Analysis Of The Effectiveness Of Metal Fiber Blended Shielding Fabric

Electromagnetic waves have been widely used in radar, communications, medical industry, and other electronic devices in the broadband range of 0Hz~400GHz.

The microwave refers to the electromagnetic wave whose frequency is about 300MHz~300GHz, that is, the wavelength is in the range of 1m~1mm.

Microwave has the characteristics of high frequency, wide frequency band, short wavelength, high directivity and high resolving power.

Microwave shielding fabrics are mainly used to shield electromagnetic radiation from frequencies ranging from tens of MHz to GHz.

The research shows that the metal fiber blended fabric has good shielding effect on microwave radiation.

On the basis of analyzing the shielding mechanism of metal fiber shielding fabric, the distribution of metal fiber in yarn, the content of metal fiber and

The relationship between tightness and shielding effectiveness of fabrics is discussed.

Shielding mechanism of 1 electromagnetic shielding fabric

The fabric without shielding material has a volume resistivity of more than 1010 ohm cm and does not possess electromagnetic shielding function in itself.

The protective effect of metal fiber shielding fabric is that the metal fibers are acting.

Yarns with metal fibers interweave and form crisscross isolation nets, which can reduce the energy of electromagnetic waves to a certain extent, so as to achieve the purpose of protection.

There are 3 kinds of attenuation mechanism when electromagnetic wave is spread to shielding fabric: (1) absorption loss; (2) surface reflection loss; (3) multiple reflection loss inside fabric.

The reflection is mainly caused by the inconsistency between the medium and the wave impedance of the metal. The larger the difference between the two, the greater the reflection loss.

Absorption means eddy current effect, that is, under high frequency condition, when electromagnetic wave passes through the shield, eddy current is generated on the shield surface, and eddy current produces countermagnetic field to counteract the original interference magnetic field, and at the same time, it also generates heat loss, which reduces the electromagnetic wave energy and achieves the shielding effect.

Shielding effectiveness of shielding fabric SE refers to the ratio of the field strength of a measured point to the same measured point after shielding without shielding. The unit is dB.

The relationship between shielding effectiveness SE and pmission coefficient T is:

SE=20log1T (1)

In the case of plane wave, the pmission coefficient of the whole metal mesh is:

T=s{0 265 x 10-2Rf+j[0.265 * 10-2Xf+0.333 * 10-8flnsa-1.5]} (2)

Type (2), s is metal mesh spacing (m), a is metal wire mesh radius (m), Rf is the net resistance per unit length of wire mesh (ohm /m), Xf is reactance per unit length of wire mesh (/m /m), f is frequency (Hz).

According to formula (2), the shielding effectiveness of shielding fabric can be calculated by SE.

Compared with the measured shielding effectiveness curve, the theoretical calculation curve and the measured shielding effectiveness curve are the same, and the calculated value is larger than the measured value. However, with the increase of the test frequency, the two trend is closer to the two.

This is due to metal.

With the increase of frequency, the resistance and reactance of the conductive network will increase, so the reflection loss of electromagnetic wave will decrease.

At the same time, according to the shielding theory, the holes or gaps with a certain depth can be regarded as waveguides, while the waveguide can attenuate the electromagnetic waves propagating under certain conditions.

The hole or slot in the shielding fabric is equivalent to a waveguide operating below the cut-off frequency (Fco).

The cut-off frequency can be calculated from the following formula:

Fco=3 * 10112l (3)

Type: l is the thread of hole or seam, unit mm.

Because the cut-off frequency is determined by the size of the hole or suture in the shielding fabric, rather than the size of the area, when the frequency increases closer to the cut-off frequency, the electromagnetic wave is increased excessively.

Between Fco/3~Fco, attenuation decreases, shielding effectiveness at Fco is close to 0dB.

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2 Relationship between metal fiber distribution and yarn structure and shielding effectiveness

Compared with the fabric made of stainless steel short fiber blended yarn and stainless steel filament, the shielding efficiency of the fabric is quite different, as shown in Figure 1.

W1 is a stainless steel filament blended fabric with a diameter of 50 m, and W2 is a stainless steel short fiber blended fabric with a diameter of 8 m and a length of 38~80mm.

W1 and W2 specifications are shown in Table 1.

It can be seen from the table that the metal fiber content of W1 is much higher than that of W2. However, in Figure 1, the shielding effectiveness of W2 is obviously better than that of W1.

This reflects the best shielding effectiveness of fabrics made of different stainless steel under different frequency bands.

When the frequency is relatively low, as shown in Figure 1 below 500MHz and relatively high frequency band, the shielding effectiveness of the fabric made of stainless steel staple blended yarn is better than that of stainless steel filament blended yarn when 2000MHz is higher than that of the stainless steel.

In addition, the structure of stainless steel filament blended yarn has a great influence on the shielding effectiveness of the fabric.

In covered yarns, stainless steel filaments are regularly coated with short fibers; in the strands, stainless steel filament and staple are intertwined; in core spun yarns, stainless steel filaments are located inside the staple fibers; therefore, in core spun yarns, the conductive mesh formed by stainless steel filament has smaller holes or sutures, followed by ply yarns, and the largest covered yarn.

According to the electromagnetic shielding theory, the important reason for the permeability of the pores is that the impedance at the seams or holes has changed, especially when the frequency is higher.

Because the pore affects the distribution of the power line and flux density line on the metal network, it interrupts the high-frequency induction current path, resulting in electrical discontinuity, resulting in the decrease of shielding effectiveness.

From the shielding mechanism of fabrics, we know that the cut-off frequency Fco of electromagnetic waves polarized in the direction of pore length mainly depends on the size of the long edge rather than the short side. Therefore, the shielding effectiveness of the pore line through the smallest core spun yarn is better, followed by the strand, and the wrapped yarn is worse.

From this we can see that the amount of metal fiber on the unit area is not necessarily good shielding effectiveness, because the distribution of metal fiber and the structure of yarn also have an effect on shielding effectiveness.

3 Effect of metal fiber content and tightness on shielding effectiveness

The higher the metal fiber content is, the more mixed the metal fiber is.

The greater the arrangement density, the better the shielding effectiveness of the fabric.

However, when the metal fiber content increases to a certain extent, with the increase of the metal fiber content, the shielding effectiveness of the fabric appears saturation in some test frequency bands, and some of them even show a significant decrease in shielding effectiveness.

At present, some data have been interpreted accordingly. It is considered that the reason that the shielding effectiveness increases with the increase of metal fiber content is due to the decrease of the diameter of the blended yarns with the increase of the metal fiber content, so that when the warp and weft density is equal, the fabric tightness decreases, and the holes and gaps in the fabric increase.

However, this explanation is not in line with the actual situation.

Because the shielding effect is only the metal conductive fibers in the blended yarns, while the other textile fibers are dielectric, and they do not have the electromagnetic interference resistance in the testing frequency band. Therefore, such an explanation is actually taking into account the other textile fibers in the fabric subjectively to the electromagnetic effect. The conclusion is that the yarn diameter decreases, the holes and gaps increase, so the electromagnetic wave is increased too much and the shielding effectiveness decreases. It is therefore considered that the tightness of the fabric is directly related to the shielding effectiveness of the fabric. When the metal fiber content is used to improve the shielding effectiveness of the fabric, the total tightness of the fabric will not be reduced, otherwise the shielding effectiveness will decrease.

At the same time, there is a direct relationship between fabric tightness and fabric shielding effectiveness, which is also inconsistent with the actual situation.

Because the pores between the conductive mesh of metal fiber always decrease with the increase of metal fiber content or the density of metal fiber blended yarn, but not increase.

According to the basic theory of electromagnetic interference and the coupling theory of small holes, in the near field (r< lambda /2), when the distance between the two apertures is large enough, the RF energy generated by the coupling of two holes can be ignored. At this time, only the holes in the region of A= pi R2 (r= /2 /2) are applied, such as the size of the orifice decreases, so that the area of PI R2 contains more holes, shielding effectiveness will decrease.

When this factor plays a leading role, the overall shielding effectiveness begins to decrease.

Therefore, even when the metal fiber content increases to a certain extent, even the phenomenon of shielding effectiveness decreases obviously.

4 Conclusion

(1) metal fiber distribution and yarn structure have great influence on fabric shielding effectiveness.

The shielding effectiveness of the blended fabric with stainless steel filament or staple fiber is different in different test frequency bands. In some bands, the shielding effectiveness of the former may be high, and the shielding effectiveness of the latter is higher in some frequency bands. The yarn structure also has a great influence on shielding effectiveness.

(2) the total fabric tightness is not directly related to the shielding effectiveness of the fabric. The more metal fiber content in the fabric is, the better. When the content reaches a certain level, the overall shielding effectiveness will decrease. It shows that the metal fiber blended yarn has the best state in the density, and the shielding frequency is related to the size of the hole between the holes in the conductive mesh of the fabric.