What effect does the experimental table have on the measurement in EMC testing

With the promotion and implementation of electromagnetic compatibility specifications in various countries, people are increasingly concerned about the electromagnetic compatibility of goods. Major electronic equipment manufacturers are also increasingly concerned about the electromagnetic compatibility testing of goods, especially whether electromagnetic radiation interference meets the requirements of relevant standards. In radiation disturbance tests, the impact of location on test results is very significant. The same instrument and equipment will obtain different measurement results at different test locations, so the data testing in each darkroom will vary. EN55022:2010 is a unified test specification developed in Europe to address the radio disturbance levels of information technology equipment in the field of application, stipulating disturbance limits, measurement methods, operating standards, and results. In EN's 2010: 2010 radiation disturbance test, the desktop device was placed on a non-metallic table, as shown in Figure 1. The specification only mentions the desktop size of. 5m × 1.0m, but there is no explicit specification for the material of the test bench. Due to the different dielectric constants of test benches made of different materials, the radiation disturbance test results are different. This paper quantitatively analyzes the impact of radiation disturbance measurements.

Figure 1. Table EUT Arrangement in Radiation Disturbance Test

Impact of test bench on site characteristics

EN55022:2010 requires conducting radiation disturbance tests on a wide test site. The wide test site should be flat, without empty cable racks, and surrounded by reflectors. This area is large enough to handle the antenna at the required distance, and to provide sufficient clearance between the antenna, test equipment, and reflective objects. However, with the development of society, it is difficult to find an ideal place that meets the requirements. Therefore, the radio anechoic chamber has been widely used as an alternative to a wide testing ground. The specification requires a single local attenuation NSA. This is a key indicator to confirm whether a radio anechoic chamber can achieve effective results. The radio anechoic chamber is built to simulate a width testing venue. The difference between a radio anechoic chamber and a wide area should be less than 4 dB to confirm the similarity between the two.

The impact of the test table on the test results is shown in Figure 2.

图2

Under the premise that there is no detection station in the anechoic chamber at 10m, most antennas (250 frequency)~1000MHz) are used to transmit electromagnetic waves based on the biconical antenna using a signal source (frequency 30)~250MHz. One set of site attenuation data is measured by a receiver, another set of biconical antennas, and most antennas. Calculation formula for unified site attenuation:

AN = VT - VR - AFT - AFR - Δ AFTOT

Where: VT - input voltage of transmitting antenna, dB μ V； VR - receiving antenna output voltage, dB μ V； AFT - antenna coefficient of transmitting antenna, dB; AFR - antenna coefficient of receiving antenna, dB;

Δ AFTOT - mutual impedance correction coefficient, dB (only applicable to measurements using dipole antennas with a measurement distance of 3 m, Δ AFTOT = 0）

Table 1. Site attenuation data measured by biconical and logarithmic antennas

In Table 1, Aideal is the standard normalized site attenuation value, with the deviation of Aideal minus AN, and both are less than 4 dB.

Then, the other two groups of field attenuation data were measured with foam table and wooden table, as shown in Figure 3 and Figure 4.

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

Figure 4. One of the attenuation data measurements for a wooden table field

Figure 5. Comparison of Site Attenuation Data

Epilogue

Since the dielectric constant of foam is close to that of gas and smaller than that of wood, the attenuation data measured when using a foam table in the darkroom is similar to that in the place without a detection table; When using a wooden table in a darkroom, the measured local attenuation data are significantly different from those without a detection table. Due to the large dielectric constant of the wooden table, the dark room attenuation AN of the wooden table used in Figure 5 is slightly larger. At AFT, AFR, Δ On the premise of maintaining the same AFTOT, the larger VR measured by the receiver will decrease.

Therefore, when the anechoic chamber uses auxiliary equipment such as a wooden table or a test table made of other dielectric constant materials, it can increase the field attenuation AN, resulting in a decrease in the field strength measured by the receiver, leading to differences in test results. Auxiliary equipment such as a test bench is an integral part of the effectiveness of the test site. Therefore, it is recommended to select a material with a small dielectric constant and conduct field attenuation measurements when selecting darkroom auxiliary equipment to ensure the consistency of measurement results for each darkroom.