In the broad context of Wireless Body Sensor Networks for healthcare and pervasive applications, the design of wearable antennas offers the possibility of ubiquitous monitoring, communication and energy harvesting and storage. Specific requirements for wearable antennas are a planar structure and flexible construction materials. Several properties of the materials influence the behaviour of the antenna. For instance, the bandwidth and the efficiency of a planar microstrip antenna are mainly determined by the permittivity and the thickness of the substrate. The use of textiles in wearable antennas requires the characterization of their properties. Specific electrical conductive textiles are available on the market and have been successfully used. Ordinary textile fabrics have been used as substrates. However, little information can be found on the electromagnetic properties of regular textiles. Therefore this paper is mainly focused on the analysis of the dielectric properties of normal fabrics. In general, textiles present a very low dielectric constant that reduces the surface wave losses and increases the impedance bandwidth of the antenna. However, textile materials are constantly exchanging water molecules with the surroundings, which affects their electromagnetic properties. In addition, textile fabrics are porous, anisotropic and compressible materials whose thickness and density might change with low pressures. Therefore it is important to know how these characteristics influence the behaviour of the antenna in order to minimize unwanted effects. This paper presents a survey of the key points for the design and development of textile antennas, from the choice of the textile materials to the framing of the antenna. An analysis of the textile materials that have been used is also presented.

Textile materials for the design of wearable antennas: a survey.

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

This letter presents the design of a dual-band wearable fractal-based monopole patch antenna integrated with an electromagnetic band-gap (EBG) structure. The prototype covers the GSM-1800 MHz and ISM-2.45 GHz bands. The EBG structure reduces the radiation into the human body over 15 dB. It also reduces the effect of frequency detuning due to the human body. The performance of the antenna under bending, crumpling, and on-body conditions has been studied and presented. Specific absorption rate (SAR) assessment has also been performed to validate the antenna for its usefulness in wearable applications.

Dual-Band EBG Integrated Monopole Antenna Deploying Fractal Geometry for Wearable Applications

A compact and flexible circularly polarized (CP) wearable antenna is introduced for wireless body-area network systems at the 24 GHz industrial, scientific, and medical (ISM) band, which is implemented by employing a low-loss composite of polydimethylsiloxane (PDMS) and silver nanowires (AgNWs) The circularly polarized radiation is enabled by placing a planar linearly polarized loop monopole above a finite anisotropic artificial ground plane By truncating the anisotropic artificial ground plane to contain only 2 by 2 unit cells, an integrated antenna with a compact form factor of 041 λ 0 × 041 λ 0 × 0045 λ 0 is obtained, all while possessing an improved angular coverage of CP radiation A flexible prototype was fabricated and characterized, experimentally achieving S 11 <− 15 dB, an axial ratio of less than 3 dB, a gain of around 52 dBi, and a wide CP angular coverage in the targeted ISM band Furthermore, this antenna is compared to a conventional CP patch antenna of the same physical size, which is also comprised of the same PDMS and AgNW composite The results of this comparison reveal that the proposed antenna has much more stable performance under bending and human body loading, as well as a lower specific absorption rate In all, the demonstrated wearable antenna offers a compact, flexible, and robust solution which makes it a strong candidate for future integration into body-area networks that require efficient off-body communications

Compact, Highly Efficient, and Fully Flexible Circularly Polarized Antenna Enabled by Silver Nanowires for Wireless Body-Area Networks

A new type of wearable button antenna for wireless local area network (WLAN) applications is proposed. The antenna is composed of a button with a diameter of circa 16 mm incorporating a patch on top of a dielectric disc. The button is located on top of a textile substrate and a conductive textile ground that are to be incorporated in clothing. The main characteristic feature of this antenna is that it shows two different types of radiation patterns, a monopole type pattern in the 2.4 GHz band for on-body communications and a broadside type pattern in the 5 GHz band for off-body communications. A very high efficiency of about 90% is obtained, which is much higher than similar full textile solutions in the literature. A prototype has been fabricated and measured. The effect of several real-life situations such as a tilted button and bending of the textile ground have been studied. Measurements agree very well with simulations.

Wearable Button Antenna for Dual-Band WLAN Applications With Combined on and off-Body Radiation Patterns

A novel approach to measure the dielectric constant of fabric substrate materials used for the development of wearable antennas (also called textile antennas) is presented in this paper. The technique reported here is based on the resonance method and focused on the use of microstrip patch radiator, which contains fabric material as its substrate. The accurate value of the dielectric constant of the fabric material can easily be extracted from the measured resonant frequency of the patch radiator. The dielectric constant values of six fabric materials, including jeans cotton, polyester combined cotton, and polyester, have been determined by this way. As an extended objective of this paper, initial investigations are done to study the performance/behavioral characteristics of wearable antennas in the Bluetooth industrial, scientific, and medical band. Two of the six textile antenna structures, developed to meet out the primary objective of determining the dielectric constant of fabrics, are tested, and their performance characteristics, such as impedance bandwidth, gain, efficiency, etc., are measured. In addition, another Bluetooth antenna employing polyester fabric substrate is designed considering its measured accurate value of dielectric constant and subjected to radiation pattern measurements. In general, all the measured antennas yield very good results, fulfilling the requirements for practical applications, and in particular, the third fabric antenna utilizing the accurate value of the dielectric constant determined shows superior performance characteristics compared to others, indicating the correctness of our approach. Thus, the suitability of fabric substrate materials for the development of textile antennas with microstrip patch configuration is also well demonstrated.

S Sankaralingam

Papers

Determination of Dielectric Constant of Fabric Materials and Their Use as Substrates for Design and Development of Antennas for Wearable Applications