Preliminary Study on Factors Affecting Shielding Effectiveness of Metal Fiber Blended Fabric

Preliminary Study on Factors Affecting Shielding Effectiveness of Metal Fiber Blended Fabric

Electromagnetic shielding fabric is an anisotropic shielding material, and its shielding effectiveness is affected by many factors. To obtain the shielding effectiveness value of a shielding fabric, it is mainly through experimental testing. Analysis of various factors affecting the shielding effectiveness of fabrics is conducive to deepening the understanding of the shielding mechanism of fabrics and guiding the design of electromagnetic shielding fabrics. This paper introduces the shielding mechanism and shielding effectiveness of metal fiber shielding fabrics, and discusses the main factors affecting the shielding effectiveness of metal fiber blended shielding fabrics.

1 shielding mechanism and test of electromagnetic shielding fabric

Fabrics without shielding materials do not have the function of electromagnetic screen. The protective effect of the metal fiber barrier fabric is that the metal fibers are active. The metal fiber-containing yarns are interwoven to form a criss-crossing isolation net to attenuate the energy of the electromagnetic wave to a certain extent, thereby achieving the purpose of protection.

At present, there are a variety of test methods for shielding fabric shielding effectiveness at home and abroad. The test standards are mainly "Environmental Electromagnetic Wave Sanitation Standard GB9175-88", "Workplace Microwave Radiation Hygiene Standard BG10436-89" or tested according to ASTM regulations. The test methods mainly include the far field method, the near field method and the shielded room test method. These professional testing methods are more complicated, and the instruments and equipment used are expensive. If you only need to initially determine the shielding effectiveness of the shielding fabric, you can use the following simple method: cover the mobile phone signal receiving line with shielding fabric, wait 4-6 minutes, observe the mobile phone screen. Shows how the signal strength changes.

2 factors affecting the shielding effectiveness of electromagnetic shielding fabrics

The shielding effectiveness of metal fiber blended shielding fabrics is related to many factors, such as electromagnetic radiation source, radiation distance, metal fiber type, shielding layer number, and shielding fabric structure parameters. The following mainly discusses the factors affecting the shielding effectiveness of metal fiber blended shielding fabrics from the structural parameters of the shielding fabric.

2.1 Influence of warp and weft yarn difference on shielding effectiveness

Whether the warp and weft yarns of the blended shield fabric contain metal fibers has a great influence on the overall shielding effectiveness of the fabric. Generally, when the metal fibers are contained in both warp and weft directions, the shielding effectiveness of the fabric is higher than that of the unidirectional metal fiber. This is because the warp and weft directions contain metal fibers, which constitute a relatively complete crisscross in the fabric. The metal shields the conductive mesh, which can better prevent the electromagnetic wave from propagating and thus the electromagnetic wave energy. The screen fabric composed of the unidirectional metal-containing fiber-shielding fabric is equivalent to the metal fiber arrangement in only one direction, so the shielding effectiveness is poor. The unidirectional and bidirectional metal fiber-containing shielding fabric has a great influence on the overall shielding performance. To achieve a better shielding effect, it is preferable to adopt a yarn containing metal fibers in the warp and weft directions.

2.2 The influence of organizational structure and thickness on shielding effectiveness

From the perspective of the structure of the shielding fabric, the fabric is divided into a single layer structure and a multilayer structure. For a simple single-layer structure, when the unit area contains the same metal weight, the shielding effect of the twill weave is better than that of the satin, and the shielding effect of the plain weave structure is better than the twill. Because of the high number of interlacing, the structure is tight, the number of holes and gaps is small, and the electromagnetic wave transmission is small. The satin weave has a long latitude and longitude of the fabric, and the number of interlacing is the least. The conductive shielding net is looser and the conductive performance is more. Poor, so the shielding performance is poor. The twill is bounded between the two. For multilayer fabrics, such as in heavy and weft-weighted fabrics, the layer shielding of the multilayer metal conductive screen reduces the final electromagnetic wave transmission. In general, shielding materials are relatively easy to shield the electric field, and shielding the magnetic field is much more difficult, especially for low frequency magnetic fields. Designing heavy and weft-weight multilayer shielding fabrics is a good method for shielding magnetic fields. The shielding of the single layer fabric from the electric field and the high frequency magnetic field may be carried out according to other measures to achieve the shielding purpose (such as increasing the content of the metal fiber in the fabric), but in the case of a low frequency magnetic field, a more severe diffraction phenomenon will occur. If a multi-layer shielding fabric is designed, different shielding materials can be selected for layer shielding according to requirements.

Due to the difference in fabric structure and the number of yarn counts, the thickness of the fabric is different, and the difference in thickness also affects the shielding effectiveness. When electromagnetic waves are incident on the shielding fabric, firstly, reflection and transmission occur at the surface interface. Electromagnetic waves entering the shielding fabric will be internally reflected multiple times at the interfaces inside the fabric, thereby depleting the electromagnetic wave energy, so the thickness of the fabric also has a great influence on the shielding effectiveness. .

2.3 The relationship between metal fiber distribution and yarn structure and shielding effectiveness The fabric made of stainless steel short fiber blended yarn and the fabric made of stainless steel filament blend have different shielding efficiency.

The metal fiber content of W1 is much higher than that of W2, but in Figure 1, the shielding effectiveness of W2 is significantly better than that of W1. This reflects the fact that fabrics made from different stainless steels have their best shielding effectiveness values in different frequency bands. When the frequency is relatively low, that is, lower than 500MHz, and relatively higher frequency band, that is, higher than 2000MHz, the fabric screen made by the stainless steel staple fiber blended yarn is better than the stainless steel filament blended yarn. Fabric.

In addition, the structure of the stainless steel filament blended yarn also has a great influence on the shielding effectiveness of the fabric. In the covered yarn, the stainless steel filaments regularly coat the short fibers along a spiral shape; in the strands, the stainless steel filaments and the short fibers are entangled; in the core spun yarn, the stainless steel filaments are located inside the staple fibers. In the core spun yarn, the conductive mesh formed of the stainless steel filament has a smaller hole or stitch, the strand is next, and the covered yarn is the largest. According to the electromagnetic shielding theory, an important reason for the pore conduction is that the impedance at the slit or the hole changes, and this change is particularly remarkable at a high frequency. Since the pores affect the distribution of the power line and the flux density line on the metal grid, the high-frequency induced current path is interrupted, resulting in electrical discontinuity and thus a decrease in shielding effectiveness. From the shielding mechanism of the fabric and the electromagnetic wave shielding theory, it is known that a hole or a slit having a certain depth can be regarded as a waveguide, and the waveguide can attenuate the propagated electromagnetic wave under certain conditions. The hole or slit in the shielding fabric corresponds to a waveguide operating below the cutoff frequency (Fco) (in inverse proportion to the line of the hole or slit), since the cutoff frequency is determined by the hole or slit in the shielding fabric (line length) The size of the direction, not the size of the area, increases as the frequency increases until it approaches the cutoff frequency. Between Fco/3 and Fco, the attenuation decreases, and the shielding performance at Fco is close to 0 dB. The cutoff frequency Fco of the electromagnetic wave polarized in the direction of the length of the pore mainly depends on the size of the long side of the pore rather than the short side. Therefore, the 孑L gap line is better shielded by the smallest core-spun yarn, followed by the strand and the covered yarn. Poor. It can be seen that the metal fiber content per unit area does not necessarily shield the performance, because the distribution of the metal fibers and the yarn structure also have an effect on the shielding effectiveness.