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Discussion on the radiation emission experiment of 18 m bus

Release time:

2016-05-20 00:00

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　　Both the 3-meter test and the 10-meter test are used as an open field substitute. Since the wavelength of electromagnetic wave is inversely proportional to the frequency, for 30MHz~100MHz, the wavelength is in the range of 3~10 meters, and the test distance of 3 meters falls within 1 wavelength, which belongs to the category of near field (but for the convenience of research, the industry unified to "wavelength /2π" to make the near and far field boundary). This near-field region is mainly based on induction, and induction is mainly based on magnetic field. If measured by electric field antenna, obviously the measured value will be much smaller (in fact, the interference caused by both induction and radiation is collectively called radiation-EMI). As a result, EMI measured below 100MHz can be said to be inaccurate (because magnetic fields, which affect the larger induction field, are not measured).

　　If the 3-meter method is used to test less than 100M, then the requirements on the site are very strict, because the minimum size of the 3-meter method is 9*6*6 meters, which is clearly stipulated in the standard. If the 3-meter test field is smaller than the standard size, there may be a large error.

　　If the 10-meter method is used to test, the frequency above 30M is in the far field (electromagnetic wave is dominated by electric field radiation, which can almost ignore the influence of induction). If everyone uses the 10-meter method to test, the consistency above 30MHz will be better. However, for the standard 3-meter test, the test distance of the larger test equipment (EUT) is less than 3 meters. Because the 3-meter distance is actually calculated from the geometric center of the EUT, the center of the turntable, whereas the standard test distance is calculated from the outside of the EUT, the deviation is larger.

　　That's why the 10-meter method makes more sense as an alternative test ground.

　　The 18-meter bus obviously belongs to the super large equipment, and the detection of the large equipment has always been the difficulty of electromagnetic compatibility. It is also difficult to distinguish large equipment. "GBZ 6113.205-2013 Radio Disturbance and Immunity measuring equipment and measuring methods Part 2-5 Field Measurement of disturbance emission by large Equipment" describes large equipment as follows:

　　Physically large equipment. Physically large equipment

　　A large commercial facility formed by a group of functionally linked devices that can be clearly viewed as a whole separate from the surrounding environment.

　　Note 1: Equipment may be considered large equipment when its overall size exceeds the range that can be tested on a conventional 10m test site.

　　Note 2: Large devices can be seen as isolated from the environment and other external systems through a disconnecting imaginary interface.

　　Note 3: For the purposes of this instructional technical document, components of large equipment refer to objects such as equipment, instrument units or subsystems. They are related to each other to achieve the performance goals of one or a group of functions.

　　This description is relatively vague. For example, note 1, with the development of radio anechoic chamber technology, many 10-meter anechoic chambers can achieve a clear zone size of 6 meters or even 8 meters. However, for radiation emission testing of large equipment, In the standard "YDT 1633-2007 Field Test Method for Electromagnetic Compatibility", a test distance of 30 meters is clearly recommended. See the literature as follows:

　　A.5.3 Testing Distance

　　GB4824 requires that the distance between the test antenna and the device under test is 30m.

　　If the ambient noise is too high or for other reasons, the field strength cannot be measured at a distance of 30m, then you can choose a closer distance, such as 10m or 3m.

　　When the distance changes by 10 times, a proportional coefficient of 20dB, which is inversely proportional to the distance changes, is used to normalize the measurement results to the distance specified in the standard to determine whether the requirements are met. (For example, reducing the antenna distance from 30m to 10m means increasing the emission limit specified in EN61000-6-4 by 10dB. Reducing the distance from 30m to 3m means raising the corresponding emission limit by 20dB.)

　　If the device under test is larger than 3m, be very careful when measuring at 30MHz, as the near-field effect will make the measurement larger.

　　A dark room that meets the 30-meter test is clearly an impractical plan. Space, including budget, is limited. Therefore, under the circumstances of limited space and budget, the 10m method is obviously a more scientific and reasonable scheme to test the 18m bus.

　　For the height of the test antenna, "GBZ 6113.205-2013 Radio Disturbance and Immunity measurement equipment and measurement methods Part 2-5 Field Measurement of disturbance emission by large Equipment" also has very clear instructions:

　　6.3.3 Measurement based on fit for purpose

　　It is recommended that the radiation emission measurement should be carried out according to GB/T6113.203, and the conformance should be assessed according to the corresponding limits at the distance specified in 6.1.

　　Note 1: Due to the imperfection of the measurement arrangement compared to the standard test site, such as the presence of reflections, the obtained measurements may not correspond directly to those theoretical values obtained at the standard test site and this should be taken into account when evaluating the results.

　　There are still some aspects that need further consideration.

　　It is recommended to change the height of the measured antenna within the specified range to obtain the maximum reading. When the measured distance is less than or equal to 10m, the general rules are as follows: When measuring the electromagnetic field, the antenna height should vary from 1 to 4m; when the measured distance is greater than or equal to 30m, the antenna height should vary from 2 to 6m. These changes apply to both horizontal and vertical polarization.

　　In extreme cases, where the device under test is installed high above the ground and the potentially interfered device is also located at a similar height, the above height may need to be taken into account when the measuring antenna is placed in the actual test.

　　When the position of the device under test differs greatly from that of the measuring antenna on the ground, it may be necessary to tilt the antenna in accordance with the direction of the antenna for maximum reading.

　　Note 2: The Angle of inclination should not exceed 70 degrees.

　　It is recommended to perform the tests in different locations around the test equipment. The selection of the number of locations should take into account the site condition and the physical size of the test equipment.

　　Based on the above standards, it can be seen that for the test of an 18-meter bus, the 10-meter method can meet the requirements for antenna movement within 1-6m height, including horizontal polarization and vertical polarization.

　　Test layout diagram for the locomotive

　　Layout diagram for side test

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