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Shielding effectiveness test method
Shielding effectiveness refers to the ratio of the electromagnetic field strength E1 when there is no shielding material in the same place in the electromagnetic field and the electromagnetic field strength E2 when there is a shielding material. The calculation formula is carried out according to formula (2):
Where: SE—shielding effectiveness, in decibels (dB);
E1—the electromagnetic field strength without shielding material;
E2—Electromagnetic field strength with shielding material.
Laboratory environment requirements:
a) Ambient temperature: 15℃~30℃
b) Environmental relative humidity: less than 80%
Measuring equipment and facilities
The shielding room is used for 9KHz-40GHz shielding effectiveness test. The shielding effectiveness of the shielding room should be at least 6dB greater than the corresponding shielding effectiveness level. The shielding room is well grounded, and the grounding resistance should be less than 4Ω. The shielding room should be large enough, and the dimensions are shown in Figure 2 and Figure 3 (Figure 3 is only used for 1GHz~40GHz shielding effectiveness measurement).
The test window of the shielding room is square, the side length is not less than 0.6m, the applicable frequency range is 10kHz~40GHz, the center of the square hole is not less than 1m from the ground of the shielding room, and the boundary of the square hole is not less than 0.5m from the side wall. The width of the flange on the edge of the square hole is not less than 25mm, and the flange should be conductive.
If it is only used for 1GHz~40GHz shielding effectiveness measurement, the test window size can be 0.3m×0.3m, the boundary distance from the side wall is not less than 0.1m, the square hole edge flange width is not less than 25mm, and the flange should be conductive. It should be ensured that the edge of the shielded indoor antenna is not less than 0.1m from the side wall. Shielding dark boxes can also be used.
Flange is used to fix the sample in the test. Because many mechanical factors (tolerance, stiffness, fastener position and size) of the flange design will affect the shielding effect, it is necessary to confirm that the shielding room after the flange is installed has sufficient measurement dynamics before the test The range meets the shielding effectiveness level required to be tested.
The frequency range of the measurement equipment should meet the measurement frequency requirements and meet the dynamic range requirements.
The recommended antennas for each frequency band are shown in Table 2.
Table 2 Antennas used in each frequency band
Measurement frequency point requirements
The frequency sweep method is preferred when measuring. If the point frequency method is used for testing, no less than 3 frequency points should be selected for every 10 octave in the tested frequency range, and the resonance frequency point of the shielded room should be avoided.
Shielding effectiveness measurement
The sample used to measure the shielding effectiveness by the shielding room method shall meet the following requirements:
a) The area of the sample should be larger than the size of the test window of the shielded room, and the surface of the sample should be flat;
b) If the surface of the sample is not conductive, remove the non-conductive surface at the edge of the sample to expose the conductive surface, and ensure that there is a good conductive connection between the four edges of the sample and the test window when the sample is installed.
When placing the sample on the test window of the shielding room, a conductive gasket should be installed on the flange surface of the test window. The shielding effectiveness of the conductive gasket should be greater than the shielding effectiveness required by the test by more than 10dB. The edge of the sample is sealed with conductive glue, the sample is attached to the test window, the sample is clamped with a pressure clamp or the sample is fixed with a screw to ensure a good electrical connection between the sample and the test window of the shielding room, and avoid electrical contact Defects introduce measurement deviation.
The transmitting antenna is placed outside the shielding room, and the receiving antenna is placed inside the shielding room. Try not to place metal objects irrelevant to the measurement in the shielded room. During the measurement, the position of the antenna, instrument, and other objects in the shielded room remain unchanged. When measuring the magnetic field shielding in the 10kHz~30MHz frequency band, the loop antenna adopts the coaxial method; when measuring the electric field shielding in the 10kHz~30MHz frequency band, the antenna is placed vertically, and the height of the antenna should be ensured that the bottom of the antenna rod is parallel to the bottom of the test window; In the 40GHz frequency band, the measuring antenna should be vertically polarized, and the transmitting and receiving antennas should be aligned with the center of the test window of the shielding room; the dipole antenna is preferred in the frequency range of 200MHz~1000MHz during the spot frequency method test. The distance between the antenna and the shielding material should meet the requirements of Table 3.
Table 3 Distance between antenna and shielding material
Before each test, the metal surface in contact with the sample should be removed from surface corrosion and other insulating materials.
The steps of shielding effectiveness measurement are as follows:
a) Connect the measuring equipment according to Figure 2 or Figure 3, and preheat the measuring equipment for half an hour;
b) Open the test window of the shielded room;
c) Set the appropriate output amplitude of the transmitting device, and measure the indication value of the receiving device when there is no tested sample at all test frequency points;
d) Install the test sample on the test window, and lock all pressure clamps (or special screws);
e) Keep the position of the transmitting and receiving antenna unchanged, and the output amplitude of each frequency point of the transmitting device is the same as in c), and record the indication value of the receiving device when all test frequency points have the tested sample;
f) Calculate the shielding effectiveness of the tested sample at each frequency point according to formula (2).