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Methods of measurement for shielding effectiveness of electromagnetic shielding rooms

Release time:

2023-02-17 15:09

Source:

　　Methods of measurement for shielding effectiveness of electromagnetic shielding rooms

1, what is shielding effectiveness

Electromagnetic shielding is one of the main measures of electromagnetic compatibility technology. Shielding uses a shield to block or reduce the transmission of electromagnetic energy-that is, a metal shielding material is used to close the source of electromagnetic interference so that the external electromagnetic field strength is lower than the allowable value. At the same time, external radiation interference can be prevented from entering the shielded area.

We can use the ratio of the intensity of the electromagnetic field when there is no shielding body at the same position to the intensity of the electromagnetic field after the shielding body is added to characterize the shielding effect of metal materials, which is called shielding effectiveness (Shielding Effectiveness, SE)

In the GBT12190-2001, the definition of the shielding effectiveness of the electromagnetic shielding room is: the ratio of the signal value received without the shielding body to the signal value received in the shielding body, that is, the insertion loss caused by the shielding body between the transmitting antenna and the receiving antenna.

2, reference level measurement

When there is no shielding room, the distance between the receiving loop antenna and the transmitting loop antenna is: the sum of 0.6m and the thickness of the shielding room (this is the actual distance between the two loop antennas during actual measurement); And make the two loop antennas in the same plane (coplanar method). At this time, the measured level is the reference level.

3, dynamic range dynamic range; DR

The receiving system operates in the amplitude range of the linear region.

Note 1: For SE measurement, DR is the difference between the reference level and the minimum value of the discernible signal above the background sound.

Note 2: The minimum value of the discernable signal is defined as the amplitude greater than the background noise of the test system by at least 3dB.

4-Preparations

Prior to the measurement, the relevant parties shall develop a test plan. The test plan shall include measurement frequency points, measurement locations, decision criteria (I. e. SE limits) and a list of equipment used. The equipment in the list shall be calibrated.

For the shielded room used for full anechoic chamber (FAR) or half anechoic chamber (SAC), the SE measurement shall be carried out before the installation of the absorbing material; if not, the SE measurement shall be carried out for FAR or SAC, and the measurement position shall be specified in the test plan.

Before the formal measurement, it is necessary to measure the reference level and dynamic range (DR).

5, low frequency band measurement (9kHz ~ 20MHZ)

In the low frequency band, use the electrostatic shielding loop antenna to evaluate the SE of the shielding room to the nearby magnetic field source (I. e. local source). When the frequency extends to the low end to 50Hz, the loop antenna method is also applicable. At lower frequencies, different equipment may need to be used to obtain sufficient DR. For example, the number of turns of the receive loop antenna and/or the transmit loop antenna may be increased.

　　

For single doors, loop antenna measurements shall be made at the 14 positions shown in Figure 2a) and Figure 2b). The ring face shall be perpendicular to the door seam. For horizontal door slits, the loop antenna is required to be located in the middle of the corner and the door slit; for vertical door slits, the loop antenna is required to be located at the corner, 1/3 from the top of the door and the bottom of the door. The upper and lower ends of the vertical joint shall be measured as shown in Figure 2b). For multiple doors, the above measurement positions are applied to each door, see Figure 2b) and Figure 2c).

　　

In the low frequency band (50Hz ~ 20MHz),SE can be expressed by magnetic field, see formula (1)

SEH = 20lg(H1/H2) ··············································································································

In the formula:

SEH-magnetic field shielding effectiveness, in decibels (dB);

H1-magnetic field strength (reference reading) in unshielded room, in microamperes per meter (μA/m);

H2-magnetic field strength in the shielding chamber, in microamperes per meter (μA/m).

If the indicated values of the detection instrument proportional to the magnetic field strength H1 and H2 are voltage readings V1 and V2, the formula (1) can also be more conveniently expressed as formula (2):

SEH = 20lg(V1/V2) ··············································································································

In the formula:

SEH-magnetic field shielding effectiveness, in decibels (dB);

V1-voltage reading (reference reading) in unshielded room, in microvolts (μV);

V2-the voltage reading in the shielding chamber, in microvolts (μV).

6, resonant band measurement (20MHz ~ 300MHz)

This method directly measures the influence of the electromagnetic emission source (I. e. local source) on all accessible surfaces of the shielded room. The incident field may not be a plane wave, especially at the low end of the frequency range. The geometry and physical dimensions of the shielded room may have a significant impact on the measurement results. The lowest resonant frequency of most shielded rooms is in this band (20MHz ~ 300MHz), so these frequency points should be avoided as much as possible during measurement.

 

Shielding effectiveness in the resonant band (20MHz ~ 300MHz)

In the resonance frequency band, SE can be expressed in the form of electric field strength or power, see equation (3) and equation (4)

SEH = 20lg(E1/E2) ;

SEH-electric field shielding effectiveness, in decibels (dB);

E1-electric field strength in unshielded room, in microvolts per meter (μV/m);

E2-electric field strength in the shielding chamber, in microvolts per meter (μV/m).

Or expressed:

SEH = 10lg(P1/P2) ··············································································································

SEH-electric field shielding effectiveness, in decibels (dB);

P1 -- Power in unshielded room, in milliwatt (mW);

P2-Power in the shielding chamber, in milliwatts (mW).

Shielding effectiveness in high frequency band (300MHz ~ 100GHZ)

In the high frequency band (300MHz ~ 100GHz),SE can be directly calculated by formula (2), formula (3), formula (4).

　　

Shielding Effectiveness Test of Century Huize 1m Anechoic Chamber

7. Concluding remarks

The shielding effectiveness of electromagnetic shielding room is an important basis for measuring the performance of shielding room. There will always be doors, covers, instruments, switches and other holes on the shielding body, as well as a variety of wiring penetration, these are varying degrees of destruction of the integrity of the shielding. According to the experience of Century Huize real electromagnetic compatibility system integration, the shielding effectiveness (SE) of the anechoic room/shielding room reaches more than 90dB in the frequency band of 10kHz ~ 150kHz and more than 110dB in the frequency band of 150kHz ~ 18GHz. The shielding effectiveness of the chassis used in the electromagnetic compatibility test of the Century Huize Suzhou Laboratory for civilian products is generally about 40dB; the shielding effectiveness of the chassis used for military products is usually above 60dB.

Century Wisdom (Suzhou) Testing Technology Co., Ltd

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