**1. Introduction**

Radio frequency identification (RFID) is a very popular standardized technology that is mainly employed for the identification purposes of objects or people. More precisely, an object associated with a RFID tag is remotely identified by the means of a RFID reader. The communication principle is based on the tag's load modulation of the backscattered electromagnetic wave [1–3], which implies that in most of the cases, the RFID tag is passive (i.e., it uses the transmitted energy from the reader without the need for any additional energy source). RFID is a very interesting concept that contributes to the Internet of Things (IoT) development and, more generally, it is considered as a key technology for humanity [4,5]. The advantages that are offered by RFID tags such as communication without line of sight, low cost, small size, and unique identification have made them an essential candidate for a wide range of applications, for example, logistics, retail, access and identity cards as well as wireless payment systems.

Recently, the emergence of electronic devices that can be worn in, on, or near the body called "wearables" has allowed for the possibility of recovering various physiological information from a human body and transmitting it wirelessly to a processing unit or even to a smartphone [6]. The information obtained from a wearable device can be very useful in a wide range of applications, especially in the health care sector and one of the required operations is the unique identification of the device. For this purpose, in the last years, many efforts have been undertaken in order to develop wearable RFID tags that can be associated with clothing or an accessory in a way that is non-invasive, comfortable, and invisible for the wearer. Popular considerations during the design of wearable RFID tags are usually the impact of deformation on the RFID tag's performance,

the effect of the human body's proximity to the tag's electrical and radiative properties or the tag's washability [7–11]. However, in the encountered studies, the RFID tag's topology is often kept unchanged from the conventional one (i.e., planar antenna on a substrate with properties that are specific to the application). In fact, the link between a RFID tag and the object it is associated to, is often neglected and the concept of integrating the tag into the object since the manufacturing phase is part of the "Industry 4.0" era.

One of the technologies that supports this idea is E-Thread®, in which the RFID tag's form factor is reinvented as a RFID textile yarn. The patented technique [12] consists of an automated assembling process during which the RFID chip is associated with a halfwave dipole antenna in a repeated operation. The obtained cascaded RFID tags are then wrapped by a textile material to constitute a spool of textile RFID yarns. When isolated from the spool, one RFID yarn operates in the European Ultra High Frequency (UHF) band (865.5 <sup>−</sup> 867.5) MHz and exhibits a reading range of <sup>12</sup> <sup>m</sup> [13]. The current E-Thread® RFID yarn constitutes a very interesting solution as it can be integrated within an object during the fabrication stage and offers great advantages with its slender configuration such as invisibility and comfortable for the user. However, a RFID wearable tag has to be robust to any kind of mechanical constraints such as the elongation, which is lacking in the actual RFID yarn.

In this paper, we propose an alternative solution that consists of using for the tag's antenna, a helical geometry that has similar mechanical properties to a string. A helical antenna is mainly fabricated by winding a conductive material and its geometrical parameters have an important impact on its electromagnetic properties in terms of input impedance and radiation pattern. Usually, these helical antenna properties are exploited for several scenarios such as phased antenna arrays for millimeter waves and wireless power transfer applications [14,15], wireless sensor nodes in smart agriculture [16] as well as biomedical applications [17–19]. However, to the authors' knowledge, in the literature, very few examples can be found where a helical antenna has been used in a RFID tag. For example, the study in [20] focused on the development of a helical RFID tag to be integrated into a vehicle tire. In this case, the impedance matching between the antenna and the chip was achieved using a transmission line. Meanwhile, in the study presented in [21], a helical antenna was developed for and RFID tag in which the impedance matching was achieved by tuning the geometrical parameters of the antenna.

In a previous work [22], the latter method was employed in order to design a helical antenna for the RFID tag yarn without the use of any additional elements in order to perform the impedance matching. The RFID helical tag exhibited a maximum read range at 1040 MHz*,* which is higher than the frequency of interest and 1 m of read range in the European UHF RFID band. As explained, the observed result is due to manufacturing process constraints and one of the given improvement solutions was to design a helical antenna with a spacing between turns that is higher while increasing the antenna's halflength *h*.

In this paper, the new UHF RFID helical tag-based textile yarn includes two significant improvements: (i) the helical RFID tag was designed while taking into consideration the manufacturing constraints (the nature of the employed materials and the physical dimensions' limits), and (ii) the integration of a stretchable core material as a support for the elongation. Compared to the previous version of the helical RFID tag, the suggested methodology design also allows for a manufactured structure to be obtained for which the dimensions and the electromagnetic characteristics are close to the simulated ones. This is possible through a more accurate modeling of the materials' characteristics in the design process. The rest of this paper is organized as follows. In Section 2, the topology of the helical antenna when integrated into a textile yarn is presented together with the design methodology including electrical and manufacturing specifications. Moreover, criteria for the helical RFID tag characterization using simulation and experiments are given. Section 3 highlights the simulation results in terms of reflection coefficient and radiation pattern. Moreover, the fabricated prototypes as well as the experimental characterization's results are presented. Finally, conclusions and future work are drawn in Section 4.

#### **2. Materials and Methods**

#### *2.1. Topology of the RFID Textile Yarn Integrating a Helical Antenna*

In free space, the helical antenna is characterized by its geometrical parameters, which are the diameter *D*; the half-length *h*; the turns number *N*; the pitch *s*; and the wire radius *a*, as shown in Figure 1a. As stated, these parameters impact the electromagnetic properties as follows: the diameter *D* and the pitch *s* mainly have an impact on the impedance matching while the half-length *h* and turns number *N* mainly modify the resonance frequency. Moreover, a helical antenna with a diameter much smaller than the wavelength allows a radiation pattern to be maintained with a normal mode similar to the dipole antenna of the current solution [23].

**Figure 1.** (**a**) Helical antenna configuration; (**b**) Cross section of the RFID textile yarn integrating the helical antenna.

The RFID helical tags presented in this paper were fabricated using the E-Thread® technology. The E-Thread technology consists of an automated assembling process where a dipole antenna is associated with a RFID chip for which the package was modified beforehand. On the RFID chip edges, two grooves receive two copper wires that form the tag's antenna [12]. This technique allows for several cascaded RFID tags to be obtained that can have a textile finishing during a wrapping process [13]. Furthermore, in order to obtain the helical shape, an additional step is required. This step consists of wrapping the textile material containing the cascaded RFID tags around a core material giving the helical aspect; here, a stretchable material is employed as the core of the helical antenna offering elongation capabilities. Details on the practical fabrication are given in [22]. Preliminary parametric simulations testing different dielectric constants for the used core material has allowed us to conclude that when the dielectric constant of the core increases, the impedance matching frequency shifts toward the low frequencies. Thus, it is important to identify and characterize the nature of the used material as the core during the antenna design. Indeed, any change after the manufacturing process is very difficult and may strongly deteriorate the RFID yarn.

For the simulation purpose, the textile material used for wrapping and the core material were modeled simply as dielectric materials characterized by their permittivity constant provided by the industrial partner. The dielectric constants for the employed *nylon* and *lycra* are *ε<sup>r</sup>* = 3.6 and *ε<sup>r</sup>* = 1.5, respectively. A cross section of the helical RFID textile yarn is shown in Figure 1b: *Dext* is the external diameter of the helical tag integrated in the textile; *Dint* is the diameter of the cylindrical core material; and 2*a* is the helical antenna wire's diameter.
