The microwave anechoic chamber should have an appropriate size to maintain a uniform field in a sufficient space relative to the EUT. The microwave anechoic chamber is suitable for both small electronic equipment and large electronic equipment's radiated electromagnetic field immunity test and radiated electromagnetic field emission measurement. The microwave anechoic chamber is made of absorbing materials to create a closed space, so that a pure electromagnetic environment can be created in the anechoic chamber to facilitate the elimination of external electromagnetic interference. Microwave anechoic chamber materials can be all absorbing materials, the main material is polyurethane absorbing sponge SA (for high frequency use). In addition, when testing the electromagnetic compatibility of electronic products, ferrite absorbing materials will also be used due to low frequency.
Microwave anechoic chamber construction-the radiation test conducted in these three test sites can generally be considered to conform to the propagation law of electromagnetic microwave anechoic chamber in free space.
1. All-electric microwave anechoic chamber:
The all-electric microwave anechoic chamber reduces the interference of external electromagnetic wave signals on the test signal. At the same time, electromagnetic wave absorbing materials can reduce the multipath effect caused by the reflection of walls and ceilings on the test results. It is suitable for emission, sensitivity and immunity experiments. . In actual use, if the shielding effectiveness of the shield can reach 80dB~140dB, then the interference to the external environment can be neglected, and the situation of free space can be simulated in an all-electric microwave anechoic chamber. Compared with the other two test sites, the all-electric microwave anechoic chamber has minimal reflections on the ground, ceiling and walls, minimal interference from the external environment, and is not affected by the external weather. Its disadvantage is that it is subject to cost constraints and limited testing space.
2. Semi-electric microwave anechoic chamber:
The semi-electric microwave anechoic chamber is similar to the full electric anechoic chamber. It is also a shielded six-sided box covered with electromagnetic wave absorbing material. The difference is that the semi-electric microwave anechoic chamber uses a conductive floor and does not cover the absorbing material. The semi-electric microwave anechoic chamber simulates an ideal open field situation, that is, the field has an infinite and good conductive ground plane. In the semi-electric microwave anechoic chamber, since the ground is not covered with absorbing materials, a reflection path will be generated, so that the signal received by the receiving antenna will be the sum of the direct path and the reflected path signal.
3. Typical open field:
It is a flat, open, elliptical or circular test site with uniform conductivity and no reflectors. The ideal open field ground has good conductivity and has an infinite area. The signal received by the antenna is received between 30MHz and 1000MHz. Will be the sum of the direct path and reflected path signals. However, in practical applications, although good ground conductivity can be obtained, the area of the open field is limited, which may cause a phase difference between the transmitting antenna and the receiving antenna. In the emission test, the use of the open field is the same as the semi-electric microwave anechoic chamber.
Microwave anechoic room construction equipment-In general, when ignoring the path attenuation of microwaves, it can be considered that the quietness of the anechoic chamber is mainly determined by the reflection performance of the front absorbing material at normal incidence. That is to say, when the quietness of the microwave anechoic chamber is better than -38dB at the lower limit of the frequency, it is required that the reflection performance of the front absorbing material at normal incidence is better than -38dB. Therefore, the front of the microwave anechoic chamber, the end wall facing the transmitting antenna and the other four sides of the Fresnel zone use WX-1500 absorbing materials; the selection and layout of other absorbing materials use WX-1200 absorbing materials on the wall.