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Effect of test table on measurement of radiation disturbance


  Abstract: The influence of the test table on the radiation disturbance test was studied experimentally, and the causes and solutions of the differences in test data were found. The experimental results show that the testing table with low dielectric constant should be used to reduce the uncertainty. The research has certain reference value and guiding significance for the measurement of radiation disturbance in electromagnetic compatibility test.

  Key words: radiation disturbance; Normalized field attenuation; Dielectric constant

  Introduction

  With the popularization and implementation of electromagnetic compatibility standards, people pay more and more attention to the electromagnetic compatibility of products. Major electronic equipment manufacturers also pay more and more attention to the electromagnetic compatibility test of products, especially whether the electromagnetic radiation harassment meets the relevant standards. In the radiation disturbance test, the influence of the site on the test results is very obvious. In different test sites, the same instrument will get different measurement results, so there are differences in the test data of each darkroom. EN55022:2010 is a European test standard designed to provide uniform requirements for radio disturbance levels of information technology equipment within applicable areas, specifying disturbance limits, measurement methods, operating conditions and processing requirements for results. For EN 55022:2010 radiation harassment tests, bench devices are required to be placed on non-metallic tables, as shown in Figure 1. The standard only mentions that the size of the table is usually 1.5m×1.0m, and the material of the test table is not specified. Due to the different dielectric constants of the test tables made of different materials, the results of radiation disturbance test are different. In this paper, the influence of the test table on the measurement of radiation disturbance is quantitatively analyzed.

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  1. Influence of test table on site performance

  EN 55022:2010 requires that radiation disturbance tests be conducted in an open test area. The open test site should be flat, free of overhead power lines, free of nearby reflectors and large enough to allow the antenna to be placed at the specified distance and with sufficient spacing between the antenna, the equipment under test and the reflected object. However, with the development of society, it is very difficult to find an ideal site that meets the requirements, so the anechoic chamber is widely used as an alternative site for open test sites. According to the standard, normalized field attenuation (NSA) is a key indicator of whether the anechoic chamber can achieve effective results. The anechoic chamber is built to simulate an open test site, and the difference between the normalized field attenuation of the anechoic chamber and the open field should be less than 4dB to demonstrate the degree of similarity between the two.

  The normalized field attenuation test method was adopted to verify the influence of the test table on the test results, as shown in Figure 2.

  In a 10m darkroom without a test table, a signal source is used to transmit electromagnetic waves through a double-cone antenna (frequency 30~250MHz) and a logarithmic antenna (frequency 250~1000MHz) respectively. A set of field attenuation data is measured using a receiver and another set of biconical and logarithmic antennas. Normalized site attenuation calculation formula:

  AN = VT-VR-AFT-AFR-ΔAFTOT

  Where: VT -- input voltage of transmitting antenna, dBμV; VR -- receiving antenna output voltage, dBμV; AFT -- antenna coefficient of transmitting antenna, dB; AFR -- antenna factor of receiving antenna, dB; ΔAFTOT -- mutual impedance correction coefficient, dB(only for dipole antenna measurements at distances of 3 m, other than ΔAFTOT = 0)

Table 1. Field attenuation measured by bicone antenna and logarithmic antenna

In Table 1, Aideal is the standard normalized site attenuation value, and the deviation is Aideal minus AN, both of which are less than 4 dB. Then, the other two groups of site attenuation data were measured respectively in the case of foam table and wooden table, as shown in Figure 3 and Figure 4.

北京世纪汇泽科技有限公司

北京世纪汇泽科技有限公司

 

Figure 3. One of the measurements of foam table site attenuation data                        Figure 4. One of the attenuation data measurements of the wooden table site

  The three groups of site attenuation data were compared, as shown in Figure 5.

北京世纪汇泽科技有限公司

  

 

 

 

                   

Figure 5 Comparison of site attenuation data 1

  2. Analysis of the influence of test table on the measurement results of radiation disturbance test

  Now the same signal source is placed on the foam table (as shown in Figure 6) and the wooden table (as shown in Figure 7) respectively, and the receiving antenna measures the radiation disturbance at the distance of 10 m from the signal source.

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FIG. 6 Foam table site attenuation data measurement II                                                     Figure 7 Attenuation data measurement of wooden table site II  

  The test results are shown in Figure 8.

北京世纪汇泽科技有限公司

 

 

 

 

 

 

 

                              

 

  FIG. 8 Comparison of site attenuation data II

  

  As can be seen from Figure 8, there is a great difference in the radiation disturbance test results in the frequency range of 700~900 MHz, and the difference at 800 MHz reaches 5.3dB. However, in GB/T 6113.402-2006, Part 4-2 of the Specification for Measuring Equipment and Methods of Radio Disturbance and Immunity: Uncertainty, Statistics, and Limit Modeling Uncertainty of Measuring Equipment and Facilities mentioned that the maximum allowable uncertainty value of radiation disturbance measurement is 5.2 dB. It can be seen that the difference of radiation disturbance caused by different material test tables exceeds the uncertainty specified in the standard.

  3 Conclusion

  It is not difficult to see from FIG. 5 and FIG. 8 that the frequency of maximum difference between test tables of different materials is the same, which is between 700MHz and 900MHz.

  According to the above formula for calculating normalized field attenuation, it can be concluded that:

  VR = VT-AFT-AFR-ΔAFTOT-AN

  Because the dielectric constant of foam is close to air and smaller than that of wood, the site attenuation data measured with foam table in dark room is similar to that without test table. When wooden tables are used in dark room, there is a great difference between the measured site attenuation data and that without test tables. Due to the large dielectric constant of the wooden table, the attenuation of AN in the dark room field using the wooden table in FIG. 5 is relatively large. Under the condition that AFT, AFR and ΔAFTOT remain unchanged, the maximum measurement level value VR measured by the receiver will decrease.

  Therefore, when auxiliary facilities such as wooden tables or test tables made of large dielectric constant materials are used in the radio anthems, AN attenuation of the site will increase, resulting in a decrease in the intensity of the electric field measured by the receiver, resulting in differences in test results. Auxiliary equipment such as test table is an integral part of the effectiveness of the test site. Therefore, it is suggested that materials with small dielectric constant should be selected when selecting auxiliary facilities in the dark room, and the site attenuation should be measured and compared to ensure the consistency of measurement results in each dark room.

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