The shielded room is just a big iron box, which blocks the indoor and outdoor radio signals. In general, it is the inside that can't get out and the outside can't get in, but the electromagnetic waves inside will be superimposed on the inner wall.
The anechoic chamber is based on a shielded room. The inner wall is provided with a wave absorbing material to simulate the effect of an open field. The dark room is more expensive than the shielding room and it is expensive in the dark room. The electromagnetic waves emitted into the inner wall will be absorbed, and the mixed wave effect of reflection superposition will not occur. Radiated emission interference suitable for test samples. The darkroom is generally divided into an all-electric darkroom and a semi-electrical darkroom.
Tests with low requirements for the experimental environment, such as conducted disturbances, electrostatic tests, surge tests, and lightning strike tests, are all performed through the power line. Therefore, it is sufficient to perform only in a shielded room. For space radiation and space disturbance, Space propagation harassment or anti-jamming has special requirements for space, so it needs to be conducted in a dark room to simulate the space of an open space.
All radio anechoic chambers, semi-electrical anechoic chambers, and open fields, radiation tests conducted in these three types of test sites are generally considered to be in conformity with the propagation laws of electromagnetic waves in free space.
All-electric darkroom
The full-wave darkroom reduces the interference of the external electromagnetic signal on the test signal. At the same time, the electromagnetic wave-absorbing material can reduce the influence of multipath caused by the reflection of the wall and ceiling on the test result, and is suitable for launch, sensitivity and immunity experiments. In actual use, if the shielding effectiveness of the shield can reach 80dB~140dB, the interference to the external environment can be ignored, and the free space situation can be simulated in the all-air darkroom. Compared with the other two test sites, the floor, ceiling and walls of the all-electric wave darkroom have the smallest reflection and the least interference from the external environment, and are not affected by the outside weather. Its disadvantage is that it is limited by the cost and the testing space is limited.
Semi-electrical darkroom
The semi-electrical anechoic chamber is similar to the all-electric anechoic chamber and is also a shielded six-sided box. The interior of the chamber is covered with electromagnetic wave absorbing materials. The difference is that the semi-electrical anechoic chamber uses a conductive floor and does not cover the absorbing material. The semi-anechoic chamber simulates an ideal open field scenario, ie the site has an infinitely good conductive ground plane. In the semi-anechoic chamber, since the ground does not cover the absorbing material, 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.
Open field
The open field is an elliptical or circular test site that is flat, open, well-conducted, and free of any reflectors. The ideal open field ground has good conductivity and an infinite area. The receiving antenna is received between 30MHz and 1000MHz. The signal will be the sum of the direct and reflected path signals. However, in practical applications, although good ground conductivity can be obtained, the area of ​​the open field is limited, and it may cause a 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 the semi-electrical darkroom.
Each test item of electromagnetic compatibility requires a specific test site, and the requirements of the site are the most stringent in terms of radiation emission and radiation immunity test. Since the transmission and reception of high-frequency electromagnetic fields of 80 to 1000 MHz is entirely based on the theory that spatial direct waves and ground reflected waves overlap each other at the receiving point. The site is not ideal, it will inevitably bring about a large test error.
The open test field is an important electromagnetic compatibility test site. However, due to the high cost of the open test site and its remoteness from the urban area, it is inconvenient to use it; or it is built in the urban areas, and the background noise level is large, which affects the EMC test. Therefore, indoor shielded rooms are often used instead. However, the shielded room is a metal enclosure with a large number of resonant frequencies. Once the radiated frequency and the excitation mode of the device under test cause the shielded room to resonate, the measurement error can be as high as 20 to 30 dB. Therefore, it is necessary to surround the wall and top of the shielded room. Mounted on the wave absorbing material, the reflection is greatly reduced, that is, the direct wave and the ground reflected wave are transmitted when the wave propagates, and the structure size is also based on the requirements of the open test field, so that the outdoor open field test can be simulated. This is the electromagnetic shielding. The anechoic chamber, also referred to as the EMC darkroom, has become a common EMC test site. US FCC, ANCI C63.6—1992, IEC, CISPR, and GJB152A-97, GJB2926-97 “Electromagnetic Compatibility Testing Laboratory Approval Requirements†and other standards allow the use of electromagnetically shielded semi-electrical anechoic chambers as an alternative to open test sites for EMC testing .
The EMC dark room structure is usually composed of RF shielded room, wave absorbing material, Power Supply, antenna, turntable and other parts: the RF shielded room ensures that the test is free from external interference; the absorptive material ensures the absorption characteristics of the dark room; the antenna and turntable guarantee the measured object. Test according to the conditions and conditions required by the standard; the power system guarantees test power. Auxiliary equipment such as RF screen doors, ventilated waveguide windows, cameras, lights, and power boxes should be designed to be placed outside the main reflection area as much as possible to prevent any metal parts from being exposed to the main reflection area.
The dark room floor is the only reflective surface of the electromagnetic wave. The requirements for the floor are: continuous smooth without bumps. There must not be a gap of more than 1/10 of the minimum operating wavelength to maintain the continuity of the floor. The dark room ground wire and power cord should be laid on the wall foot, do not cross the room, the wire should also wear a metal tube, and keep the metal tube and the floor good lap. In order to avoid electromagnetic wave reflections affecting the measurement error, people and test control equipment should not be in the test site. Therefore, the general EMC darkroom is composed of a test darkroom and a control room. The test darkroom houses the test antenna and the device under test. The operator and test control equipment are in the control room. If there is a high-power amplifier device, an amplifier room should also be set up to place these devices so as to avoid interference with the surrounding environment. The darkroom and the control room must each adopt an independent power supply system, use different phases of the power supply, and pass through their own filters to avoid interference from the control room through the power line into the dark room.
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