The electromagnetic shielding and reflective properties of electromagnetic textiles with pores,planar periodic units and space structures

Abstract

Examples are presented of three new methods for preparing electromagnetic (EM) textiles for a variety of applications. These are metalized textiles containing pores and meshes, textiles with planar periodic structures, and space-structured textiles. Firstly, an aluminum foil model with pores was prepared, then metalized textiles with pores, and finally silver-coated nylon net fabrics, and the relationship between shielding effectiveness (SE) and pore structure was studied. The size of the pores and the distance between them obviously influence the SE, and in particular the pores on the metalized fabrics decrease the SE. Secondly, the factors affecting the frequency selective surface (FSS) of the metal were analyzed and a FSS textile was prepared as a band-pass filter. The FSS fabric with non-conductive periodic units on the conductive fabric surface showed good resonance peaks. Finally, three types of space-structured EM textiles, including a plush fabric, a warp-knitted spacer fabric, and a velvet fabric were constructed using different conductive yarn blends. The EM reflection coefficient curves showed that these structures also obviously affected the EM properties. The three types of new EM fabrics were soft and light compared with traditional metallic EM devices. There is still, however, a long way to go to establish the exact relationship between the structure and EM properties of these new EM textiles.

Keywords

electromagnetic textiles planar periodic structure pore structure frequency selective surface

Textiles as a propagation medium for electromagnetic (EM) waves have not been widely studied, although textiles with metallic properties have a number of interesting EM properties and applications, for example as EM shielding and radar absorbing materials, new concept rapid-attack weapons in electronic warfare, and as EM stealth and compatibility materials.

In regard to the science of textile materials, a theoretical system for the conductive properties of fibers and for the antistatic properties of textiles has initially been established.^1,2 The problem of static has been solved, even in the microelectronic industry, which is very sensitive to static. Secondly, in recent years a conductive mechanism and an electrochemical theory for conductive or electroactive polymers have been established.³ A variety of polymerization technologies for conductive polymers such as polyaniline (PANI),^4–6 polythiophene (PTh),^7,8 or polypyrrole (PPy)^9–11 on the common fiber or textile surfaces have been widely reported and gradually improved in engineering terms. Thirdly, although a relatively complete theoretical system covering some aspects of dielectric constant and dielectric loss of fibers relating to EM wave transmission performance has been established.^1,2 This is limited to the dielectric properties of common fibers such as those preventing dielectric breakdown.

Bearing in mind the rapid development of information technology, the propagation performance of an EM wave through textiles deserves further study. With the improvement of individual EM protection requirements, textiles are required to provide improved EM shielding and heat–moisture comfort.

A number of papers have reported the EM shielding or reflection properties of textiles. These textiles can be prepared by electroless plating,^12,13 magnetron sputtering,¹⁴ embroidering¹⁵, or by weaving with metal or metalized fibers.^16,17 Most studies have focused on methods of preparation of planar metalized fabrics, and also their properties such as the quality of the coating layer,¹⁸ EM shielding effectiveness (SE),^19–21 and the shielding mechanism.²² These fabrics have lacked variety, either in planar or spatial structure, in other words the fabric structure has not received much attention. A number of papers have begun to appear, reporting the multilayer compound planar structure of fabrics, and the spatial EM shielding properties of fabrics has recently yielded promising results.^23–25

Experimentally, we realized that the effect of material structure on EM wave propagation performance was much more significant than that of the material itself. In the present paper we present three new experimental EM textiles and develop ideas based on design of fabric structure. These fabrics have a porous structure, are planar with two-dimensional non-conductive periodic structural units, or are space-metalized textiles. The effect of the size and distance apart of the pore units on the EM wave and reflection properties of two-dimensional and three-dimensional periodic electromagnetic textile structures have been studied.

This discussion is merely a start in the development of these new ideas. Additional and more profound research on each aspect presented in this paper will be needed to confirm the exact relationship between structure and EM properties of these new textiles. Hopefully, this may provide a richer scientific context for the development of EM textiles, particularly those with specialized EM properties.

Three methods for developing new EM textiles

The reflectivity of the EM wave on the material surface depends on its complex permittivity, and the dielectric loss of the EM wave through the material is also related to its complex permittivity, permeability and the tangent of the loss angle.²⁶ Due to the obvious differences between the EM parameters of traditional common fibers and the absorbent metallic or wave materials such as ceramics or carbonyl iron, the EM wave cannot be reflected by the surface of a traditional fiber. When the EM wave penetrates a normal fiber there will be no EM loss, in other words common textiles are transparent to EM waves. An EM wave cannot therefore be regulated by reflection or absorption when it passes through common textiles.

Fortunately the manufacturing techniques for metalized fibers or textiles is maturing and becoming more widely understood. For example, EM shielding fabrics can be produced by the metalization of textiles.²⁷ These metalized fibers and fabrics show similar conductivity and EM parameters to those of the metallic materials themselves. There are however still problems in the application of metalized textiles. For example, a metalized planar textile reflects an EM wave in the same way as the metal itself, making the target easily exposed to the formation of secondary pollution. The tight structure of metal coated EM shielding fabrics are hardly likely to provide good thermal–moisture transmission. To improve EM wave absorbing properties we might increase the amount of absorbing agent, but this would significantly increase the weight of the products. For example, the minimum area density of an absorbent coating material made from a typical carbonyl iron is 5.49 kg/m², when the reflection coefficient was less than −10 dB and bandwidth was greater than 3.9 GHz in the X-band.²⁸ It is thus difficult efficiently to control the transmission properties of an EM wave through a textile solely by taking advantage of the EM reflection and absorption properties of the material itself. In order to make the textile a good EM device we have to combine functional materials such as metallic and EM wave-absorbing materials with different structures into fiber assemblies to obtain textiles with specific EM properties. Three research approaches have been followed.

The relationship between the pore size and shape of the fabric and EM SE in highly reflective planar textiles has been investigated. In order to obtain both high SE and good thermal–moisture transmission, protective EM shielding fabrics with a large number of pores have been developed for certain frequency intervals. Meanwhile, lightweight metalized textiles based on a larger pore silver-plated nylon fiber mesh could destroy the EM emission source and hence offer low-cost and rapid attack weapons for electronic warfare.

The relationship between the planar periodic structure and frequency selectivity property of EM textiles has been studied. By arranging a one- or two-dimensional periodic conductive unit array on a non-conductive surface, or a non-conductive unit array on a conductive surface, we can obtain frequency selective surfaces (FSS), which have a band pass or band reject, and high-pass or low-pass characteristics, for certain EM waves.²⁹ Three types of repeat structural units are commonly used, either separately or in combination. One is a center connection shape, including a cross and Y-shape. A second is a hollow shape, including a circular, square or a hexagonal hollow loop. The third is a solid, including square and circular units. These make it easy to prepare FSS textiles; for example, a metalized fabric with pores could be made by electroless plating, periodic conductive units on the fabrics could be prepared by metal, either partially plated or magnetron sputtered, or by embroidering with a metal or metal-coated filament. These fabric FSS are light and flexible compared with traditional metal FSS.

EM textiles with three-dimensional structures have been developed and their reflective properties investigated. These fabrics are three-dimensional, including velvet and plush fabrics, and have a spacer fabric of metalized fibers, and they exhibit characteristic EM wave reflection or frequency selective penetration properties.

Experimental methods

Materials

Aluminum foils with various pore size

The commercial aluminum foils employed were 0.19 mm thick. Two series of circular pores were punched in the foils, and the distances between the centers of adjacent pores and the pore diameters of two series of pores are shown in Table 1.

Table 1.

Description of aluminum foil samples

Sample series	Series 1			Series 2
Pore distance, mm	10			2.5	5	7.5	10	15
Pore diameter, mm	1	2	3	2.5

Fabric samples

Experimental fabrics with either two-dimensional periodic structure units or a three-dimensional structure were produced, as shown in Table 2.

Table 2.

Description of fabric samples

No.	Name	Components	Area density(g/m²)	Method	Corresponding picture
1	fabric 1# with pores	copper/nickel-coated PET	180	electroless plating	Figure 4, 1#
2	fabric 2# with pores	copper/nickel-coated PET	180		Figure 4, 2#
3	fabric 3# with pores	copper/nickel-coated PET	180		Figure 4, 3#
4	fabric FSS	copper-coated PET	175		Figure 10(a)
5	plush fabric	silver-coated nylon/PET	224		Figure 13
6	velvet fabric	cotton/stainless steel fiber	–	handmade	Figure 15
7	spacer fabric DY007	silver-coated nylon/PET fabric	264		Figure 16
8	spacer fabric DY009	silver-coated nylon/PET fabric	263	warp knitting	Figure 16
9	spacer fabric DY010	silver-coated nylon/PET fabric	289		Figure 16

Test methods

EM properties

Equipment includes an Agilent E8257D signal generator, 7–9 GHz parabolic antenna, 1–18 GHz double-ridged horn antenna, spectrum analyzer and signal receiver E7405A, flange coaxial testing apparatus.

Shielding effectiveness (SE)

The SE is the ratio of power without (p₁) and power with (p₂) a shielding sample.³¹ Considering the environment, the SE can be calculated in dB using Equation (1). The SE was measured by the shielded enclosure method at 4∼18 GHz high frequency and by the flanged coaxial method at 100 KHz ∼1.5 GHz low frequency. Both the SE and EM reflection coefficient ( $S_{11}$ ) were tested in the EM Compatibility Laboratory of the Beijing University of Technology.

SE = 10 lg \frac{p_{1}}{p_{2}} = 10 lg \frac{p_{1}^{'} {- p}_{0}}{p_{2}^{'} {- p}_{0}},

(1)

where P′₁ was the testing power without shielding sample, P′₂ was the testing power with shielding sample, and p₀ was the testing power of the environment.

The EM reflection coefficient (S₁₁)

The test principle of the arch method is illustrated in Figure 1. The EM wave signal from the transmitting antenna is reflected by the sample on the reflective aluminum board and received by the antenna. The ratio of the receiving power (p_τ) and the transmitting power (p_t) gives the reflection coefficient (S₁₁):

S_{11} = 101 g \frac{p_{τ}}{P_{t}}

(2)

Figure 1.

The arch method testing system.

Regarding the environment, S₁₁ could be calculated using Equation (3):

S_{11} = 10 \lg \frac{p_{1}^{τ}}{p_{t}} - 10 \lg \frac{p_{2}^{τ}}{p_{t}} = (p_{_{1}}' - p_{t}') - (p_{2}' - p_{t}') = p_{_{1}}' - p_{2}' < lowered >, < / lowered >

(3)

where

p_{1}'

and

p_{2}'

are the receiving power with and without the sample on the aluminum plate.

The size of the sample is 180 mm × 180 mm. Under unspecified conditions, the angle between the transmitting and the receiving antenna is 15°, and the angle between each antenna and the sample normal center is 7.5°. The distance between the transmitting antenna and receiving antenna is 4 m.

Results

The development of an EM textile with pores, and the relationship between the pore size and the SE

The theoretical relationship between pore size and SE

According to the principle of the pore coupling of EM compatibility, when the pore size is much smaller than the wavelength (λ), the pore is equivalent to the electric and the magnetic dipole.

For conductor plate shielding materials, SE can be calculated by the Schelkunoff formula (4) based on the transmission theory, SE_A (dB) being the absorbing loss of the shielding materials, SE_R (dB) is the single reflection loss on the surface of the shielding materials, and SE_M (dB) was the multi-reflection loss inside the shielding materials:³¹

SE = {SE}_{A} + {SE}_{R} + {SE}_{M}

(4)

{SE}_{A} = 20 \lg [\exp (t / σ_{r} {f μ}_{r})] = 131.43 t \sqrt{{f μ}_{r} σ_{r}}

(5)

{SE}_{R} = 168.2 + 10 \lg (σ_{r} / σ_{r} {f μ}_{r} f r)

(6)

{SE}_{M} = 20 \lg [1 - \exp (- 2 t / - 2 t δ)] = 10 [1 - 2 \times 10 - 0.1 {SE}_{A} \cos (0.23 {SE}_{A}) + 10 - 0.2 {SE}_{A}]

(7)

where f(Hz) is the EM wave frequency,

μ_{r} and σ_{r}

are respectively the relative permeability and relative conductivity of the shielding materials; t (m) is the thickness of the shielding materials and δ is the skin depth. From Equation (7), SE_M is related to SE_A and can be ignored when SE_A is greater than 15 dB or

(t / σ_{r} {f μ}_{r} g) >> 1

Considering the total metal plate without pores, its SE_t at distant field and plane wave can be expressed by Equation (8). When the distance between the shielding material and the EM field source was greater than $(λ / σ_{r} {f μ}_{r} 2)$ , the EM field can be the distant field and the EM wave can be the plane wave.

{SE}_{t} = 20 \lg \frac{1}{T_{t}} = 131.43 t \sqrt{{f μ}_{r} σ_{r}} + 168.2 + 10 \lg (σ_{r} / σ_{r} {f μ}_{r} f r)

(8)

In the case of the metal plate or foil with circular pores, its SE can be calculated by the empirical Equation (9), if ∑q = F and $D = λ$ :³¹

SE = 20 \lg (1 / T_{r} {f μ}_{r} T) = 20 \lg \frac{1}{(T_{t} + T_{h})} = 20 \lg \frac{1}{(T_{t} + 4 n (q / σ_{r} {f μ}_{r} F) \frac{3}{2})}

(9)

where D is the diameter of the circular pore, q is the area of each pore, n is the number of pores, and F is the total area of the shielding metal foil, T_t is the transmission coefficient of the total shielding metal and is shown in Equation (8), and T_h is the transmission coefficient of the pores on the shielding metal foil.

Combining Equations (8) and (9), the SE of the metal plate with pores is not only related to the EM parameters ( $μ_{r} {, σ}_{r}$ ),the thickness (t) of the metal plate and the frequency, but is also inversely proportional to the third power of the side length of square pores, or the diameter of round pores, or to the length of the gap. These have been applied in the EM leakage control of the metal plate with pores.³¹ The pores on the shielding materials would decrease the SE, but shielding materials with both good SE and proper pore structures can be engineered. For example, the 2.45 GHz EM microwave will not leak, even with multiple holes in the metal window of a household microwave. In Equations (8) and (9) a number of factors will in fact affect SE. Even for a conventional metal plate with a pore structure it is still not possible to construct a theoretical equation to express properly the relationship between the pores and the SE.

The SE of the metal mesh is obviously different from that of the metal plate with pores on its surface. For a plane wave in a distant EM field, the SE of the metal mesh depends mainly on the reflection loss. According to transmission theory,³⁰ it is given by the empirical equation (10).

SE = 20 \lg \frac{1}{T} = 20 \lg \frac{1}{s \times \sqrt{(0.265 \times 10 - 2 R_{f}) 2 + [0.265 \times 10 - 2 X_{f} + 0.333 \times 10 - 8 f (In \frac{s}{a} - 1.5)] 2}}

(10)

where T is the transmission coefficient, s is the distance from the adjacent metal wire of diameter a and AC resistance R_f, X_f is reactive resistance of the metal wire, and f is the EM wave frequency. The SE of the metal mesh is thus related to the diameter of the wire, the distance between the wires, and AC resistance and reactive resistance per unit length.

Aluminum foils with pores

In order to establish how the pores affect the SE of thin and soft conductive materials, firstly the aluminum foils were punched by hand with an array of pores. Figure 2 shows the SE of 0.19 mm thick aluminum foil in which the diameter of circular pores was respectively 1, 2 and 3 mm, and the pores were 10 mm apart.

Figure 2.

The SE of aluminum foils of similar pore distance but different pore diameter.

The EM wave length for 1800 MHz was 16.5 cm, which was much greater than the pore diameter. According to Equation (9), the larger the diameter of the pores on the aluminum foil, and hence the larger the area of the pores, the lower the SE. This applies to the data in Figure 2.

The SE of aluminum foils having the same diameter pores but different pore distance is illustrated in Figure 3. The larger the pore distance, the fewer the number of pores in a similar area of foil and the larger the SE, which agrees with the data in Figure 3. The data curve of 7.5 mm pore distance was very close to the data curve for a 10.0 mm pore distance, which could have been due to measurement or sample preparation error. In Figures 2 and 3 the SE data were difficult to estimate or calculate from the equation. The two aluminum foil experiments demonstrated that SE could be controlled by changing the size and arrangement of the pores on the shielding plate.

Figure 3.

The SE of aluminum foils with similar pore diameter but different pore distance.

The development of a shielding fabric with pores

In order to develop shielding fabrics with both good SE and thermal–moisture transmission properties, we prepared three kinds of metalized fabrics with pores, based on the above theory and experimental work. Figure 4 shows three different kinds of weave diagram, corresponding to the fabrics illustrated below in Figure 5. The surfaces of the three fabrics were electrolessly plated using metallic copper and nickel.

Figure 4.

Weave diagrams of fabrics with pores.

Figure 5.

Metalized fabrics with pores.

The relationship between pore size and SE of the fabrics is shown in Figure 6. Sample 0# was a fabric with a plain tight weave. It was difficult to measure the exact pore size and area, and it was also difficult to estimate the pore area from the weave diagram. From Figure 6 and Equation (9), it is seen that in the sequence from fabrics 1# and 3# to 2# the pore area became smaller and the SE higher, indicating that we could select certain weave diagrams to produce shielding fabrics with a certain SE and good thermal–moisture transmission properties. If necessary, a thermoplastic synthetic fabric with a more complex pore structure and arrangement could be produced by laser drilling.

Figure 6.

The SE curves of metalized fabrics with pores.

The development of an EM textile net with mesh

A silver-coated nylon mesh fabric with different mesh sizes was developed. When the mesh fabric with a 5 mm side length of the square mesh was 0 m and 2 m distant from the launch antenna; the SE curves are shown in Figure 7. The SE was about 5 dB at 0 m close to the EM field and about 10 dB at 2 m distant from the EM field. Though none of the parameters in Equation (10) changed with distance, the wave in the near field was spherical and Equation (10) was not applicable, meaning that many of the waves would bypass the sample and the SE would decrease.

Figure 7.

The SE of silver-coated nylon mesh fabric with a 5 mm square mesh: (a) 0 m interval from flexible network to the antenna, and (b) 2 m interval between flexible network and antenna.

According to Equation (10), the SE will vary by changing the size of the mesh. The SE obtained by computer simulation of a net with different square mesh and 0 m interval from the net to antenna is shown in Table 3 at 7–9 GHz frequency. When s = 5 mm the SE was 5.1∼6.7 dB, which was only slightly greater than the measurement value, but the trend in the measurement value and the simulation value were similar.³⁰

Table 3.

The maximum and minimum simulated values of SE with different sized square mesh

Size (mm)	Maximum (dB)	Minimum (dB)
1	50.0	52.3
2	30.6	32.8
3	21.8	24.0
4	16.3	18.6
5	6.7	5.1
10	3.3	2.1
15	1.8	1.1
20	1.1	0.7
25	0.8	0.4

From Table 3, when the side length of the square mesh was 1 mm the SE would be 50 ∼ 52.3 dB. For silver-coated nylon net with square mesh of 5 mm side length, the area density of a net with 75 dtex nylon filament and 20% silver coating was only 36 g/m². The net was so light it could be launched using a net gun. This was an effective method to limit the signal source without any traces.

At the same time the SE would change with the frequency, according to Equation (10). The simulated SE of a shielding net made of 75 dtex silver-coated nylon filament is shown in Figure 8. The square mesh size was 1 mm. On the one hand, in everyday life the EM radiation would be seriously harmful to humans at a low frequency of 30 MHz∼3 GHz,³⁰ whereas the shielding mesh fabrics had high SE, around 25 dB. On the other hand, the SE was lower at higher frequencies, 16∼18 GHz. Each mesh behaved as a waveguide tube, so the mesh fabric had a cut-off frequency. According to cut-off waveguide tube theory, an EM wave with frequency higher than the cut-off frequency would pass through the fabric mesh as a high-pass filter. The mesh fabric would also have good SE and thermal–moisture transmittance.

Figure 8.

Simulated SE of silver-coated nylon mesh fabric with 1 mm square mesh.

The performance and development of the EM textiles with a planar periodic structure

Preparation method for the FSS textile

Based on FSS theory, a periodic array of the non-conductive units (or pore units) on a planar conductor surface could cause a band-pass or a high-pass effect, and a periodic array of conductive units (or a resistance element when conductivity was poor) on a planar non-conductive surface could cause a band-stop or low-pass effect. An EM wave would therefore pass selectively through a FSS. A FSS was thus an ideal choice for the useful signal and the inhibition of an interference signal. The fine control of EM waves is more useful than for simply blocking and shielding. In the EM field an FSS made of metallic materials has been extensively researched.²⁹ On the other hand, the formation of textiles in which the EM wave frequency selectively penetrates has not so far received attention.

Using normal textile preparation methods a FSS textile was not difficult to produce. All the factors could be designed in, mixtures of base materials with different EM parameters were available, including the shape or size of units on the surface, unit array density and mode. FSS textiles would be softer and lighter than those made from traditional metallic materials.

For high-pass effect FSS textiles, non-conductive structural units on conductive fabrics could be obtained by laser drilling on the metalized synthetic material, printing the mask pattern to block the metalization process and forming a periodic non-conductive unit. Many methods can be used to metalize the textile, including magnetron sputtering and electroless plating, metal coating, or weaving with metal or metalized fibers. The conductive periodic structural units on an ordinary fabric could be produced in a similar manner, including partially electroless plating, electroplating or magnetron sputtering, partly coating or printing a conductive polymer, or by computer-based embroidering with conductive fibers. These processing methods are relatively simple, combining traditional textile processes and new metalization technologies. The following experiments describe how EM textiles with a planar periodic structure can be obtained with a significant resonance peak at a given frequency.

The theoretical frequency response characteristics of FSS with periodic circular shape

A number of factors affect the resonance frequency, including the shape and size of FSS units, the EM parameters of the medium, and the polarization and incident angle of the EM wave.²⁹ Due to the complexity of the FSS structure and EM transmission, the equivalent circuit model is only suitable for simple FSS units such as grids, circular or rectangular units The frequency resonance properties of the FSS can be analyzed from the equivalent circuit model.

Considering the size of the periodic units, for the circle-type patch structure units in Figure 9 the equivalent circuit model is shown in Figure 10. For an LC oscillating circuit, the resonance frequency is calculated from Equation (11).³²

f = \frac{1}{2 π \sqrt{L \cdot C}}

(11)

When

d' << a

, L (inductance) and C (capacitance) can be derived using Equations (12) and (13).^33,34

C = \frac{ɛ_{0} 2 a cos θ}{π} [\ln \frac{2 a}{π d'} + \frac{1}{2} (3 - 2 cos 2 θ) (\frac{a}{λ}) 2]

(12)

L = \frac{μ_{0} a cos θ}{2 π} [\ln \frac{2 a}{π d'} + \frac{1}{2} (3 - 2 \cos 2 θ) (\frac{a}{λ}) 2]

(13)

Figure 9.

The circular patch-type structure.

Figure 10.

The circular model of the equivalent circuit.

When the EM wave is vertically incident to the surface, the resonance frequency (f) at low frequencies of FSS (when $\frac{a}{λ} << 1$ ) is given by Equation (14).³²

f = \frac{1}{2 π \sqrt{L \cdot C}} = \frac{1}{2 π \frac{a}{π c} \sqrt{\ln \frac{2 a}{π d'} \cdot \ln \frac{2 a}{π d}}},

(14)

where a is the distance from the center of one periodic unit to an adjacent periodic unit, d is the width of the circle, and d′ is the distance between adjacent periodic units. For the circular shape in Figure 9, d = R₁–R₂, and d′ = 2(a/2–R₁). From the model equation, the resonance frequency of FSS is related to the size and arrangement of the periodic units. For the shape of FSS units, the diameter of round units is about 1/3 of the resonance wavelength.²⁹

Reflection properties of the band-pass filter fabric

Figure 11 shows the fabric FSS with ring-type repeating units. These circular non-conductive units on the conductive fabric surface can be produced by first printing a passivating solution onto the circular units, followed by electrolessly plating metal over the whole surface. The outer and inner diameter of the ring shown in Figure 11 were 8 mm and 6 mm, respectively; the periodic distance was 20 mm and the periodic unit distance d′ was 4 mm. The wavelength of 2–18 GHz was 15 cm∼16.7 mm and a/λ was not significantly below 1. The resonant frequency can be calculated from Equations (11), (12), and (13). The FSS reflection coefficient (S₁₁) curve obtained by the arch method, and the simulation curve by numerical analysis, are shown in Figure 12. There are two resonant peaks, at 9.1 GHz and 15 GHz, for the simulation curve and at 8.92 GHz and 15.3 GHz for the testing reflection curve. The two were thus in good agreement, but the 15 GHz peak was not the main resonance peak. The sample could allow the 7∼11 GHz EM wave near the resonance peaks at 9.1 GHz to pass through and at the other frequency it would fail to pass. The FSS fabric showed good frequency selective properties.

Figure 11.

The band-pass filter fabric and its structural units, with the insulating unit.

Figure 12.

Reflection coefficient curve of the band-pass filter fabric.

The conductive elements with periodic structure in the insulating surface of textiles, or the non-conductive elements with periodic structure in the conductive surface, could be made by a number of methods. The resonance frequency could be controlled by choosing periodic unit sizes and shapes before preparing the flexible band-pass filter fabric with frequency selectivity. It could be used for accurate control of the EM signal transmission and energy transfer in order to transfer the signal at a particular frequency.

The performance and development of the EM textile with a three-dimensional periodic structure

The EM reflection properties of a silver-coated plush fabric

The three-dimensional periodic structure units in the fabric would acquire a more effective frequency selective performance. There are several mature technologies for producing a three-dimensional periodic structure in textiles.

Figure 13 shows the plush fabric produced from silver-coated nylon filaments on the plush loom. The length of the plush was 8 mm. The silver-coated nylon filaments were consolidated onto the bottom surface of the fabric forming independent upright U-shaped conductive units.

Figure 13.

The plush fabric formed by silver-coated fibers.

Figure 14 shows the reflection coefficient ( $S_{11}$ ) of the plush fabric where the angle between the transmitting and receiving antennae was 15°, 30°, or 45°. When the frequency was above 14 GHz, the $S_{11}$ of the plush fabric became increasingly lower. Most of the incident EM wave entering the fluff was repeatedly reflected before the energy was consumed, and only a very small proportion of the EM energy was reflected back to the receiver. Clearly the EM wave was being absorbed by the plush fabric, based on the theory that $S_{11}$ is related to fiber size.³² The fiber size affects the incident impedance of the EM wave on the surface of the plush fabric, and showed that the plush fabric exhibited frequency selectivity penetration.

Figure 14.

The reflection coefficient curve of the plush fabric containing silver-coated fiber.

EM reflection properties of cotton/stainless steel velvet fabrics

Pores 5 mm apart were punched in the fiberglass–epoxy substrate. The 184 dtex cotton/stainless (weight ratio 60/40) fiber was manually fed through from the pores on one fiberglass–epoxy substrate to the pores on a second substrate, the distance between the two substrates being 10 mm. Cotton/stainless fibers between the two fiberglass–epoxy substrates were then cut and two velvet fabrics with 5 mm plush formed, as shown in Figure 15.

Figure 15.

The handmade cotton/stainless velvet fabric.

The reflection coefficient curve of the velvet fabric is illustrated in Figure 16. It will be seen that the attenuation of reflectivity still reached −5 dB over 7∼17 GHz, although the sample had a slight plush and a low stainless content. The velvet fabrics had less mass than absorbent materials at the same level of attenuation.

Figure 16.

The reflection coefficient curves of cotton/stainless velvet fabric.

EM reflection properties of a spacer fabric composed of silver-coated fibers

The above velvet and plush fabrics had an upright open-ended plush. Warp-knitted spacer fabrics with silver-coated nylon filament spacer yarns were prepared, the two end surfaces comprising polyester fibers. The spacer yarns were consolidated into the two end surfaces. In order to achieve the best scattering effect and lowering the S₁₁, the consolidation manner of the conductive spacer yarns was modified. The conductive spacer yarns were grouped and each group arranged in a circular shape. On the lower surface of the spacer fabric, the spacer yarns were consolidated to give a smaller circumference, and on the upper surface they formed a larger circumference, giving an upwardly open and relatively flat truncated cone. The EM wave could then be repeatedly scattered within the cone and the reflectivity in any particular direction would be decreased.

The spacer fabric sample is illustrated in Figure 17, and Figure 18 shows the reflection coefficient curves of the spacer fabric, designed according to the above concept at different frequencies. Each differed in three respects, including the density of the conductive spacer yarns, the manner of consolidation, and the thickness of the fabric. The peak of the reflection coefficient curves of the spacer fabric with no more than 300 g/m² area density actually fell to −20 dB ∼ −30 dB. The bandwidth was up to 20 GHz when the reflection coefficient was below −5 dB, which is far greater than that of common absorbent materials.

Figure 17.

The spacer fabrics with silver-coated fibers.

Figure 18.

Reflection coefficient curves of spacer fabric with silver-coated nylon fibers giving different consolidation. The thickness of the samples DY007, DY008, and DY010 was respectively 4 mm, 4.2 mm, and 3.8 mm.

Conclusions

Three new types of EM textiles were prepared by different methods. One was from metalized fabrics with pores and metalized mesh fabric, since their pores size and arrangement could affect their SE on the EM-shielding theory analysis and the practical testing. The metalized fabrics with pores would be used in EM-shielding clothing and in EM weaponry. Another was FSS fabric with periodic structural units on the metalized fabric surface. The FSS fabric had a typical resonance frequency similar to the metal FSS, and its resonance frequency could be designed based on the transmission theory and special periodic structural units. The FSS fabric could form a soft high-pass or low-pass filter.

The third was 3-dimensional fabrics with metalized fibers woven in. These were plush velvet and spacer fabrics.

All three types had a low reflection coefficient and wide band width in the range 2∼18 GHz. They could be used as lightweight and wide frequency wave-absorbent materials. The three were quite different from traditional metallic EM devices. They were soft and light in weight and easy to bend. Due to the complexity of the transmission of the EM wave, even in metallic EM devices, the present paper is able to give only a simplified analysis.

There is still a long way to go to develop soft lightweight EM textiles, and to establish the relevant theoretical basis.

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The electromagnetic shielding and reflective properties of electromagnetic textiles with pores,planar periodic units and space structures

Abstract

Keywords

Three methods for developing new EM textiles

Experimental methods

Materials

Aluminum foils with various pore size

Fabric samples

Test methods

EM properties

Shielding effectiveness (SE)

The EM reflection coefficient (S11)

Results

The development of an EM textile with pores, and the relationship between the pore size and the SE

The theoretical relationship between pore size and SE

Aluminum foils with pores

The development of a shielding fabric with pores

The development of an EM textile net with mesh

The performance and development of the EM textiles with a planar periodic structure

Preparation method for the FSS textile

The theoretical frequency response characteristics of FSS with periodic circular shape

Reflection properties of the band-pass filter fabric

The performance and development of the EM textile with a three-dimensional periodic structure

The EM reflection properties of a silver-coated plush fabric

EM reflection properties of cotton/stainless steel velvet fabrics

EM reflection properties of a spacer fabric composed of silver-coated fibers

Conclusions

References

The EM reflection coefficient (S₁₁)