How to consider the factors of electromagnetic compatibility test to build anechoic chambers

1、 Overview:

An anechoic chamber is also called an anechoic chamber, and some anechoic chambers are also called microwave anechoic chambers There is no reflection room, etc. The function of anechoic chamber is to avoid the influence of external electromagnetic waves, make the measurement activities not affected by the external electromagnetic environment, and avoid the test signal radiation from generating interference sources, polluting the electromagnetic environment, and affecting other electronic products. There are many problems that must be considered when building anechoic chambers. The details of various enterprises, enterprises or detection centers are different, so the anechoic chambers built are also different. What factors should be considered in the construction of anechoic chamber?

2、 Design anechoic chamber according to the purpose:

General anechoic chamber can be divided into: electromagnetic compatibility test anechoic chamber and antenna test anechoic chamber.

1. The key of EMC test radio room is to replace the wide field. This is a place for EMC testing. According to the standard, it is generally designed as a semi-anechoic chamber. In addition to the road surface, the other five sides of the darkroom are pasted with absorbing materials, and the road surface is reflective metal sheet. It is characterized by a wide frequency range. The national standard generally stipulates that the frequency range is 30MHz~1GHz. At present, most of the frequencies are guaranteed to be 30MHz~18GHz. The military standard frequency range is 30MHz~40GHz. The main indicators include: shielding effectiveness, site uniformity, normalized site loss and transmission loss. The electromagnetic compatibility test anechoic chamber is divided into the 3 meter method, the 10 meter method and the 5 meter method to standardize the anechoic chamber. Each enterprise, enterprise or testing center can select the appropriate anechoic chamber according to its own capital situation, available land area and common test object specifications, without copying the methods of other units.

2. Antenna test radio darkroom simulation refers to the electromagnetic environment in free space. All the absorbing materials of the hexahedron in the radio anechoic chamber are pasted in the main reflection area, which has better absorbing characteristics than other areas. The antenna applicable to the radio anechoic chamber test is generally in the microwave frequency band, so the antenna test of the radio anechoic chamber is also called the microwave anechoic chamber. Ideally, there should be no electromagnetic reflection at every position of the anechoic chamber, which is the principle of building an antenna to test the radio anechoic chamber. Although no matter how effective the customization is, how sound the building is, and how high the quality is, it is obviously impossible to have no electromagnetic reflection at every location. Therefore, when designing the anechoic chamber for antenna testing, first design a static area according to the efficient specification, frequency range and antenna characteristics of the antenna to be tested. The electromagnetic environment in the static area shall meet the needs of the antenna to be tested.

3、 Anechoic chamber characteristic antenna test:

1. Each enterprise, enterprise or testing center can design the frequency range and static area characteristics of the darkroom according to their own needs. Different from the electromagnetic compatibility test anechoic chamber, the frequency range of the antenna test anechoic chamber has specific provisions, and has the characteristics of static area. The standing wave characteristics. The transmittance of the hexahedral wall absorbing material of the anechoic chamber is higher than that of the electromagnetic compatibility test anechoic chamber. Especially for low sidelobe antennas, antennas with special requirements should be pasted by scientific calculation, unique design, scientific selection and appropriate absorbing materials.

2. The antenna test darkroom has no strict requirements for electromagnetic shielding. Some do not even need to design a separate shield to shield, but directly paste the absorption material from the wall. The shielding and absorption effect of electromagnetic wave can meet the requirements. Of course, it depends on the electromagnetic environment of the darkroom. Different electromagnetic environments have different regulations. If the electromagnetic environment around the address of the pre-built anechoic chamber is weak, which may affect the test results, or if the radiation power is large during the antenna test, which may affect the surrounding electromagnetic environment, it should be considered to build a suitable shield for shielding.

4、 Overview of common darkroom technical standards:

1. Electromagnetic compatibility test

The technical standards for electromagnetic compatibility test anechoic chamber and military standards have clear provisions. Especially in the international EMC darkroom standard, the test standard has been clearly specified. The following technical standards are for readers' reference: 1) Electromagnetic shielding characteristics frequency range shielding efficiency 14KHz ～ 1MHz ≥ 60dB, 1MHz ～ 1000MHz ≥ 90dB, 1GHz～18GHz(40GHz)≥80dB。

 2) The site uniformity is 1.5m × 75% of the field strength error on the 1.5m imaginary vertical plane needs to be between 0dB and+6dB.

3) Normalized field loss and transmission loss The error between normalized field loss and transmission loss and theoretical value is within ± 4dB.

2. There is no uniform standard for the performance requirements of antenna test anechoic chamber (microwave anechoic chamber). Generally speaking, the frequency range shall meet the requirements of the unit. In the static area of the customized antenna test anechoic chamber, the larger reflection level is 45dB lower than the main beam level, and the uniformity of the transverse and longitudinal fields is better than 1.5dB. The above performance requirements are only used to test the performance requirements of the darkroom. For high-performance antennas, low-sidelobe antennas or antennas with special requirements, the performance requirements of anechoic chamber should also be based on the actual performance requirements. If the antenna test anechoic chamber is used for far-field test, it should be noted that the anechoic chamber space meets the far-field test conditions.

5、 Design other darkroom facilities:

It is important to understand the specifications and shielding characteristics of the darkroom. In addition to absorbing electromagnetic baud, the design or use of other facilities is also very important. The quality of the characteristics of the help equipment will also impair the performance of all darkrooms. The power line and power line filter consider the power type and high-power load of the darkroom in the future, such as detection equipment. AC 380 is applied to test equipment, auxiliary equipment and its lighting system V.220V.110V, DC power supply, variable frequency power supply, etc. In particular, the EMC test darkroom has strict electromagnetic shielding regulations. If there are certain defects, it will also bring great inconvenience to the test work.

1. Test turntable. Power line wiring and power line wiring shall be equipped with necessary test turntable in the darkroom, and its characteristics shall meet the test requirements. Pay attention to separate the power cord from the power cord and properly shield it to prevent impact.

2. The signal interface board and the waveguide ventilation window signal interface board are used to assemble the signal transfer terminals of the anechoic chamber and the main control room, the signal connection between the amplifier room or the detection room for other purposes, the total installation and location of the signal interface board, and the type of connection terminals therein. The total number should be considered. The anechoic chamber must be equipped with the waveguide ventilation window. For the exhaust fan or air conditioning unit, pay attention to the frequency range and shielding efficiency of the waveguide ventilation window.

3. The darkroom monitoring system shall be assembled in the darkroom according to the size of the darkroom. The total number of cameras and the number and characteristics of monitors shall be designed and installed. It is convenient for observation and inspection. Supervise the working status of the test system and equipment to be tested.

4. The lighting facilities selected for fire alarm devices and lighting facilities shall not produce interference signals and shall be easy to disassemble and maintain. The fire alarm equipment shall not affect the overall characteristics of the darkroom.

5. Environmental protection requires that some adhesives are inevitably used for darkroom construction, especially for the production of absorbent materials adhesives. Some dark rooms will emit a pungent smell, which will adversely affect the physical and mental health of the tester. Therefore, environmental protection requirements must be put forward to the darkroom contractor as one of the acceptance regulations.

6、 Darkroom function test:

The shielding characteristics, site uniformity, normalized site loss and transmission loss of the electromagnetic compatibility test anechoic chamber shall meet the national standards and national standards, and shall be tested and ensured to meet the standards. There is no uniform test standard for antenna test anechoic chamber, but the agreed test standard. It is worth noting that the antenna used to detect the anechoic chamber should have very similar characteristics to the antenna prepared in the anechoic chamber in the future. Different antennas are used for darkroom detection (for example, the total width of antenna lobe is different). The test results will be very different. The application of narrow lobe antenna test is better than that of wide lobe antenna test, which means that the application of narrow lobe antenna test meets the standard, while the application of wide lobe antenna test does not necessarily meet the standard. Therefore, in the darkroom bidding, the type of detection antenna used in darkroom testing, including receiving antenna, should be specified in advance to prevent unnecessary disputes in darkroom acceptance.