Electromagnetic properties of electro-textiles for wearable antennas applications

Abstract

Wearable, textile-based antennas get more and more attention for body-centric communications because it could be easily worn on body and integrated into clothes. Electro-textiles (e-textiles) are used as antenna patch and ground plane. The electromagnetic properties of the textiles play important roles in antenna design and performance. This paper focuses on the study of the electromagnetic properties of e-textiles for wearable antennas applications and mainly discusses the electromagnetic properties of e-textile cell and the influences of different woven densities and different e-textile materials to antenna performances. Simulation and measurement results show that if the e-textiles adopt woven pattern, then when the distance between two conductive fibers is within 2 mm, the e-textiles could be regarded as metal plane to design antennas. In addition, the results show that metal-plated woven fabric could be used as metal plane to design antennas, while non-woven fabric shows distinct differences.

Keywords

wearable antenna / electro-textile (e-textile) / electromagnetic properties

Introduction

Electro-textiles (e-textiles) have appeared for a period of time; however, they are mainly used as electromagnetic shielding materials [1]. In recent years, as the development of the fourth generation of mobile communications (4G), wearable antennas, especially textile antennas, get more and more attention [2-5]. E-textiles are used as antenna patch and ground plane, while nonconductive textiles constitute antenna substrate. This type of antenna could be easily worn on body and integrated into clothes. The electromagnetic properties of e-textiles play important roles in antenna design and performance. In Ref. [6], Ouyang et al. proposed a waveguide-based cavity method to measure the high-frequency properties of e-textiles. In Ref. [7], Banaszczyk et al. discussed the current distribution within woven e-textile sheets using a computer program. They took the conductivity of the fibers, the contact resistance between the fibers, the contact angle between the fibers and the electrodes, and the size and the aspect ratio of the textile sheets into account. However, the inductance is not discussed. In Ref. [8], Ouyang et al. compared two different e-textile antennas made of the same e-textile material but of different structure. To the best of our knowledge, in this area, no published paper has been found in China yet. This paper focuses on the study on the electromagnetic properties of e-textiles for wearable antennas applications, mainly studies the electromagnetic properties of e-textile cell, and calculates the basic resistance and inductance in a grid as well as discusses the influences of different woven densities and e-textile materials to antenna performances.

Study on electromagnetic properties of e-textile cell

In general, there are two main methods that could rend a textile material electrically conductive: 1) by applying a conductive coating on the surface of a non-conductive textile after it is formed and 2) by incorporating conductive fibers (e.g., via interweaving or via embroidery) into the textile structure. Any textile structure, including knitted, woven, or nonwoven textiles, can be thus made electrically conductive [6,8-10].

Because the textile adopting woven pattern has more intersections and is tighter, which means less distances between metal grids, as shown in Fig. 1, it is adopted in this paper to study the electromagnetic properties of e-textiles.

We assume that the e-textiles are woven pattern, with woven density N ppi, and the conductive fibers are made of copper, with diameter of 80 μm. In this case, the distance between two conductive wires could be expressed as

(1)d={\displaystyle \frac{\mathrm{25.4}-\mathrm{0.08}N}{N-\mathrm{1}}}mm\mathrm{.}

The calculated distances between conductive fibers at different woven densities could be seen in Table 1.

Wearable antennas are often used in the ISM band, especially 2.4 GHz, so we take f=\mathrm{2.4}\mathrm{G}\mathrm{H}\mathrm{z} into account. While f=\mathrm{2.4}\mathrm{G}\mathrm{H}\mathrm{z}, \lambda =c/f=\mathrm{125}\mathrm{m}\mathrm{m}, at this time \lambda /\mathrm{10}=\mathrm{12.5}\mathrm{m}\mathrm{m}, the electrical dimensions of the e-textile cells are electrical small, so we should construct lumped-parameter electric circuit models.

As described above, if we take the woven density N=50 ppi, the distance between conductive fibers is d=\mathrm{437}\mathrm{\mu }\mathrm{m}. The skin depth of copper at 2.45 GHz is \delta =\mathrm{1}/\sqrt{\mathrm{\pi }f{\mu }_{\mathrm{0}}\sigma }=\mathrm{1.35}\times {\mathrm{10}}^{-\mathrm{6}}\mathrm{m} (\sigma =\mathrm{5.8}\times {\mathrm{10}}^{\mathrm{7}}\mathrm{S}/\mathrm{m}, {ϵ}_{\mathrm{r}}=\mathrm{1}, {\mu }_{\mathrm{r}}=\mathrm{1}). At this time, r_{\mathrm{w}}/\delta \approx \mathrm{30} (r_{\mathrm{w}} is the radius of the conductive fiber).

According to the formula used to calculate the per-unit-length resistance of solid wire [11],

(2)r_{\mathrm{h}\mathrm{f}}={\displaystyle \frac{\mathrm{1}}{\mathrm{2}{r}_{\mathrm{w}}}}\sqrt{{\displaystyle \frac{{\mu }_{\mathrm{0}}}{\pi \sigma }}}\sqrt{f}(r_{\mathrm{w}}\gg \delta ).

We could get that the per-unit-length resistance of the fiber is \mathrm{4.54}\times {\mathrm{10}}^{\mathrm{4}}\mathrm{\Omega }/\mathrm{m}.

Consider higher frequencies where r_{\mathrm{w}}>\mathrm{2}\delta, the per-unit-length external inductance is larger than the per-unit-length internal inductance by a factor of 10, and above this frequency, the difference increases since the external inductance remains constant with increasing frequency, but the internal inductance decreases as \mathrm{1}/\sqrt{f}. Consequently, the impedance of the internal inductance is usually much smaller than the impedance of the external inductance, and we may therefore neglect the internal inductance in the model.

Assuming that the two wires are separated sufficiently (s/r_{\mathrm{w}}>\mathrm{5}) (s is the distance between the two wires), according to the formula that is used to calculate the per-unit-length external inductance of two parallel wires [11],

(3)l={\displaystyle \frac{{\mu }_{\mathrm{0}}}{\mathrm{\pi }}}\ln {\displaystyle \frac{s}{r_{\mathrm{w}}}}\mathrm{H}/\mathrm{m}\mathrm{.}

We could get that the per-unit-length inductance is 0.76 H/m.

As discussed above, the resistance in a grid length is R=\mathrm{19.8}\mathrm{\Omega }, while the inductance is L=\mathrm{330}\mathrm{\mu }\mathrm{H}.

Influences of different woven densities to antenna performances

To study the influences of different woven densities to antenna performances, we use a full-wave electromagnetic simulation software HFSS based on FEM to simulate antennas consisted of metal grids at different woven densities. We simulated metal plane antenna, 0.5-mm-long gird antenna, 1-mm-long grid antenna, 1.5-mm-long grid antenna, and 2-mm-long grid antenna, respectively [12]. The results of return loss and radiation with different woven densities are shown in Figs. 2 and 3 and Table 2.

From the simulation and comparison above, we could see that if the e-textiles adopt woven pattern, then when the grid length is less than 2 mm, and the bandwidth, radiation pattern, and radiation efficiency are nearly the same as the metal plane antenna; however, when the grid length reaches 2 mm, there is an obvious offset in the frequency band as well as decreased radiation efficiency.

Influences of different e-textile materials to antenna performances

To study the influences of different e-textile materials to antenna performances, we adopt four different materials to design and fabricate four microstrip patch antennas [12]. All the geometry parameters of the four antennas are the same, and FR-4 with relative permittivity of 4.4 is used as the substrate, while copper is used as the ground plane. Copper, Nora (Ni/Cu/Ag-plated Nylon fabric with surface resistance<0.03 Ω/m² and total thickness 0.1 mm nominal), Kassel (Cu/Ag-plated Nylon fabric with surface resistance<0.03 Ω/m² and total thickness 0.110 mm nominal), and Koln (Cu-plated Nylon non-woven fabric with surface resistance<0.02 Ω/m² and total thickness 0.25 mm nominal) are adopted as the antenna patch, respectively. The four fabricated antennas are shown in Fig. 4.

We measure the four antennas using a vector network analyzer HP 8752C, and the results are shown in Table 3.

From the above comparison and analysis, we could see that the antenna adopting Cu/Ag-plated Nylon fabric e-textile shows the most similar performance to the metal patch antenna, while the antenna adopting Ni/Cu/Ag-plated Nylon fabric also shows similar performance to the metal patch antenna; however, the antenna adopting Cu-plated Nylon non-woven fabric has distinct difference from the metal patch antenna.

Conclusion

This paper studies the electromagnetic properties of e-textiles for wearable antennas applications and mainly focuses on the influences of different woven densities and materials to antenna performances. According to the analysis and experiments mentioned above, we could conclude that if the e-textiles adopt woven pattern, then when the distance between two conductive fibers is within 2 mm, we could regard e-textiles as metal plane to design antennas. In addition, metal-plated woven fabric could be used as metal plane to design antennas, while non-woven fabric shows distinct differences.

References

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