This paper will review the evolution of wearable textile antennas over the last couple of decades. Particular emphasis will be given to the process of embroidery. This technique is advantageous for the following reasons: (i) bespoke or mass produced designs can be manufactured using digitized embroidery machines; (ii) glue is not required and (iii) the designs are aesthetic and are integrated into clothing rather than being attached to it. The embroidery technique will be compared to alternative manufacturing processes. The challenges facing the industrial and public acceptance of this technology will be assessed. Hence, the key opportunities will be highlighted.

https://www.mdpi.com/2079-9292/3/2/314/pdf?view=inline

Embroidery and related manufacturing techniques for wearable antennas: Challenges and opportunities

Wearable antennas have gained much attention in recent years due to their attractive features and possibilities in enabling lightweight, flexible, low cost, and portable wireless communication and sensing. Such antennas need to be conformal when used on different parts of the human body, thus need to be implemented using flexible materials and designed in a low profile structure. Ultimately, these antennas need to be capable of operating with minimum degradation in proximity to the human body. Such requirements render the design of wearable antennas challenging, especially when considering aspects such as their size compactness, effects of structural deformation and coupling to the body, and fabrication complexity and accuracy. Despite slight variations in severity according to applications, most of these issues exist in the context of body-worn implementation. This review aims to present different challenges and issues in designing wearable antennas, their material selection, and fabrication techniques. More importantly, recent innovative methods in back radiations reduction techniques, circular polarization (CP) generation methods, dual polarization techniques, and providing additional robustness against environmental effects are first presented. This is followed by a discussion of innovative features and their respective methods in alleviating these issues recently proposed by the scientific community researching in this field.

/pdf/wearable-antennas-a-review-of-materials-structures-and-5bzxx4cp70.pdf

Wearable Antennas: A Review of Materials, Structures, and Innovative Features for Autonomous Communication and Sensing

We present a new selection of E-fibers (also referred to as E-threads) and associated embroidery process. The new E-threads and process achieve a geometrical precision down to 0.1 mm. Thus, for the first time, accuracy of typical printed circuit board (PCB) prototypes can be achieved directly on textiles. Compared to our latest embroidery approach, the proposed process achieves: 1) 3 × higher geometrical precision; 2) 24 × lower fabrication cost; 3) 50% less fabrication time; and 4) equally good RF performance. This improvement was achieved by employing a new class of very thin, 7-filament, Elektrisola E-threads ( diameter ≈ 0.12 mm, almost 2 × thinner than before). To validate our approach, we “printed” and tested a textile spiral antenna operating between 1-5 GHz. Measurement results were in good agreement with simulations. We envision this textile spiral to be integrated within a cap and unobtrusively acquire neuropotentials from wireless fully-passive brain implants. Overall, the proposed embroidery approach brings forward new possibilities for a wide range of applications.

Fabrication of Textile Antennas and Circuits With 0.1 mm Precision

A comprehensive review concerning the geometry, the manufacturing technologies, the materials, and the numerical techniques, adopted for the analysis and design of wideband and ultrawideband (UWB) antennas for wireless applications, is presented. Planar, printed, dielectric, and wearable antennas, achievable on laminate (rigid and flexible), and textile dielectric substrates are taken into account. The performances of small, low-profile, and dielectric resonator antennas are illustrated paying particular attention to the application areas concerning portable devices (mobile phones, tablets, glasses, laptops, wearable computers, etc.) and radio base stations. This information provides a guidance to the selection of the different antenna geometries in terms of bandwidth, gain, field polarization, time-domain response, dimensions, and materials useful for their realization and integration in modern communication systems.

/pdf/wideband-and-uwb-antennas-for-wireless-applications-a-1yabyj5rhz.pdf

Wideband and UWB Antennas for Wireless Applications: A Comprehensive Review

An in-depth survey of wearable antennas for offbody radio links in the very high-frequency (VHF) and ultrahigh-frequency (UHF) bands (below 1 GHz) is presented. The review encompasses distinctive specifications, design criteria and challenges, material characterization procedures, antenna topologies and technologies, multiantenna systems, dedicated testing procedures, applications, and future trends.

https://biblio.ugent.be/publication/7045516/file/7045521.pdf

Wearable Antennas for Off-Body Radio Links at VHF and UHF Bands: Challenges, the state of the art, and future trends below 1 GHz.

In this work, we examine the use of reconfigurable intelligent surfaces (RISs) to create alternative paths from a transmitter to a receiver in millimeter-wave (mmWave) networks, when the direct link is blocked. In this direction, we evaluate the end-to-end signal-to-noise ratio (SNR) expression of the transmitter-RIS-receiver links that take into account the transmitter-RIS and RIS-receiver distances and enables us to acquire important insights regarding the RIS position that maximizes it. Finally, the insights are corroborated by numerical results.

Optimal Reconfigurable Intelligent Surface Placement in Millimeter-Wave Communications

Α feasibility study of using components such as conductive Velcro tape and zip fastener to change the polarisation of a textile patch antenna from linear (LP) to circular (CP) is presented. The antennas are designed to operate at 2.3–2.5 GHz frequency band, are made using felt substrate and conductive cloth and their size is 100.4 mm × 74.2 mm × 4.26 mm. It is proved, via simulations and measurements, that such components are strong candidates as means for polarisation reconfiguration of textile antennas. Both Velcro and zip textile patch antennas at the LP mode yield axial ratio (AR) larger than 10 dB at the operating frequency range. In the CP mode, the Velcro antenna gives AR equal to 3.1 dB at 2.35 GHz and the zip antenna 1 dB at 2.31 GHz. Repeatability measurements of interchanging polarisation modes (LP–CP) for both antennas have been conducted presenting acceptable repeatability. Great advantage of both antennas (Velcro and zip) is that the polarisation can be changed at will by a simple manipulation (detach the Velcro or unzip); thus, providing to the user the flexibility of adjusting the antenna characteristics depending on wearable application and environment of operation.

https://ietresearch.onlinelibrary.wiley.com/doi/pdf/10.1049/iet-map.2019.0951

Use of conductive zip and Velcro as a polarisation reconfiguration means of a textile patch antenna

This paper examines the far-field patterns of a monopole antenna mounted on different phantoms operating at the 5.8 GHz band. A 40cm height oval cross-section human body phantom is a good approximation for this analysis. The cost of the human equivalent liquid and the weight of the phantom can be reduced by using a hollow version of the proposed human body phantom.

Evaluation of a human body phantom for wearable antenna measurements at the 5.8GHz band

This paper examines the design, realization and evaluation of a lightweight and low cost hollow oval cross-section torso phantom appropriate for wearable antenna performance assessment. The phantom consists of an empty inner space (hollow) surrounded by a shell with double plastic walls between which there is a tissue simulating liquid. The phantom’s plastic shell is made of a low loss cast acrylic and the liquid is a commercially available one with properties calibrated for the frequency range of 2 - 6 GHz. The proposed phantom is compared, through simulations, with a full liquid torso phantom and a heterogeneous anthropomorphic voxel phantom. Additionally, the fabricated phantom is compared with human bodies and a homogeneous anthropomorphic solid phantom, through measurements. The phantom performance is tested in terms of electric field distribution of a wearable antenna on its surface and the path loss between two wearable antennas, on either side of the phantom. It is proved that the hollow phantom performance approximates the full liquid phantom when an RF absorbing material is placed in the central hollow region. The phantom performance in terms of S11 wearable antenna measurements is evaluated and found in good agreement with real human bodies in the examined frequency range (2 - 6 GHz). The far field wearable antenna performance of the proposed phantom shows deviation in gain less than 1.5 dB, compared with anthropomorphic phantom.

/pdf/design-realisation-and-evaluation-of-a-liquid-hollow-torso-hinzevk477.pdf

Aris Tsolis

Papers

Embroidery and related manufacturing techniques for wearable antennas: Challenges and opportunities

Optimal Reconfigurable Intelligent Surface Placement in Millimeter-Wave Communications

Use of conductive zip and Velcro as a polarisation reconfiguration means of a textile patch antenna

Evaluation of a human body phantom for wearable antenna measurements at the 5.8GHz band

Design, realisation and evaluation of a liquid hollow torso phantom appropriate for wearable antenna assessment