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Layer-to-Layer Angle Interlock 3D Woven Bandstop Frequency Selective Surface.

1. INTRODUCTION

During the last decade, there has been an increasing interest in frequency selective surfaces (FSS) due to the large number of applications in which they can be employed [1], such as absorbers [2], artificial magnetic conductors [3,4], electromagnetic shielding, among others. The electromagnetic shielding can be achieved thanks to the bandpass [5,6] or bandstop [7,8] responses of the FSS, depending on the design of the periodic structure.

Different approaches have been used to develop FSS, from conventional designs based on resonators implemented over rigid substrates [9-11], FSS based on 3D printing [12], the use of high permittivity ceramic materials in order to avoid conductive materials [13], thicker multiband designs based on substrate integrated waveguide (SIW) technology [14] or multilayered designs [15]. The before mentioned alternatives present different advantages, such as the possibility of achieving high precision details in the fabricated prototypes. Nevertheless, none of them are flexible or washable, and their dimensions are constrained by the prototyping machine or 3D printer, leading to a limited number of unit cells in the manufactured design.

For this reason, during the last few years, there has been an increasing interest in flexible circuits and antennas, based on textile technology, thanks to the possibility of integrating them in special garments and automobile upholsteries, or developing comformable structures for biomedical applications. In the literature, a variety of solutions to develop Textile Integrated Circuits (TIC) has been proposed. Embroidered techniques [16-20], non-woven solutions [21-23], designs based on using several fabrics with different electromagnetic behaviour [24-26] or inkjet printed patterns over textile substrates [27-30] are some of the most cited solutions. More recently, there has been an increasing interest in the development of TIC based on Substrate Integrated Waveguide (SIW) technology [31,32].

Some of the aforementioned solutions have also been developed to integrate the FSS into the textile. As an example, in [33], a flexible and portable textile-reflectarray backed by an FSS is proposed using two separate embroidered fabrics connected through Velcros. However, this solution requires two fabrics and is not appropiate for low profile applications. In [34], an inkjet printed FSS over a fabric is presented, whereas in [35], an FSS is proposed employing the screen printing technology. These procedures present the difficulty of achieving a thin and conductive printed layer over an uneven substrate, leading to the necessity of double-printing the layout or using intermediate impermeable coatings between the fabric and the ink, increasing the number of required subprocesses. Furthermore, the ink must be properly printed on the fabric to avoid its deterioration due to the wear out.

With the aim of pushing textile components to the microwave range, a weaving-based procedure to design, simulate and manufacture RF textile-integrated structures has already been thoroughly discussed in [36-38]. Moreover, this weaving-based procedure presents the advantage of avoiding posterior adhesives, coatings or sewing procedures to manufacture the flexible prototypes. For applications such as momentarily shielding a room from a certain frequency, a flexible woven structure can work as a smart curtain, which is easy to install, uninstall, keep or transport. Moreover, industrial weaving looms allow the automatically manufacturing of very large prototypes, in comparison with standard prototyping machines. For this reason, flexible textile-integrated FSS can overcome the problems attributed to standard FSS, opening a new field of research.

In this paper, a flexible fully textile-integrated bandstop FSS is designed. It can be manufactured with existing industrial textile machinery, making it suitable for mass production in larger dimensions than conventional FSS based on rigid substrates. The proposed FSS is consists of a layer-to-layer angle interlock 3D woven fabric whose top layer presents periodic cross shaped conductive resonators and its performance is independent of the incident wave direction and polarisation.

The paper is organised as follows. In Section 2, the structure of the FSS will be explained. In Section 3, a description of the employed materials will be presented. In Section 4, the translation into a woven FSS will be discussed. In Section 5, the simulated data will be presented. In Section 6, the fabrication process of the FSS will be explained. In Section 7, the experimental validation will be presented and discussed.

2. STRUCTURE OF THE FSS

A flexible bandstop FSS is proposed to work at 3.75 GHz with a 0.8 GHz bandwidth. The FSS has been designed from a unit cell to which periodic boundary conditions (PBC) have been applied, leading to an ideal infinite FSS. Figure 1(a) represents an overview of the unit cell and the boundary conditions, as well as a side view of the proposed unit cell where the different layers can be identified. The tangential component of the magnetic field, ht, in the two boundaries which are parallel to the YZ-plane is zero and the tangential component of the electric field, et, in the two boundaries which are parallel to the XZ-plane is zero, leading to a PBC environment. The unit cell has been illuminated with a plane wave, whose polarisation axis is vertically oriented (Y-axis), propagating in the k direction from port 1 to port 2, so that the [S.sub.21] parameter can be calculated.

The unit cell is composed of three layers, leading to a total height of the unit cell denoted by H. The top layer of the unit cell is composed of a cross-shaped resonator, whose side and width are LT and WT, respectively, as it can be seen on the upper part of Figure 1(b). The middle and bottom layers are identical and completely dielectric, as depicted on the bottom part of Figure 1(b). The total length of the square-shaped unit cell is L. All the mentioned dimensions are summarised in Table 1.

The proposed FSS is fully integrated in textile; therefore, the conductive and dielectric elements are manufactured using different types of threads. The employed materials will be explained in Section 3 and the translation into a woven prototype will be detailed in Section 4. For this reason, the dimensions summarised in Table 1 have been calculated once a sample of the textile substrate has been electromagnetically characterised, as it will be explained in Section 4.

3. DESCRIPTION OF THE EMPLOYED MATERIALS

Different materials have been employed for the design of the FSS, for both the weft and the warp directions. The weft direction coincides with the width of the fabric and, consequently, with the width of the loom, while the warp direction coincides with the length of the fabric. The warp threads are previously assembled in the loom, whereas the weft yarns are successively inserted in the woven structure during the fabrication process. In the proposed FSS, the warp direction coincides with the direction of the F-axis. Consequently, the weft direction is oriented in the X-axis direction. In this section, a description of the employed materials and, therefore, the structure of the woven substrate is presented.

3.1. Description of the Electrically Conductive Materials

Electrically conductive Shieldex 117f17 2-ply yarns [39], with a resistance of 3.9[ohm]/cm have been used for the conductive warp and weft threads. The ply number means the number of initial threads that are twisted around one another to create a single and stronger thread. These threads consist of two plies and each ply is formed by 17 filaments and weights 117dtex. The diameter of the Shieldex 117f17 2-ply yarns is [d.sub.Shieldex] = 0.3 mm. In the upper right corner of the Figure 2 a sample of a Shieldex 117f17 2-ply yarn is depicted.

3.2. Description of the Dielectric Materials

For the dielectric parts of the FSS two different types of threads have been used. For the weft threads, high tenacity black polyethersulfone (commonly known as PES) 550dtex, formed by 28 filaments, has been employed. The diameter of the PES yarns is [d.sub.PES] = 0.3 mm. For the warp threads, white polyethylene-terephthalate (commonly known as PET or polyester) 76/24, which means that it is composed of 76 filaments with 24dtex, has been used. The diameter of the PET yarns is [d.sub.PET] = 0.1 mm. Figure 2 depicts a sample of the woven substrate, where the black PES is depicted, and a magnification of a warp PET yarn is detailed in the bottom right corner.

4. DESIGN OF THE WOVEN FSS

A layer-to-layer angle interlock 3D woven bandstop FSS is proposed. The FSS has been designed from a unit cell to which PBC has been applied, leading to an ideal infinite FSS. Nevertheless, a finite version composed of 8 x 8 unit cells has also been simulated to compare the simulations with the measurements. First, a sample of the dielectric substrate has been manufactured and electromagnetically characterised in order to design the FSS and, therefore, translate it into a woven prototype.

The substrate of the FSS is a multilayered fabric composed of three layers of weft PES threads interwoven using warp PET yarns, leading to a satin woven structure. Due to the air gaps between the threads, a sample of the substrate has been manufactured and electromagnetically characterised using an Agilent Technologies 85072A Split Cylinder Resonator and its relative dielectric permittivity and loss tangent have been found to be [[epsilon].sub.subs] = 1.75 and tan[([delta]).sub.subs] = 0.0037, respectively. Figure 2 represents the top view of a sample of the woven substrate.

As previously mentioned, the woven prototype is composed of three layers: the top layer is a dielectric layer in which the conductive cross-shaped resonators are inserted, and the middle and bottom layers are identical and completely dielectric layers. Two dielectric layers have been employed in order to provide the prototype with more consistency. The three layers are identical, except for the positions where the dielectric threads have been substituted by conductive threads to implement the resonators. The conductive resonators placed in the top layer are translated into 5 Shieldex threads as shown in Figure 3(a), which will be simulated as 5 conductive strips, separated a distance [s.sub.T] between centres and whose widths are [d.sub.T] = [d.sub.Shieidex], as it can be seen in Figure 3(b). The middle and bottom layers are completely dielectric as depicted in Figure 3(c). The mentioned dimensions are summarised in Table 1.

Nevertheless, Figure 4 depicts different detailed versions of the translation of the unit cell into a woven prototype. Figure 4(a) represents the detailed uniform cross-shaped resonator which would be structured in a conventional rigid substrate. Figure 4(b), Figure 4(c) and Figure 4(d) represent, respectively, a detailed view of the unit cell translated using 3, 5 and 9 strips corresponding to 3, 5 and 9 conductive threads. While the dimension denoted by [d.sub.T] remains constant in the non-uniform resonators, the distance [s.sub.T] is modified as summarised in Table 2. In Section 5, the different electromagnetic behaviours of the presented versions is explained.

5. SIMULATIONS

The proposed design has been analysed using the 3D full-wave High Frequency Structure Simulator (HFSS) software. First, different versions of the translation into a woven prototype have been simulated as depicted in Figure 5. These versions have been simulated as infinite FSS using the set-up previously described in Figure 4(a). Consequently, the more strips employed in the translation, the higher attenuation achieved. Nevertheless, instead of using the translation proposed in Figure 4(d) with 9 strips, the translation depicted in Figure 4(c) with 5 strips has been employed. This decision has been made due to manufacturing specifications.

Then, an infinite FSS modelled as one unit cell (described in Section 4) with PBC has been simulated under normal incidence conditions and the result has been compared with the simulation of a finite structure. Then, the behaviour of the finite FSS in terms of angle of polarisation and angle of incidence has been studied.

An infinite array of resonators has been simulated applying PBC to a unit cell placing two ports at a distance of D/2 = 0.75 m from each side of the FSS, respectively, as previously illustrated on the upper part of Figure 1(a). A finite version 8 x 8 unit-cell has also been simulated in the middle between two horn antennas separated a distance D as depicted in Figure 6(a). The simulated [S.sub.21] parameter, normalised by the response in the absence of the FSS, is shown in Figure 6(b). The dimensions of the simulated horns are identical to the dimensions of the real horns which have been used for the experimental validation. For this reason, the non-shaded and the shaded range of frequencies represent, respectively, the frequencies in which a single-mode operation of each different horn is guaranteed. This is applicable for the all of the following graphics.

To study the behaviour of the designed FSS in terms of the angle of polarisation, the FSS has been simulated rotating it around its Z-axis, from [theta] = 0[degrees] to [theta] = 45[degrees] as depicted in Figure 7(a). The simulated [absolute value of ([S.sub.21])] parameter for the different angles of polarisation is shown in Figure 7(b). The minimum value of the [absolute value of ([S.sub.21])] parameter is modified with the angle of polarisation, however the attenuation level remains over 10dB. Consequently, the FSS presents an stable performance in terms of the angle of polarisation.

To study the behaviour of the designed FSS in terms of the angle of incidence, the FSS has been simulated rotating it around its F-axis, from [phi] = 0[degrees] to [phi] = 45[degrees] as depicted in Figure 8(a). The simulated IS211 parameter for the different angles of incidence is shown in Figure 8(b). Although the minimum value of the [absolute value of ([S.sub.21])]. The simulated FSS presents a stable performance in terms of its angle of incidence.

6. FABRICATION PROCESS

A finite 8x8 unit cells FSS has been manufactured using an industrial MuGrip loom. The manufacturing process has been based on satin weaving. The conductive warp yarns are previously mounted in the loom and the conductive weft threads are inserted in the fabric during the fabrication process leading to a conductive lattice in the top layer of the fabric fabric, instead of the required cross-shaped resonators, thanks to an auxiliary layer as depicted in Figure 9(a). In Figure 9(a), the magenta and blue colored segments of conductive thread correspond, respectively, to the segments of warp and weft conductive threads which are interwoven in the auxiliary layer. In Figure 9(b), a schematic drawing of the woven structure after the cutting process is depicted, in which the leftover conductive segments have been removed and the cross-shaped resonators are represented.

With the aim of translating the schematic drawing in Figure 9 into woven patterns, Figure 10 is presented. Figure 10(a) represents the front view of the weft threads and the side view of the warp patterns. In the segment which corresponds to the separation between two adjacent resonators the conductive threads must be interrupted in order to avoid a short circuit. For this reason, during this segment, the conductive warp threads are interwoven in the auxiliary layer and, subsequently, interwoven again in the main fabric. An automatic cutting process is then required to remove the auxiliary layer, leading to the representation depicted in Figure 10(b). In this representation, the weft threads corresponding to the positions of the top layer beneath the auxiliary layer have been left loose, after the cutting process.

The described woven patterns lead to a layer-to-layer angle interlock weaving [40-43], due to the individually woven layers of weft threads through undulated warp threads. In order to, not only connect the different layers to create a multilayered structure, but also avoid the presence of loose weft threads after the cutting process, binders must be employed. Binders are threads which, instead of present a woven pattern in an individual layer, they connect different layers to conform a compact woven structure, as depicted in Figure 10(c). These threads must be dielectric, in order to achieve the separation between the conductive resonators avoiding the short circuits. Due to the necessity of using these binders also between the conductive threads, 5 conductive strips have been employed, instead of 9, in the translation into the woven prototype. By the use of the auxiliary layer and the binders, the different cross-shaped resonators can be achieved as schematically depicted in Figure 10(d).

A finishing process is required after removing the fabric from the loom in order to achieve the requirements. Figure 11 depicts a generic finishing process which starts with the insertion of the fabric in liquid chemicals, then needling the fabric and finally applying heat to dry it. The manufactured FSS has been heated at 185[degrees] on the frame of the stenter machine.

The top view of the manufacture prototype can be seen in Figure 12(a), whereas the bottom view is depicted in Figure 12(b). A magnification of a unit cell from the top layer is shown in Figure 12(c), whereas its analogous bottom view is depicted in Figure 12(d). The white woven lines in the before mentioned figures are due to the binders.

7. EXPERIMENTAL VALIDATION

The prototype has been experimentally validated using different setups and employing two horn antennas, horn #1 and horn #2, respectively, which are depicted in Figure 13, together with their dimensions in centimetres. The horn #1 presents a single-mode operation until a frequency equal to 3.9 GHz, whereas the horn #2 presents its analogous from 3.9 GHz to 5.75 GHz.

All the set-ups are composed of a wooden canvas, in which the prototype has been assembled and allows the required movements for the validation under different angles of incidence, two pairs of horn antennas (for the aforementioned different ranges of frequencies) mounted in rotary platforms for the validation of the different angles of polarisation and a vector network analyser. The [S.sub.21] parameter between the horn antennas, for each set-up, calibrated by using the response between the antennas without the FSS, has been measured using an Agilent N5247A PNA-X vector network analyser and compared with the simulations.

The FSS has been experimentally validated under normal incidence conditions using the setup shown in Figure 14(a). The comparison between simulations and measurements can be seen in Figure 14(b). The manufactured FSS provides a bandstop behaviour, [absolute value of ([S.sub.21])] < -10dB, which provides a 0.6 GHz stopband centered at 3.75 GHz, with a 10dB minimum attenuation and a 27dB maximum attenuation. The predicted simulated data agree with the measurements, although the differences may be due to the approximations taken into account in the simulations. In the simulations, conductive flat strips are employed for the resonators, whereas the manufactured conductive woven paths are slightly longer due to the weaving curves. For this reason, the measured range of working frequencies is slightly lower than the simulated one, although greater.

The FSS has been experimentally validated for different angles of polarisation, [theta] [member of] [0[degrees]-45[degrees]], using the set-up shown in Figure 15(a) in which the antennas have been mounted on the rotary platforms. The comparison between simulations and measurements is depicted in Figure 15(b). The manufactured FSS presents a bandstop behaviour, consequently, the FSS exhibits a stable performance in terms of the polarisation angle. This represents an interesting point for the potential use of the proposed FSS as a wall cover to filter undesired frequencies.

The FSS has also been experimentally validated for different angles of incidence, [phi] [member of] [0[degrees]-45[degrees]], using the set-up shown in Figure 16(a) with a rotary canvas. The comparison between simulations and measurements can be seen in Figure 16(b). As predicted, the manufactured FSS has a bandstop behaviour. Therefore, the FSS also exhibits a stable performance in terms of the angle of incidence, reinforcing its applicability in the aforementioned wall covering.

8. CONCLUSION

A novel flexible fully textile-integrated bandstop FSS has been presented. A methodology to implement a layer-to-layer angle interlock 3D woven structure and its subsequent cutting procedure to achieve the independent resonators have been throughly explained.

To validate the design, a 8 x 8 unit-cell prototype has been manufactured and experimentally characterised under different conditions, including different angles of polarisation and angles of incidence. A good agreement between simulations and measurements has been achieved, experimentally verifying the theoretically predicted behaviour of the textile structure. Therefore, the manufactured FSS exhibits a stable performance in terms of the angle of polarisation and the angle of incidence.

The manufacturing procedure using industrial textile machinery provides the possibility of manufacturing flexible and large FSS, as opposed to other alternatives based in conventional substrates, solving the fabrication problems of large shielding surfaces.

ACKNOWLEDGMENT

This work has been supported by Gobierno de Espana TEC2015-72110-EXP, TEC2016-80815-P and FPU14/00016 grant, and by the Gobierno del Principado de Asturias (PCTI)/FEDER-FSE under projects IDI/2016/000372 and IDI/2017/000083.

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Leticia Alonso-Gonzalez *, Samuel Ver-Hoeye, Miguel Fernandez-Garcia, and Fernando Las-Heras

Received 17 April 2018, Accepted 24 June 2018, Scheduled 17 July 2018

* Corresponding author: Leticia Alonso-Gonzalez (lalonso@tsc.uniovi.es, alonsoleticia@uniovi.es).

The authors are with the Signal Theory and Communications Area, Department of Electrical Engineering, University of Oviedo, Campus Universitario de Viesques, Edificio Polivalente, despacho 0.8.07, Gijon E-33203, Spain.

Caption: Figure 1. Schematic drawing of the proposed unit cell, conductive materials (yellow) and dielectric materials (grey). The dimensions are summarised in Table 1. (a) Overview of the unit cell with the boundary conditions (D = 1.5 m) and side view of the unit with the notation of the layers. (b) Top and bottom layers of the unit cell and the dimensions.

Caption: Figure 2. Woven substrate with 2 conductive threads and magnifications: a Shieldex thread and a dielectric PET warp thread.

Caption: Figure 3. Schematic drawing of the proposed unit cell and its dimensions. The dimensions are summarised in Table 1. (a) Top view. (b) Detailed view of the dimensions (top view). (c) Bottom view.

Caption: Figure 4. Schematic detailed drawing of the different versions of the proposed unit cell and its dimensions. (a) Uniform. (b) Three strips. (c) Five strips. (d) Nine strips.

Caption: Figure 5. Simulated [absolute value of ([S.sub.21])] parameter for the different versions of the unit cell proposed in Figure 4 using the infinite FSS (set-up in Figure 1(a)).

Caption: Figure 6. Normal incidence. (a) Schematic drawing of the simulated set-up (D = 1.5 m). (b) Simulated [absolute value of ([S.sub.21])] parameter for an infinite FSS (set-up in Figure 1(a)) and for a finite FSS in (a).

Caption: Figure 7. Angle of polarisation. (a) Schematic drawing of the simulated set-up. (b) Simulated [absolute value of ([S.sub.21])] parameter and the influence of the angle of polarisation on the finite FSS performance.

Caption: Figure 8. Angle of incidence. (a) Schematic drawing of the simulated set-up. (b) Simulated [absolute value of ([S.sub.21])] parameter and the influence of the angle of incidence on the finite FSS performance.

Caption: Figure 9. Schematic drawing of the woven structure before and after the cutting process. (a) Top layer before the cutting process. Magenta and blue colored segments represent, respectively, warp and weft segments of threads which have been left unwoven from the main fabric for the subsequent cutting procedure. (b) Top layer after the cutting process.

Caption: Figure 10. Schematic drawing of the woven structure based on a layer-to-layer angle interlock 3D fabric. (a) Side view of the woven structure with using an auxiliary layer. (b) Side view of the woven structure with the loose threads, after cutting the auxiliary layer. (c) Side view of the woven structure using binders to avoid the loose threads. (d) Top view equivalence.

Caption: Figure 11. Schematic drawing of the finishing process with a stenter machine.

Caption: Figure 12. Manufactured prototype and magnification of a unit cell. (a) Top view of the prototype. (b) Bottom view of the prototype. (c) Magnification and dimensions of the top view of the unit cell. (d) Magnification and dimensions of the bottom view of the unit cell.

Caption: Figure 13. Horn antennas employed for the measurements and their dimensions in centimetres. (a) Top view. (b) Overview.

Caption: Figure 14. Normal incidence. (a) Measurement set-up using horn #1 (D = 1.5 m). (b) Measured (solid line) vs. simulated (dashed line) [absolute value of ([S.sub.21])] parameter.

Caption: Figure 15. Angle of polarisation. (a) Measurement set-up using horn #2. (b) Measured (solid line) vs. simulated (dashed line) [absolute value of ([S.sub.21])] parameter and the influence of the angle of polarisation on the finite FSS performance.

Caption: Figure 16. Angle of incidence. (a) Measurement set-up using horn #2. (b) Measured (solid line) vs. simulated (dashed line) [absolute value of ([S.sub.21])] parameter and the influence of the angle of incidence on the finite FSS performance.
Table 1. Dimensions of the top layer and the unit cell of the FSS.

Dimensions (mm)

Top layer                                             Unit cell

[L.sub.T]         [W.sub.T]   [d.sub.T]   [s.sub.T]   L    H
35                    5          0.3         1.2      45   1

Table 2. Dimensions of the different versions of the translation into
a woven prototype.

Dimensions (mm)

Figure        4(a)       Figure       4(b)       Figure

[W.sub.T]   [s.sub.T]   [d.sub.T]   [s.sub.T]   [d.sub.T]
5               0          0.3         2.4         0.3

Dimensions (mm)

Figure        4(c)       Figure       4(d)

[W.sub.T]   [s.sub.T]   [d.sub.T]   [s.sub.T]
5              1.2         0.3         0.6
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Article Details
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Author:Alonso-Gonzalez, Leticia; Ver-Hoeye, Samuel; Fernandez-Garcia, Miguel; Las-Heras, Fernando
Publication:Progress In Electromagnetics Research
Article Type:Report
Geographic Code:1USA
Date:Jun 1, 2018
Words:5548
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