## 3 Manufacturing Techniques for Wearable Antennas
### Flexible Textile Wearable Antennas
#### 3.2.3 Embroidery  
The traditional embroidery process creates aesthetic shapes using colored threads on a base textile material. By using specialist conducting threads, antennas can be embroidered onto the base textile. Although the basic principles are the same, the technology has advanced to enable a digital image to be directly embroidered using a computer aided embroidery machine (Figure 5). The conducting threads must exhibit suitable flexibility and strength so as not to be broken by the high tensions required in the embroidery machine.  
Of great importance before designing and embroidering the antenna is knowledge of the properties (_i.e.,_ DC resistance, conductivity) of the conductive yarns to be used [34]. Once the conductive yarns are characterized then it is easier to find techniques to improve the performance of the antenna; for  
Figure 5: Computer aided embroidery machine at Loughborough University.  
example using a higher density of stitching. Conductive threads with high resistance yield poor S21 results in transmission line measurements [34]. After the characterization of the conductive yarns and of the textile substrate, the design of the embroidered textile antenna (Figure 6) comes next. The design and simulation method to model wearable antennas composed of embroidered conductive threads is presented in [35]. A detailed simulation method is required to model the directional and discrete nature of the threads [35] (Figure 7).  
The challenges of fabricating embroidered antennas are outlined in [36]. Such challenges include: selecting the most suitable conductive thread in terms of conductivity; strength; flexibility and the assessment of the behavior of the conductive threads when they are stitched to form an approximately continuous object so as to improve the efficiency of the patch antenna [36]. The effect of the stitch direction and stitch density on the performance of the antenna has been extensively studied and described in [37]. The understanding of the current flow on a designed patch antenna is a fundamental requirement to choose the stitch direction (Figure 8). Higher antenna efficiencies can be obtained by ensuring the principle current flow is parallel to the stitch direction. This can lead to further challenges when the design is required to also work at higher modes where the current is in the perpendicular direction and for more complicated designs where the current flows in different directions.  
Figure 6: Embroidered patch antenna.  
Figure 7: Embroidered patch antenna magnified to observe the air gap between the conductive threads.  
The effect on antenna performance caused from different stitching geometries is studied and presented in [38]. The effect of the stitch spacing on the gain and directivity was examined. Generally, the closer the stitching spacing, the higher the antenna efficiency will be. However, this comes at the expense of reduced flexibility and increased length of thread which directly leads to higher manufacturing costs.  
The effect of the type of stitching on the dipole antenna performance is described and demonstrated in [39]; where dipole tag-antennas embroidered with different thread densities and two different stitch patterns were considered. The embroidering aspects of Ultra High Frequency (UHF) Radio-Frequency Identification (RFID) antenna were presented in [40]. Professor [PERSON] and his colleagues have contributed to the field of embroidered antennas and RF electronics [41, 42, 43, 44, 45, 46, 47, 48]. Throughout these publications novel conformal embroidered antennas and novel embroidered sensors for medical applications are presented. Novel embroidered microstrip lines were developed and the fabrication procedure of e-fiber transmission lines was proposed. An ultra-wideband embroidered textile antenna is presented in [49]. Due to the usage of conducting thread, the antenna is flexible; can facilitate washing and can be an attractive solution for a wearable device.