insufficient absorption of the polyurethane foam-based absorbing material in the low frequency range. The transition layer is made of polyurethane foam containing a mixed wave absorbing agent. Manufactured, it plays an impedance matching role for the above two absorbing materials. Therefore, composite absorbing materials have the characteristics of small size, ultra-wideband, and high performance. For example, the use of a 0.3m thick composite absorbing material can achieve all the absorption properties of a nearly 3m thick polyurethane foam-based absorbing material, which greatly expands the effective use of the echo-free anechoic chamber. Composite absorbing materials can be used not only in EMC shielded semi-dark rooms and shielded dark rooms, but also in occasions such as ship, airport, tower, etc.
It should be pointed out that a stealth material with good absorption performance is only a necessary condition for a non-echo anechoic chamber to have a good absorbing effect, only when the overall design of the anechoic chamber is also scientific, and its shape, size, and scattered area are fully considered. Only when the requirements such as the quietness of the quiet zone are required, can the absorption effect of the echoless anechoic chamber reach the best.