Stitched Ground Planes for Textile Antenna Application: An Experimental Study

This paper presents an application of stitched ground plane for microstrip patch antenna design. In this work Matlab interface to computer embroidery techniques were used to implement the felt and denim substrates on microstrip patch antenna. These antennas were simulated using a commercial full 3D electromagnetic CST Microwave Studio 2019. A method to optimize the stitch patterns with conductive thread for antenna ground plane for 2.45 GHz industrial, scientific, and medical (ISM) band and 5 GHz wearable wireless local area networks (WLAN) frequencies was achieved. Rigid and flexible wearable antennas (microstrip patch antennas) were fabricated using the stitched ground plane. The electrical resistance was reduced between the meshes during the stitching design process. Results in terms of bandwidth, radiation patterns and reflection coefficients (S 11 ) are presented.

rectangular microstrip patch antennas are fed by a microstrip line with inset feed. The inset feed is used for microstrip patch antenna to improve the impedance match with planar feed configuration. The input impedance of microstrip patch antenna depends on the feed position for probe feed or the microstrip feed inset [7] [8] [9] [10] [11].
A patch antenna with enhanced gain, bandwidth hand directivity has been developed by using micromachining technique for RFID applications. For compacting the antenna, dielectric constant of 11.9 is taken. Performance of the antenna has been collated while using single dielectric layers, after that they used two dielectric layers of substrate (silicon/air) and then expanded the substrate layers to three (silicon/air/silicon). Finally, it was concluded that better results in terms of bandwidth, gain and directivity can be achieved using multilayer substrate [12].
Several authors for the design of textile antennas used different conductive materials and dielectric substrates. Textile antenna has been fabricated by various methods including weaving, knitting, lamination, printing, and embroidery [13]- [18]. An embroidery technique was chosen because of the integration of garments with stitched antennas. Embroidery is fast and flexible in creating new patterns and make integrating radio frequency systems into clothing/garments for wearable applications. This focuses on providing a better interconnection between the mesh nodes and reducing the discontinuities of the stitched patch.
This work also optimizes stitched patterns and reduces overstitching of the mesh. One specific advantage of stitched antennas is their simplicity of construction. They can be retrofitted to existing garment designs and there is a high level of flexibility. The work develops methods for making use of two flexibility by considering a set of problems with many variables and seeking optimal solutions with methods.
This paper presents the method of implementing stitched ground plane design using computerized aided design (CAD) software-controlled embroidery machine. This technique was interfaced with MATLAB to produce desired fabric-based The rest of this paper is structured as follows: the designed method is discussed in Section 2. The mechanism of embroidery is also addressed in Section 3. In Section 4 the return losses and radiation patterns are presented. Section 5 concludes the work.
Microstrip patch antenna width The actual length L of the microstrip patch antenna is given as Ground dimension: for a good and practical designs, a finite ground plane is Open Journal of Antennas and Propagation needed apart from having an essential substrate. The dimension of the ground plane may be given as

Input Impedance
The transmission-line model represents the patch as a low-impedance line whereby the width of the patch determines the impedance and the effective dielectric constant. A microstrip patch consists of parallel plate radiation conductance and capacitive susceptance that loads both the radiating edges of the patch. The radiation conductance for a parallel-plate radiator is defined as [9]: The computed value of G was 2.49 mΩ and R e the input resistance at the edge, is 200 Ω with y 0 inset distance. The capacitive susceptance in relation to the effective strip extension is given as Microstrip antenna is fed from the edge using an inset line where the gap on either side of strip line is equal to its width. The feed location y 0 was computed from Equations (9)-(13) using a radian angle measure.

Antenna Structure
This section presents a simulation study of a set microstrip patch antennas with different ground plane configurations. Simulations were performed using CST Microwave Studio. This software package uses the FIT method for electromagnetic simulations. The structure and dimension of the proposed microstrip patch antenna is inspired from Balanis [7]. The design geometry is shown in Figure 1.  The dimensions of the microstrip patch antennas were approximated from equations [8]. The next patch antenna was designed to operate at 5 GHz and the dimensions are listed in Table 1. The proposed antennas are constructed with two different substrates: FR4 substrate and fabric substrates (denim and felt).
The patch and the inset line are made on the same layer.

Return Losses
The return loss is similar to the VSWR that shows matching between feeding system, transmission lines and the antenna. To obtain a perfect matching between the feeding and antenna; Γ = 0, RL = infinity, no power is reflected back.
When Γ = 1, RL = 0 shows that all the incident power is reflected. From the practical applications, VSWR = 2 is acceptable limit for a good antenna, this corresponds to RL of 9.54 dB. RL greater than 10 dB is acceptable [7].

( )
Return Loss 20 log Γ dB = − (14) ( ) where the Input impedance a a a Z R jX = + , R a is the resistance and X a is reactance Z l is the characteristic impedance of the transmission line feeding the antenna 50 Ω.

Concept of Computerized Stitching Technique
Usually textile antennas take the form of planar structures that conform to the human body or follow underlying shape of the garment (e.g. a shirt pocket).
Several material properties influence the working of a textile antennas. Permittivity Open Journal of Antennas and Propagation and the thickness of the substrate determine the bandwidth and efficiency of a planar microstrip antenna. The use of textiles in antenna design requires characterization of their properties. The accurate characterization of textile substrate properties is fundamental before the antenna design. The conductivity of the ground plane and of the patch is an essential critical factor in determining the efficiency of the antenna [6]. Over the years, many techniques and materials have been employed in producing textile antennas. Textile antennas can be fabricated by several methods, but embroidery is examined in this work. A digital embroidery machines at Loughborough University was used in stitching the ground plane for microstrip patch antenna designs (see Figure 2). Embroidery was chosen as the method for fabrication of stitched antennas because digital embroidery is fast and flexible in pattern generation and the integration of high frequency systems into clothing. Stitch antennas do not require glue or cutting or lamination processes only stitching the design onto fabric. Antenna designs are converted into a format compatible with an embroidery machine for production. In embroidery patterns are superimposed onto an existing fabric and a stitch design is created. This technology provides high speed, ease of manufacturing, accurate and easily modified stitch antennas. Antennas are automatically integrated into fabric through this process which reduces cost of production. Conductive threads are expensive. A sample quantity of Amberstrand silver yarn costs £1 per meter [3] and [18]. Embroidery uses specialist conducting thread that have a polymer core and coated with a silver/nickel. Several authors have addressed the issue of fabricating textile antennas using embroidery [3] and [14]- [19], but Matlab interface to computer embroidery was used in this design. This method is a unique approach to antenna fabrication.
Embroidery was used to incorporate RF functions into garments and clothes.
The more closely the stitch space the better the electrical connectivity between neighboring stitches and the better the antenna performance.   forming a lock stitch. In the process of embroidery, small portion of oil was applied to Liberator thread and Amberstrand so that the threads would not unravel or break during the stitching process. This makes the thread filament stick together and reducing the unravel loss. Single-layer embroidery was used to realize higher conductive textile surfaces giving a lower DC resistance [3] and [17] [18].
In all the fabrications of wire antennas, planar wire antennas and microstrip antennas single layer stitching densities were employed. There are detailed works relating to fully wearable textile antennas using computerized embroidery techniques at different frequencies [19].

Predicted and Measured Results
In this section, copper tape of thickness 0.035 mm has been used design the Open Journal of Antennas and Propagation The ground planes represented in Figure 5 were designed and fabricated in this work. Figure 5(a) is a solid patch on meshed ground plane (MPG1) of dimension 2mm × 2mm. Figure 5(b) is a solid patch on a solid ground plane (PA1). The solid ground planes were replaced using felt and denim. Figure 5(c) antenna PA3 is a solid patch and meshed ground (using conductive thread on denim material).

Comparison of Patches with Meshed and Solid Ground Planes
The measured input reflection (S 11 -parameter) of the antenna samples MGP4 and PA1 is shown in Figure 7.

Solid Patch with Solid Ground (PA1) and Meshed Patch with Meshed Ground
Antenna sample PA1 is a solid patch with solid ground plane and antenna sam-

Simulated Surface Current for Meshed Ground Plane
The surface current is strong at the feed point and along the feed length. Currents flow along the vertical lines and the horizontal lines shows an increase in current path as shown in Figure 9 and lowering the fundamental resonant frequency. The current distribution on the triangular mesh is uniformly. The triangular ground plane current paths are longer causing the solid patch antenna with a meshed ground plane to radiate at lower frequency than the solid patch with solid ground plane [22].
The simulated 3-D far field pattern, which controls the antenna's radiation pattern, is illustrated in Figure 10. As can be shown from the figure, the region with the highest gain of 6.83 dB is coming from the radiating element of the antenna and slowly settles down to 0 dB further away from the radiating structure. Open Journal of Antennas and Propagation

Conclusions
This were experimental and have been tested from reported literature.
Using the Amber strand with a property of a higher conductivity thread gives a better continuity across the mesh nodes. The method of stitching via the interface reduces the surface disjointedness across the mesh nodes of the ground plane. The interface method reduces the stitching density and electrical resistance between mesh nodes making the antennas to be flexible and wearable. This design has also produced a compact density without breakage along the paths.
Integration of the ground plane into antennas reduces the cost of production.
The designed antenna can be integrated into human clothing with sensors for wearable applications.

Conflicts of Interest
The author declares no conflicts of interest regarding the publication of this paper.