Electromagnetic compatibility semi-electric wave anechoic chamber construction methods and requirements

The full anechoic chamber reduces the interference of external electromagnetic wave signal to the test signal. At the same time, electromagnetic wave absorbing materials can reduce the multi-diameter impact of wall and ceiling reflection on the test results. It is applicable to emission, sensitivity and anti-interference experiments. In practice, if the shielding efficiency of the shield can reach 80dB~140dB, then the interference to the external environment can be ignored, and the free space can be simulated in the full anechoic chamber. Compared with the other two test sites, the floor, ceiling and wall of the full anechoic chamber have the least reflection, the least interference from the external environment, and is not affected by the external weather. Its disadvantage is that the test space is limited and limited by cost.

Semianechoic chamber

Semi-anechoic chamber is also called electromagnetic compatibility anechoic chamber. The semi-anechoic chamber is similar to the full-anechoic chamber, and is also a six-sided box with shielding design, which is covered with electromagnetic wave absorbing materials inside. The difference is that the semi-anechoic chamber uses conductive floor and does not cover absorbing materials. The semi-anechoic chamber simulates the ideal open field, that is, the field has an infinitely good conductive plane. In a semi-anechoic chamber, because the ground is not covered with absorbing material, a reflection path will be generated, so the signal received by the receiving antenna will be the sum of the direct and reflected path signals. It is currently popular at home and abroad and an ideal EMC test site. Because this darkroom is an alternative to the open field. Therefore, the standard measurement method of open field is fully utilized.

Open area test (OAT) is an oval or circular test site, flat, open, uniform conductivity, without any reflector. The ideal open field mask has good conductivity and infinite area. The signal received between the receiving antennas at 30 MHz~1000MHz will be the sum of the direct and reflected path signals. However, in practice, although good ground conductivity can be obtained, the limited area of the open field may lead to the phase difference between the transmitting antenna and the receiving antenna. In the launch test, the use of open field is the same as that of semi-anechoic chamber.

The following focuses on the common half-wave electromagnetic compatibility anechoic chamber in China:

EMC semi-anechoic chamber is mainly used to measure electromagnetic radiation disturbance and electromagnetic radiation sensitivity, rather than an open test field for electromagnetic interference. Because the test environment of the semi-anechoic chamber needs to simulate the electromagnetic wave transmission conditions of the open test field (that is, there are only direct waves and ground reflected waves in the electromagnetic wave transmission), the size of the anechoic chamber should be based on the structural requirements of the open test field, generally divided into 10 standard m methods, 5m methods and 3m methods. There are many kinds of semi-anechoic chambers, which have great differences in functions, structural forms, material selection, installation forms, etc. However, which scheme is adopted mainly depends on the type of test required by the user, the size of the space of the object to be tested, and the test level. First, we should define the frequency range of the test, whether it is military standard test or civil standard test, manufacturer's internal pre-compatibility and diagnostic test, or third-party certification test; Secondly, we should make clear the maximum space that the test object may occupy, the main application standards of the laboratory, its own financial situation and possible expansion and upgrading needs in the future, so as to select a suitable and feasible test site and instrument and equipment configuration scheme.

There are two basic ways to build a semi-anechoic chamber: assembled and welded

The fabricated housing consists of formed steel plates (made of two curved plates on four sides) and bolted modules. There are conductive pads between any two adjacent shielding modules to ensure excellent RF shielding and electrical continuity. The method has the advantages of light structure, convenient assembly, convenient maintenance and disassembly in the future, and short construction period.

The welding shielding system is composed of large steel plates welded together to form a tight RF sealing body. The advantages of welded packaging are durability, high shielding performance, but not easy to disassemble, and suitable for fixed places. Due to the different types of shield plates of darkroom manufacturers and the different connection methods and processes between shield plates, in order to ensure good electrical contact and sealing, it is necessary to understand and compare the shield plates and their connection methods in detail, and combine the requirements of the actual construction period.

The construction of an emc laboratory is a systematic engineering based on scientific design of interface relations: test equipment → semi-anechoic chamber → parent building design. For the implementation of specific projects, the sequence is just the opposite. It has gone through the construction of the parent building → installation of semi-anechoic chamber → installation and commissioning of test equipment. Therefore, before building a semi-anechoic chamber, the parent building must be built first, including the floor area, structural bearing capacity, space size, power supply, air supply, air conditioning, water proofing, etc. First of all, the design of the parent building should consider sufficient ceiling height to meet the requirements of semi-anechoic chamber, conduction test shielding room, shielding amplifier room and other auxiliary facilities. In order to facilitate the installation, maintenance and ventilation of the darkroom, sufficient space must be reserved between the parent building and the darkroom. The floor of the parent building mainly bears the weight and load of all shields and installation parts, and special attention should also be paid. The civil ground needs to be laid carefully, and the ground waviness should not exceed a few millimeters within a few meters, as well as a high moisture-proof requirement. In addition, the turntable in the darkroom needs to be installed in a concrete pit with a certain depth, and the terrain is low, so waterproof treatment must be done. Before installing the darkroom, there is usually a high residual humidity requirement for the bearing ground. Each laboratory should also provide a grounding point with low grounding resistance. Because there are many problems with the interface between the darkroom and the parent building, and domestic construction companies have little experience to learn from some of these requirements, it is very necessary for civil engineering design units, construction units and darkroom manufacturers to communicate as much as possible.

The construction of anechoic chamber involves several aspects:

1. Size of anechoic chamber.

You should know that the anechoic chamber you want to build is a standard 10-and-a-half meter anechoic chamber or a standard 3-and-a-half meter anechoic chamber or an anechoic chamber with customized size according to your site requirements; Regardless of whether the anechoic chamber matches the shielded control room and amplifier room, what is their length, width and height, and which direction they are built in the anechoic chamber.

2. Type, size and quantity of PSD.

PSD is divided into different types, such as single-pole, double-pole and three-pole, including electric, pneumatic and manual. You should choose according to the convenience of future test equipment and personnel access and the shielding performance, durability, reliability, ease of maintenance and other factors of the screen door.

3. Type and connection mode of shield plate.

Due to the different types of shielding plates used by darkroom manufacturers to build darkrooms, the connection methods and processes between shielding plates are also different. In order to ensure good contact tightness and electrical continuity, attention should be paid to understanding and comparing the connection mode between the selected shield plate and the shield plate.

4. Shielding performance of shielded room.

The manufacturer of anechoic chamber shall provide specific shielding performance indicators (give specific values), including shielding performance indicators of magnetic field, electric field, plane wave and microwave in different frequency bands (from low frequency band to high frequency band).

5. Wave absorbing materials.

Pay attention to the manufacturer and model of the absorbing material (including ferrite absorbing material and sharp crack absorbing material); Pay attention to the composition of absorbing materials and their high temperature, high humidity and flame retardancy; Understand the long-term deformation and absorbing performance of various absorbing materials in different frequency bands, and understand their laying methods, positions and functions in the anechoic chamber.

6. Power line and signal line filters.

Consider the power supply type and maximum power of the darkroom in the future, such as whether the test equipment, test equipment, auxiliary equipment and lighting system should use 380v, 220v and 110v AC voltage, whether there are test and test equipment that need DC power supply, how many of them are, whether there are special test equipment and auxiliary equipment that need a special filter, etc, So that the supplier can prepare these power lines and signal line filters in advance.

7. Guide plate and waveguide ventilation window.

The darkroom needs to be installed with a waveguide ventilation window, which is connected with the exhaust fan or air conditioning system. Pay attention to the frequency band design of the waveguide ventilation window, and its shielding performance should meet the shielding performance of the shielding room. The guide plate is used to install terminals or signal interfaces, and cannot be installed directly through the shielding wall; The installation quantity, position and terminal quantity and type of guide plate shall be discussed with the test equipment supplier.

8. Fire alarm device and lighting facilities.

Pay attention to selecting lighting facilities that will not produce any radio interference; The installation of fire alarm device shall not affect the performance of darkroom.

9. Darkroom automation device

The automatic devices in the darkroom include antenna lifting tower, turntable and monitoring system, as well as the controller that can control them. The selection of antenna lifting tower shall consider whether the manufacturer, model and adapter are installed with antenna and supported by test equipment (that is, whether the test software of the connected test equipment supports its driver); The turntable shall also be supported by the test equipment, and the size and bearing capacity of the turntable shall be determined according to the size of the test equipment, taking into account the rotation angle and positioning accuracy of the turntable; The monitoring system includes cameras installed in the darkroom and monitors placed in the control room. The number of cameras and monitors can be selected according to actual use. The camera shall be selected to work normally in a wide frequency range and high field intensity without generating radio frequency interference signals, and installed in a no-fire area.

10. darkroom performance test and warranty training

Because the anechoic chamber is not only suitable for the emi in emc, but also can be used to test the radiation immunity of ems. Therefore, the performance test of the anechoic chamber should include the shielding performance test after the completion of the anechoic chamber shield, the field attenuation test after the completion of the anechoic chamber and the field uniformity test. These tests shall be carried out in accordance with the contract requirements and relevant international standards for darkroom construction and acceptance. Select well-known testing institutions recognized by authoritative institutions. The contract shall include the warranty of darkroom and the warranty and training of supporting equipment and parts.