Analysis of shielding effectiveness in different kinds of electromagnetic shielding fabrics under different test conditions

Abstract

This study used the DR-SO4 window method to test the shielding effectiveness of silver-plated fiber functional fabric, copper–nickel duplicate coating fabric, and stainless steel fiber-blended-type fabric. These electromagnetic shielding fabrics exhibited different levels of shielding effectiveness under different polarization directions. In the same frequency, the shielding effectiveness difference between the vertical polarization wave direction and horizontal or 45° polarization wave direction is higher in silver-plated fiber functional fabric and copper–nickel duplicate coating fabric than that in stainless steel fiber-blended fabric. The radiation distance of 1.5 m has great influence on the shielding effectiveness of the three fabrics. These fabrics show a repeated and intersected change in wrinkle degrees of 1# and 2#. The fabrics in the wrinkle degree of 2# have higher shielding effectiveness than that of 3#. The wrinkle recovery properties of electromagnetic shielding fabrics also affect their shielding effectiveness. The shielding effectiveness of copper–nickel duplicate coating fabric with low wrinkle recovery property considerably changes. This research provides a basis for the design of electromagnetic shielding fabrics.

Keywords

electromagnetic shielding fabrics polarization wave directions radiation distance wrinkle degree of fabric

With the rapid development of the electronics industry, all kinds of military and civilian electronic devices and equipment have also developed. Electromagnetic radiation and interference cause exposure in humans’ living space to electromagnetic environment pollution during the working process of electronic devices and equipment. The thermal and nonthermal effects of electromagnetic waves produce different degrees of adverse effects on the human body at different frequencies.¹ Electromagnetic shielding fabrics (EMSFs) exhibit a certain shielding effect on electromagnetic waves and can be used to develop a kind of electromagnetic protective clothing that protects the human body. Thus, the shielding effectiveness (SE) of EMSFs must be studied in a wide frequency range.

In recent years, many studies on EMSFs were conducted. EMSFs can be divided according to different technologies into the following: metal-blended fabrics; metal-metalized fabrics; and shielding coating fabrics.² Blended yarns of stainless steel and polyester staple fibers are initially produced by ring spinning and then woven into various structures to facilitate the weaving of stainless steel wires and reduce material costs.³ Currently available EMSFs, which are made of stainless steel fiber and polyester, cotton, or bamboo carbon, and other short fibers, are typically used to fabricate blended-woven fabric and common fabrics. EMSFs with silver-plated fiber exhibit a good antistatic property, electrical conductivity, and electromagnetic shielding property.^4,5 Copper–nickel duplicate coating fabrics display strong corrosion resistance and low price.⁶ Ortlek et al.⁷ showed that the density of conductive hybrid yarn in a fabric structure is an important parameter that affects the electromagnetic shielding characteristics of woven fabrics. Su and Chem⁸ reported that a dense structure has a high SE. In this study, the influence of metal fiber content per unit area on SE was analyzed, and the underlying mechanism was discussed when the weft and warp densities were changed. Liu et al.⁹ demonstrated the positive correlation of metal fiber content per unit area and SE. Another experimental study indicated that the SE increases with increasing conductive stainless steel fiber ratio in nonwoven fabrics at different frequencies.¹⁰ The three commonly used fabrics for electromagnetic protective clothing are silver-plated fiber functional fabrics, copper–nickel duplicate coating fabrics, and stainless steel fiber-blended fabrics. At present, the main methods for testing the SE of EMSFs are the near-field method, far-field method, and shielding enclosure method.¹¹ The SE of EMSFs can also be detected through the shielding box method,¹² which provides more consistent results with the actual test environment and the living environment than the other shielding test methods. Moreover, the shielding box method has low cost and is therefore economical. In this method, electric field polarization, radiation distance, and fabric wrinkle degree are important factors that affect the SE of fabrics. Gupta et al.¹³ analyzed the SE of woven fabrics made of conducting yarn of different conductivity levels; this work also evaluated subsequent conversions of yarn into an ultra-lightweight material through a free-space microwave measurement system in microwave frequency (8–18 GHz) for vertical and horizontal polarizations of electromagnetic waves. Liu et al.¹⁴ analyzed the influence of the hole on the SE of the samples according to vertical or horizontal maximum sizes and polarization wave directions. Li et al.¹⁵ established a three-dimensional (3D) simulation model based on the theory of electromagnetic shielding by using multi-physics-coupled software COMSO. A simulation test was conducted using 304 austenitic stainless steel filaments on different polarization directions of electromagnetic waves; the result showed that the SE of fabric was affected significantly by different polarization directions. Qi et al.¹⁶ explained the effect of the polarization direction of incident wave on SE; the fabric showed the highest SE when the polarization direction of the incident wave was parallel to the fabric fiber and the lowest SE when the angle of incident wave polarization direction and fabric fiber was 45°. Xiuchen and Liu¹⁷ designed a simulation experiment on the basis of the shielding mechanism of electromagnetic shielding clothing and investigated the influence of radiant frequency and radiant distance on the SE of the clothing. Dvurechenskaya and Zieliński¹⁸ measured two types of newly manufactured shielding materials (nonwoven and fabric) by using an improved free-space transmission technique at 2–5 GHz depending on electromagnetic wave electromagnetic polarization and antenna to specimen distance; the metal components of EMSFs reduced their wrinkle recovery property.¹⁹ Shao et al.²⁰ reported that the formation of the folded core material increased the metal surface area of the unit area, thereby improving the capability to absorb electromagnetic waves from a macro point of view. However, these studies are single contents and did not compare silver-plated fiber functional fabrics, copper–nickel duplicate coating fabrics, and stainless steel fiber-blended fabrics carried under different directions of electric field polarization waves, radiation distances, and fabric wrinkle degrees.

This research selected silver-plated fiber functional fabrics, copper–nickel duplicate coating fabrics, and stainless steel fiber-blended fabrics for testing of SE within a wide frequency range of 1–18 GHz. The change regulations of the three EMSFs were compared and analyzed according to the experimental results under different directions of electric field polarization waves, radiation distances, and wrinkle degrees. The SE of EMSFs was analyzed under different conditions. The results provide a basis for producing EMSFs and electromagnetic protective clothing possessing high SE.

Experimental details

Theoretical analysis

According to the standard ASTM D4935-2010,²¹ SE is defined as the ratio of power or voltage received with the electromagnetic shielding material and without the electromagnetic shielding material with logarithmic representation under the same excitation level. The calculation formulas are as follows

SE = 10 lg \frac{P_{1}}{P_{2}} orSE = 10 lg \frac{V_{1}}{V_{2}}

(1)

where P₁ and V₁ are the power and voltage values, respectively, without the electromagnetic shielding material under constant excitation level. Here P₂ and V₂ are the power and voltage values, respectively, with the electromagnetic shielding material.

Experimental instruments

The experimental equipment employed a DR-SO4 window method SE test box, DR6103 broadband double-ridge horn antenna, and AV3629D microwave vector network analyzer (Microwave VNA). The connection of the test equipment is shown in Figure 1.

Figure 1.

Electromagnetic shielding effectiveness test equipment.

Fabric parameters

The study selected silver-plated fiber functional fabric, copper–nickel duplicate coating fabric, and stainless steel fiber-blended-type fabric. The silver-plated fiber functional fabric was 100%-plated silver fiber of woven fabric, and the silver ions were implanted by vacuum sputtering on the basis of nylon. The copper–nickel duplicate coating fabric was coated with metals, including plate copper and then nickel on the base of polyester, to prevent the oxidation of copper. The stainless steel fiber-blended-type fabric was blended from stainless steel filament, stainless steel staple fiber, polyester filament, and cotton staple fiber-blend stainless steel fiber. A kind of yarn structure made filaments as the trunk, and short fibers were inserted to surround the structure. Weft yarns were compounded by long and short stainless steel fiber yarns. Warp yarns were made by the mutual alternate permutation of stainless steel fiber, long staple fiber composite yarn, and polyester filament composite yarn. The surface features of these three fabrics were observed by a scanner, as shown in Figure 2. The related parameters of EMSFs are shown in Table 1.

Figure 2.

Surface features of the three fabrics: (a) silver-plated fiber functional fabric; (b) copper–nickel duplicate coating fabric; (c) stainless steel fiber-blended-type fabric.

Table 1.

Fabric parameters.

Fabric name	Fabric weave		Density (yarn/10 cm)		Weight (g/m²)	Thread count (tex)	Component content
Fabric name	Name	Fundamental weave	Weft	Warp	Weight (g/m²)	Thread count (tex)	Component content
Silver-plated fiber functional fabric	Complex twill weave		472	377	75	7.8	100% silver-plated fabric
Copper–nickel duplicate coating fabric	Complex twill weave		431	324	44	8.5	20% copper/10% nickel/70% polyester
Stainless steel fiber-blended-type fabric	Twill weave		510	253	210	32	30% stainless steel fiber/ 30% polyester/40% combed cotton

Test method—shielding box method

SE is an important parameter to reflect the performance of electromagnetic shielding materials. The test method is related to whether the SE of electromagnetic shielding materials can be evaluated objectively and accurately. A test affects the results with different test methods. According to the principle of electromagnetic radiation shielding, the SE of metal fiber shielding materials mainly depends on the reflection loss. The SE (also known as the “attenuation level”) and the shielding efficiency of fabric are used to characterize the performance of fabric against electromagnetic radiation. Different kinds of EMSFs were tested by the shielding box method that followed the national standard GJB6190-2008.²² The SE is a difference value between field strength and power measured by the receiving signal device testing with and without the barrier of anti-electromagnetic radiation fabric, which is the testing principle of the shielding box test method. The shielding box is shown in Figure 3.

Figure 3.

Shielding box. RF: radio frequency.

Experimental sample preparation

The sample size was 40 cm × 40 cm. From each EMSF five samples were selected to test the SE of the three kinds of EMSFs in terms of different directions of electric field polarization wave and radiation distance. Each sample was tested 10 times. A total of 50 test results were obtained. Then, the SEs of different samples were obtained after averaging the 50 results. Finally, the SE curve of each sample was drawn by average value.

In the effect of different wrinkle degrees, each EMSF was divided into three test samples named as 1#, 2#, and 3#. Firstly, the surface of 1# was kept flat. Then, test samples 2# and 3# were kneaded into a ball by hand, and 2# test sample was larger than 3#. The kneaded test samples were spread, and they formed different degrees of wrinkles on the surface. Therefore, the same areas of EMSFs had different wrinkle degrees and numbers of concave–convex parts. The smaller the ball is, the lower the depth will be. The more concave–convex parts are, the more apparent the wrinkle will be. Therefore, the degree of the wrinkle of the 2# test sample was less than that of the 3# test sample. The wrinkle degree test samples of 1#, 2#, and 3# ranged from large to small, which were flat EMSF 1#, slightly wrinkled EMSF 2#, and deeply wrinkled EMSF 3#. Furthermore, the wrinkle degrees of each kind of EMSF were 1#, 2#, and 3#, which made the same five samples. In making the same wrinkle samples, we kneaded them into same balls by hand. We selected the same number, height, and depth of concave–convex parts. The samples that did not meet these requirements were eliminated. We kept the samples in the same wrinkle degree consistently. Each sample was tested 10 times. A total of 50 test results were obtained. Then, the SEs of different samples were obtained after averaging 50 results. Finally, the SE curve of each sample was drawn by average value. The EMSFs had different surface morphologies of the macro and micro because of the influence of wrinkles. An EPSON scanner was used to scan the macroscopic surface of the EMSFs. A Keyence VK-X110 shape measurement laser microscope was used to scan the microscopic height distribution of EMSF surfaces, which was enlarged 200 times. The surface wrinkle state of the EMSFs is shown in Table 2.

Table 2.

Surface wrinkle state of electromagnetic shielding fabrics (EMSFs).

Fabric name	Macroscopic surface of EMSFs			Microscopic height distribution of the surface of EMSFs
Fabric name	1#	2#	3#	1#	2#	3#	Remarks

The wrinkle recovery property of EMSFs was measured with a Digitel Wrinkle Recovery Tester YG541L according to the ISO 2313 standard. The sample size was 40 mm × 15 mm. The load weight was 10 N. The compression area was 20 mm × 15 mm. The compression time was (300 + 5) s and the relaxation time was (300 + 5) s. The number of each sample was 20, namely 10 samples of warp and weft directions. Each direction of EMSFs was tested with five obverse samples and five reverse samples. Each sample was cut into 40 mm ×15 mm, as shown in Figure 4. The wrinkle recovery angles were tested after 5 min of pressure relief.

Figure 4.

Schematic diagram of different polarization wave forms.

Results and discussion

The effects of different directions of the electric field polarization wave

The electromagnetic wave propagates in a certain direction in several cases. Different polarization wave directions have different influences on the object of SE due to anisotropy. We should focus on it in practical applications. Given the ground as a reference system, a vertical polarization wave occurs when the electric field E is perpendicular to the ground. Meanwhile, when E is parallel to the ground, it becomes the horizontal polarization wave. The angles between the launch horn antenna and the ground are 0°, 45°, and 90°. Three kinds of polarized wave forms are shown in Figure 5. The standard deviation values and the average values of the test results are listed in Table 3. To calculate the average values of test results, the SE curves are drawn in Figures 6 –8.

Figure 5.

Schematic diagram of different polarization wave forms.

Table 3.

Standard deviation values and average values of electromagnetic shielding fabrics.

Frequency/ GHz Value Name	Silver-plated fiber functional fabric						Copper–nickel duplicate coating fabric						Stainless steel fiber-blended-type fabric
	0°		45°		90°		0°		45°		90°		0°		45°		90°
	S	A/dB	S	A/dB	S	A/dB	S	A/dB	S	A/dB	S	A/dB	S	A/dB	S	A/dB	S	A/dB
1	3.44	67.2	2.17	63.4	3.12	41.9	17.03	54.2	5.56	62.4	1.05	38.7	0.66	36.8	10.09	48.1	2.86	35.2
2	0.08	62.1	0.88	62.1	3.56	46.6	9.26	55.1	3.94	56.3	2.05	41.4	1.94	44.0	6.27	48.1	2.11	40.1
3	3.47	70.8	4.91	75.9	6.50	45.5	9.71	67.5	8.68	62.1	2.24	51.7	10.74	49.6	8.17	46.4	5.85	45.6
4	2.54	61.3	1.73	62.4	3.92	40.0	8.71	57.6	10.64	57.4	4.46	38.9	8.92	41.4	8.47	40.1	3.65	29.2
5	1.46	62.4	2.95	63.9	1.45	32.9	7.49	56.5	10.33	60.1	6.45	29.0	9.52	41.2	10.88	40.7	3.68	18.5
6	3.33	64.5	2.24	55.6	9.30	45.3	13.57	53.8	6.23	53.4	15.70	42.0	5.59	35.1	10.99	39.1	4.24	31.6
7	3.04	59.8	5.25	56.6	4.85	33.5	21.18	44.3	5.79	50.0	4.55	35.5	1.71	23.3	9.53	35.5	4.90	19.5
8	1.68	62.7	2.75	55.9	4.32	30.3	16.60	51.1	5.03	49.1	1.63	28.0	1.26	32.2	7.80	38.4	4.91	33.3
9	3.87	59.4	2.33	63.7	7.47	40.7	11.72	54.5	9.31	54.8	4.16	27.7	8.36	35.6	10.93	36.8	4.24	28.3
10	2.24	63.5	3.61	50.5	3.96	36.5	13.49	53.0	8.51	47.7	5.12	36.9	5.47	34.5	6.81	35.5	1.09	31.5
11	4.52	61.3	5.08	58.0	1.92	33.5	6.92	53.8	8.54	52.3	0.58	35.7	9.88	39.8	8.52	37.9	1.43	34.6
12	5.53	54.4	3.52	62.7	1.40	28.4	13.58	51.0	3.02	63.2	8.72	35.5	5.77	33.4	19.55	43.8	3.18	23.1
13	4.43	55.8	1.75	55.3	6.70	29.7	13.10	43.1	11.32	54.3	2.95	24.3	2.68	27.1	12.12	33.8	4.21	18.8
14	2.64	54.3	5.95	60.5	4.01	22.8	10.06	49.9	5.55	50.1	1.52	22.1	9.44	33.3	14.04	35.7	3.33	22.3
15	2.86	49.6	0.90	46.6	1.92	22.6	2.51	46.5	6.74	43.8	3.44	21.0	14.47	32.6	10.84	27.5	4.92	24.4
16	2.47	55.7	2.13	47.9	4.36	34.0	9.43	47.7	4.22	48.1	9.56	38.1	9.97	30.2	13.93	31.7	12.38	29.2
17	1.42	53.2	2.35	46.0	5.51	49.4	6.44	49.9	4.53	42.9	7.75	41.1	13.46	33.5	10.92	29.0	2.65	29.7
18	2.29	40.7	3.23	41.1	4.24	41.6	3.11	37.3	5.60	37.8	6.59	40.3	6.94	29.4	9.55	29.6	2.75	29.0

Note: S represents the standard deviation value, and A represents the average value.

Figure 6.

Silver-plated fiber functional fabric shielding effectiveness (SE) curve.

Figure 7.

Copper–nickel duplicate coating fabric shielding effectiveness (SE) curve.

Figure 8.

Stainless steel fiber blended-type fabric shielding effectiveness (SE) curve.

In Figures 6 –8, the SEs of the three kinds of EMSFs have great fluctuation. They are lower than horizontal and 45° polarization wave directions for the SE of the three kinds of EMSFs in the vertical polarization wave direction, and they have a minimum value that is about 20 dB in the frequency band of 13–16 GHz. When the difference value of the SE values of the three kinds of EMSFs in the vertical polarization wave direction was subtracted from that in horizontal or 45° polarization wave direction, the silver-plated fiber functional fabric and copper–nickel duplicate coating fabric had a great difference value, and the stainless steel fiber-blended fabric had a small difference value. However, the curve change of all the SEs of EMSFs was repeated and intersected in the horizontal and 45° polarization wave directions. The SE of silver-plated fiber functional fabric was mainly 50–70 dB in the horizontal and 45° polarization wave directions but was 20–50 dB in the vertical polarization wave direction. In the horizontal polarization wave direction, the SE of the copper–nickel duplicate coating fabric was mainly 45–60 dB. Furthermore, the SE of the stainless steel fiber-blended-type fabric was mainly 20–40 dB.

For this phenomenon, the reasons for each kind of EMSF that had different warp and weft yarn densities were analyzed. The difference of the warp and weft yarn density could cause different size pores in the unit length of different directions. Thus, it resulted in inconsistent quantity and arrangement of the metal fibers that are distributed in the unit length in all directions. The difference between the three kinds of fabrics led to the change of SE. The test angle of the test sample could be changed to make the warp and weft yarns parallel or perpendicular to the ground. Therefore, the effect of the polarization wave direction adopted the orientation analysis on the SE of EMSFs. This experiment provides the dense direction of yarn arrangement, as shown in Figure 9. Whether the yarn is warp or weft, the direction is perpendicular to the relatively tight yarn arrangement, which is the dense direction of yarn arrangement. The vertical polarization wave direction is perpendicular to the dense direction of yarn arrangement. The 45° polarization wave direction is a 45° angle with the dense direction of yarn arrangement. Meanwhile, the horizontal polarization wave direction is parallel to the dense direction of yarn arrangement.

Figure 9.

Definition of the direction of the dense arrangement and polarization wave of yarns.

According to the definition of the dense direction of the arrangement of yarns, the SEs of the three kinds of EMSFs were significantly different in horizontal, 45°, and vertical polarization wave directions when the frequencies were the same. When the polarization directions and the direction of the dense arrangement of yarns were in the same direction, the EMSF had a high SE. When the polarization direction was perpendicular to the direction of the dense arrangement of yarns, the EMSF had a low SE. In the unit area of EMSFs, the dense direction of the arrangement of yarns had a higher density than those of warp yarns. The number of metal fibers of the dense direction of yarn arrangement was more than that of the warp direction in unit length. In the dense direction of yarn arrangement, the distance between the metal fibers was small, and then the contact between the metal fibers was sufficient, which made the direction of the yarns to have good conductivity. The angle of vertical polarization wave direction with the polarization direction of the electric field was small, which caused the direction component of the electromagnetic waves to increase in that direction. The SE of EMSF increased as well. Another explanation for the high density of the dense direction of yarn arrangement was the large number of metal fibers per unit length and small pore size. Thus, the EMSFs can resist the strength of the electric field cutting. The direction had high SE. On the contrary, the SE of EMSF became small.

Effects of different radiation distances

The radiation distance was the distance between the radiation source and the radiation object. The electromagnetic wave was ubiquitous in real life. The radiation distance was different to different radiation objects, and the influence was also different. The influence of different radiation distances on the radiation object should be considered. In this experiment, the influence of different radiation distances on the SE of EMSFs was tested by changing the distance between the horn antenna and the EMSFs. The distances were 0.5, 1, and 1.5 m (see Figure 10). The standard deviation values and the average values of test results are listed in Table 4. The SE curves are drawn in Figures 11 –13, according to the calculated average values of the test results.

Figure 10.

Schematic diagram of radiation distance.

Table 4.

Standard deviation values and average values of electromagnetic shielding fabrics.

Frequency/ GHz Value Name	Silver-plated fiber functional fabric						Copper–nickel duplicate coating fabric						Stainless steel fiber-blended-type fabric
	0.5 m		1 m		1.5 m		0.5 m		1 m		1.5 m		0.5 m		1 m		1.5 m
	S	A/dB	S	A/dB	S	A/dB	S	A/dB	S	A/dB	S	A/dB	S	A/dB	S	A/dB	S	A/dB
1	0.44	60.4	1.60	59.9	3.44	66.8	2.49	57.7	1.41	57.1	17.03	51.6	9.33	47.1	12.27	45.4	0.66	36.9
2	2.26	66.7	1.73	62.5	0.08	62.1	5.63	60.0	3.47	60.8	9.26	53.6	4.67	50.6	11.72	47.6	1.94	43.7
3	1.79	65.8	1.78	65.2	3.47	70.6	7.73	60.4	6.67	59.0	9.71	66.1	8.97	45.0	8.48	45.0	10.74	47.9
4	0.63	64.5	2.41	61.4	2.54	60.9	10.10	55.6	8.90	55.2	8.71	56.3	8.24	39.0	8.74	39.6	8.92	40.0
5	3.74	64.8	5.93	67.8	1.46	62.5	6.86	55.7	14.50	63.1	7.49	55.4	11.22	39.3	10.93	40.5	9.52	39.8
6	3.22	64.9	2.08	58.6	3.33	64.2	11.35	57.4	6.55	54.9	13.57	51.8	8.66	39.5	11.64	37.6	5.59	34.2
7	1.10	63.6	6.18	62.1	3.04	59.4	9.83	56.1	11.04	59.3	21.18	41.0	4.55	43.5	12.17	37.9	1.71	23.5
8	5.08	68.3	5.70	65.6	1.68	62.8	11.15	56.0	3.89	56.3	16.60	48.7	10.29	36.6	13.67	41.0	1.26	32.1
9	1.27	64.0	6.37	68.1	3.87	58.9	6.29	59.1	7.92	55.8	11.72	52.7	9.89	44.7	10.56	39.0	8.36	34.3
10	0.93	62.9	5.51	63.6	2.24	63.3	8.23	55.6	6.48	53.8	13.49	51.0	11.82	37.8	11.51	37.4	5.47	33.6
11	3.02	60.5	3.12	64.8	4.52	61.1	8.34	55.5	12.55	57.5	6.92	53.3	11.85	36.9	10.47	36.1	9.88	38.3
12	1.34	58.3	14.89	72.3	5.53	54.1	6.84	53.0	11.07	57.1	13.58	49.1	12.78	35.1	12.74	35.1	5.77	32.6
13	2.00	61.2	3.75	62.2	4.43	56.5	5.47	54.5	12.77	53.8	13.10	41.1	13.76	36.5	11.52	32.5	2.68	26.7
14	4.13	66.9	7.91	59.6	2.64	54.2	10.68	58.9	3.11	50.8	10.06	48.3	13.92	36.9	14.44	35.1	9.44	31.8
15	2.16/	62.5	1.58	60.7	2.86	50.0	10.23	51.5	10.01	50.7	2.51	46.6	11.65	31.7	12.40	30.4	14.47	30.5
16	10.82	67.1	1.62	61.2	2.47	55.5	8.67	50.3	8.83	52.4	9.43	46.3	11.33	31.7	12.67	32.8	9.97	28.7
17	3.76	59.5	2.11	51.7	1.42	53.2	9.43	50.4	6.44	47.5	6.44	48.8	10.63	32.9	11.10	32.1	13.46	31.4
18	1.91	56.0	4.59	53.0	2.29	40.7	5.61	48.8	6.59	51.1	3.11	37.3	11.38	33.2	13.35	35.6	6.94	28.4

Note: S represents standard deviation values, and A represents average values.

Figure 11.

Silver-plated fiber functional fabric shielding effectiveness (SE) curve.

Figure 12.

Copper–nickel duplicate coating fabric shielding effectiveness (SE) curve.

Figure 13.

Stainless steel fiber-blended-type fabric shielding effectiveness (SE) curve.

As shown in Figure 11, the SE of silver-plated fiber functional fabric of the three kinds of radiation distance mainly floated in 50–70 dB in the whole frequency range. In the frequency range of 12–16 GHz, the SE of silver-plated fiber functional fabric was higher at 1 and 0.5 m radiation distances than that at 1.5 m. In Figure 12, the copper–nickel duplicate coating fabric SE of 1 and 0.5 m radiation distances were close, had a decreasing trend, and were higher than that of 1.5 m in the frequency range of 6–16 GHz. The SE of 1 and 0.5 m radiation distances mainly floated in 50–60 dB. The SE of the radiation distance of 1.5 m mainly floated in 40–50 dB. As shown in Figure 13, the stainless steel fiber-blended type fabric SE of 0.5 and 1 m radiation distances were close, which had a decreasing trend. The SE of 1.5 m radiation distance was lower than those of 1 and 0.5 m radiation distances in the frequency ranges of 6–8 and 12–14 GHz. In the whole frequency range, the maximum value was obtained for the SE of the three kinds of EMSFs in the radiation distance of 1.5 m. Furthermore, this value showed a sharp increase and decrease change in the frequency band of 11–13 GHz. Figure 11 shows 6–8 GHz, and Figure 12 indicates 12–14 GHz. Meanwhile, Figure 13 presents 6–8 GHz. Therefore, these phenomena were caused by the electromagnetic parameters of the fabric itself, which changed with the frequency and the influence of the secondary field around the EMSFs. In this frequency band, the electrical conductivity of the electromagnetic parameters of the fabric increased dramatically because of the frequency change and secondary field of stainless steel fiber, which increased the overall conductivity of the fabric. Thus, the SE of the EMSFs improved.

Effects of different fabric wrinkle degrees

The effect of wrinkle on SE was tested by changing the wrinkle degrees of the three kinds of shielding fabric. We set the flat EMSF 1#, slightly wrinkled EMSF 2#, and deeply wrinkled EMSF 3# in this experiment. The test distance was 1.5 m. The standard deviation values and average values of the test results are listed in Table 5. To calculate the average values of the test results, the SE curves are drawn in Figures 14 –16.

Table 5.

Standard deviation values and average values of electromagnetic shielding fabrics.

Frequency/ GHz Value Name	Silver-plated fiber functional fabric						Copper–nickel duplicate coating fabric						Stainless steel fiber-blended-type fabric
	1#		2#		3#		1#		2#		3#		1#		2#		3#
	S	A/dB	S	A/dB	S	A/dB	S	A/dB	S	A/dB	S	A/dB	S	A/dB	S	A/dB	S	A/dB
1	2.73	66.2	8.50	59.8	2.76	54.6	17.03	51.6	2.27	67.4	1.28	58.1	0.66	36.9	11.78	45.7	1.88	36.5
2	1.68	62.8	2.54	59.5	1.15	56.5	9.26	53.6	0.48	61.5	0.83	54.4	1.94	43.7	6.39	45.6	2.65	39.1
3	3.28	71.0	4.66	63.0	4.91	58.1	9.71	66.1	4.26	67.3	1.69	53.0	10.74	47.9	6.82	43.0	0.67	38.0
4	2.52	60.8	2.93	60.4	4.41	52.9	8.71	56.3	2.55	59.2	0.42	48.1	8.92	40.0	8.64	36.9	0.99	32.3
5	3.52	61.5	2.34	63.0	7.09	54.8	7.49	55.4	2.29	63.4	0.59	47.6	9.52	39.8	9.59	35.2	1.36	30.3
6	3.03	62.7	2.66	58.5	7.40	54.9	13.57	51.8	2.65	59.1	0.70	47.2	5.59	34.2	8.52	35.8	3.35	27.5
7	2.87	59.3	11.06	60.8	4.09	47.3	21.18	41.0	4.16	65.3	0.50	42.5	1.71	23.5	6.87	33.1	2.72	31.2
8	1.74	62.8	5.32	60.9	4.59	52.3	16.60	48.7	0.28	58.3	2.16	49.4	1.26	32.1	8.09	36.3	0.76	30.5
9	4.77	57.2	4.85	60.8	5.04	51.2	11.72	52.7	0.94	64.4	1.00	44.8	8.36	34.3	6.02	36.0	0.25	32.0
10	2.35	62.0	5.51	59.0	2.62	55.6	13.49	51.0	8.72	65.9	4.06	48.1	5.47	33.6	9.28	34.0	1.06	28.8
11	4.16	59.5	1.92	58.8	4.33	59.7	6.92	53.3	4.31	63.2	6.34	48.0	9.88	38.3	7.19	32.6	1.17	27.0
12	5.79	55.6	4.21	51.3	2.80	47.5	13.58	49.1	2.29	51.9	1.94	47.5	5.77	32.6	12.81	34.3	1.61	24.4
13	4.06	56.8	4.74	58.5	6.09	51.5	13.10	41.1	3.95	60.0	3.11	43.5	2.68	26.7	8.52	29.0	0.56	22.9
14	2.50	54.5	4.26	57.0	8.57	49.0	10.06	48.3	5.81	55.8	2.74	43.0	9.44	31.8	13.57	28.3	0.57	20.2
15	5.29	52.5	6.34	57.4	6.68	46.4	2.51	46.6	4.72	59.6	1.04	40.5	14.47	30.5	11.37	25.6	0.40	18.6
16	2.61	54.2	5.88	53.5	6.87	44.9	9.43	46.3	4.37	54.9	0.62	40.8	9.97	28.7	10.07	29.7	0.38	22.6
17	1.73	53.0	5.47	59.1	6.60	49.6	6.44	48.8	5.65	51.1	2.04	42.5	13.46	31.4	10.01	29.5	0.24	23.1
18	2.06	40.4	4.99	46.9	4.83	41.1	3.11	37.3	2.09	50.2	2.94	38.1	6.94	28.4	8.97	26.9	2.40	23.6

Note: S represents the standard deviation value, and A represents the average values.

Figure 14.

Silver-plated fiber functional fabric shielding effectiveness (SE) curve.

Figure 15.

Copper–nickel duplicate coating fabric shielding effectiveness (SE) curve.

Figure 16.

Stainless steel fiber-blended-type fabric shielding effectiveness (SE) curve.

As can be seen from Figure 14, the SE of the silver-plated fiber functional fabric 3# is lower than those of 1# and 2# and is a conductive grid structure. The warp and weft yarns were interlaced and also easily broken because of large tension. The silver powder of the surface of silver-plated fiber functional fabric easily fell after being folded. After the fabric was wrinkled, the warp and weft yarn arrangement structure of the fabric was deformed, as shown in Figures 17 and 18. The yarn structure was pressed, which made it uneven. Apparent holes were observed in the warp and weft yarns, which were the main reason for the decreased SE of EMSFs. The holes in the fabric are shown in the yellow circles in Figure 18. Given the wrinkle of EMSFs, the yarns slipped and produced more holes between the warp and weft yarns, which decreased the SE of the EMSFs. That is to say, the holes of fabrics became equivalent to the opening holes that caused electromagnetic leakage in the metal shield.

Figure 17.

Before the silver-plated fiber functional fabric became wrinkled.

Figure 18.

After the silver-plated fiber functional fabric became wrinkled. (Color online only).

As presented in Figure 15, the SE of copper–nickel duplicate coating fabric wrinkle degree 2# was higher than that of 1#. As shown in Figures 19 and 20, copper–nickel duplicate coating fabric warp and weft yarns were crimped, and they buckled each other, which made their surface concave and convex, respectively. Copper–nickel duplicate coating fabric has a metal coating structure and produces multiple planes to increase the reflectivity of electromagnetic wave after folding. At the same time, the reflection loss of the electromagnetic wave increased on the surface of EMSFs. Therefore, the SE of EMSFs also relatively improved. However, with the fabric wrinkle degree that increased to a certain degree (wrinkle degree 3#), the yarns moved and the coating metal fell off. The porosity of the EMSFs increased, and the conductivity decreased. Finally, the SE of EMSFs was reduced. Thus, the SE of copper–nickel duplicate coating fabric in the wrinkle degree of 3# was lower than that in the wrinkle degree of 2#. As shown in Figure 16, the SE of stainless steel fiber-blended-type fabric mainly floated in 20–40 dB. The SE of stainless steel fiber-blended-type fabric in the wrinkle degree of 3# was lower than those in the wrinkle degrees of 1# and 2#. As presented in Figures 21 and 22, the stainless steel filaments moved, and stainless steel staple fibers stuck out from the yarn surface, which unevenly arranged the stainless steel materials. This result explains the reason very well that the SE of stainless steel fiber-blended-type fabric decreased with the wrinkle degree of 3#.

Figure 19.

Before the copper–nickel duplicate coating fabric became wrinkled.

Figure 20.

After the copper–nickel duplicate coating fabric became wrinkled.

Figure 21.

Before the stainless steel fiber-blended-type fabric became wrinkled.

Figure 22.

After the stainless steel fiber-blended-type fabric became wrinkled.

Another reason explains the different effects of wrinkle on the SE of the three kinds of EMSFs. The three kinds of EMSFs have different wrinkle recovery properties. Their wrinkle recovery angles are shown in Table 6.

Table 6.

Wrinkle recovery angles of the three kinds of electromagnetic shielding fabrics.

Fabric name	Average value of warp recovery angle/°	Average value of weft recovery angle/°	Total average value of recovery angle/°
Silver-plated fiber functional fabric	106.38	89.38	97.88
Copper–nickel duplicate coating fabric	99.64	91.34	95.49
Stainless steel fiber-blended-type fabric	116.93	114.79	115.86

As shown in Table 6, the three kinds of EMSFs have different wrinkle recovery properties. The wrinkle recovery property of copper–nickel duplicate coating fabric was poorer than those of silver-plated fiber functional fabric and stainless steel fiber-blended-type fabric. Copper–nickel duplicate coating fabric was easily wrinkled, and recovered flatness after being wrinkled was difficult. Thus, the change of the SE of copper–nickel duplicate coating fabric was greater than those of silver-plated fiber functional fabric and stainless steel fiber-blended-type fabric between different wrinkle degrees. Meanwhile, the three kinds of the SE of EMSFs were lower in the wrinkle degree of 3# than those of 1# and 2# in most frequency bands. Therefore, wrinkle recovery properties had a certain influence on the SE of the three kinds of EMSFs.

Conclusions

In the frequency band of 1–18 GHz, different polarization directions had a great influence on the SE of EMSFs. EMSF had a high SE when the polarization directions and direction of the dense arrangement of yarns were in the same direction. When the polarization direction was perpendicular to the direction of the dense arrangement of yarns, the EMSF had a low SE. Compared with the difference values for the SE value of the three kinds of EMSFs in vertical polarization wave direction minus that in horizontal or 45° polarization wave direction, the difference values of silver-plated fiber functional fabric and copper–nickel duplicate coating fabric were all high but that of stainless steel fiber-blended fabric was small.

For the SE of the three kinds of EMSFs at a radiation distance of 1.5 m, the value in the whole frequency range was at maximum and lower than 1 and 0.5 m in most frequency ranges. The SEs of the three kinds of EMSFs alternately increased and decreased at the radiation distances of 1 and 0.5 m. Individual frequency bands greatly varied.

The SE values of three kinds of EMSFs in the wrinkle degree of 3# were lower than those in the wrinkle degrees of 1# and 2#. In the wrinkle degrees of 1# and 2#, the SE values of the three kinds of EMSFs were repeated, and change intersected. These results influenced the SE of the three kinds of EMSFs, particularly wrinkle recovery properties. The wrinkle recovery property of copper–nickel duplicate coating fabric was poorer than those of silver-plated fiber functional fabric and stainless steel fiber-blended-type fabric. Thus, the wrinkle recovery property had a great effect on the SE of copper–nickel duplicate coating fabric.

Footnotes

Acknowledgements

The silver-plated fiber functional fabric was produced by Jiaxing Microwave Shielding Material Company of China. The copper–nickel duplicate coating fabric was produced by Qingdao Zhiyuan Xiangyu Functional Fabric Company of China. The stainless steel fiber-blended-type fabric was produced by Jiaxing Microwave Shielding Material Company of China.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed the receipt of the following financial support for the research, authorship, and/or publication of this article. This work was supported by the National Natural Science Foundation of China (No. 61671489 and No. 61471404).

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