*2.2. Design Specifications*

In order to design a helical antenna for the RFID textile yarn, electrical specifications have to be guaranteed. In addition to these conditions, manufacturing constraints in terms of dimensioning are imposed by the manufacturing process.

#### 2.2.1. Electrical Specifications


#### 2.2.2. Manufacturing Constraints

In order for the RFID helical tag design to be compatible with the E-Thread® manufacturing process, some of the helical antenna's geometrical parameters have to respect certain limitations (which for the most part are therefore fixed according to manufacturing constraints):


Hence, the geometrical parameters of the helical antenna that can be varied in order to design a helical RFID tag while meeting the specifications are: the half-length *h* and the turns number *N*. Table 1 summarizes the variable and the fixed geometrical parameters.


**Table 1.** Geometrical parameters of the helical antenna integrated into a textile yarn.

#### *2.3. Helical Antenna's Simulated Structure*

All the presented simulations were performed using CST Microwave Studio 2018, electromagnetic simulation commercial software.

The described helical RFID tag was configured in 3D and a full view is shown in Figure 2a. In addition, a vertical cross section is illustrated in Figure 2b. The pitch *s* and the diameter *D* that strongly impact the impedance matching of the helical antenna have been fixed for manufacturing constraints and thus, only the resonance frequency can be

modified. For this purpose, the number of turns *N* and the half-height *h* are simultaneously varied in order to obtain a resonance frequency in the UHF RFID band.

**Figure 2.** Helical antenna in 3D. (**a**) Full view of the structure. (**b**) Vertical cross section of the structure.

#### *2.4. Characterization of the Helical RFID Tag*

Here, the designed helical RFID tag was characterized in two ways. First, by simulation, and more precisely by evaluating its impedance matching and its radiation pattern. Second, the tag was evaluated by experimental tests through the measurements of the read range and by estimating its robustness to stretching.

#### 2.4.1. Helical RFID Tag's Impedance Matching

Unlike other RF scenarios in which the antenna's impedance has to be matched to 50 Ω, in RFID, the antenna's impedance has to be matched with the IC's impedance. The impedance matching is evaluated through the complex power wave reflection coefficient Γ, which can be expressed as in Equation (1):

$$
\Gamma = \frac{Z\_{\text{chip}} - Z\_{\text{ant}}\ast}{Z\_{\text{chip}} + Z\_{\text{ant}}} \tag{1}
$$

where *Zant* is the helical antenna's input impedance.

#### 2.4.2. Helical RFID Tag's Read Range

In most applicative contexts of UHF RFID, the read range is a very important criterion to describe the performance. In order to compare the experimental result to the one obtained by simulation, the read range can be calculated using the theoretical expression obtained from the Friis transmission equation:

$$R = (^{\lambda}/4\pi) \cdot \sqrt{\frac{P\_t G\_t G\_r \chi \tau}{P\_{th}}} \tag{2}$$

where *λ* is the wavelength; *P<sup>t</sup>* is the power transmitted by the reader; *G<sup>t</sup>* is the reader's antenna gain; *G<sup>r</sup>* is the tag's antenna gain; *χ* is the polarization loss; *Pth* is the tag's activation threshold that represents the power needed for the IC to start operating; and *τ* is the power transmission coefficient defined as:

$$
\pi = 1 - |\Gamma|^2 \tag{3}
$$

It is worth noting that the quantity *PtG<sup>t</sup>* represents the equivalent isotropic radiated power (EIRP). Its maximum value depends on the geographical location, for instance, the value imposed by the European Telecommunications Standards Institute (ETSI) is 3.28 W, whereas the tag's activation threshold is specific to the chosen IC.

In the presented work, the Voyantic Tagformance commercial test bench [25] was used to measure the read range.

#### 2.4.3. Helical RFID Tag's Robustness in Terms of Stretching

In order to measure the helical RFID tag's tolerance to elongation, the Voyantic Bench test was also used after performing some modifications in order to correspond to our application. More precisely, both the antenna extremities are attached to a basic textile filament that is wound around two spools. As shown in Figure 3, the spools' rotation, clockwise and counter clockwise, allows for the application of an elongation on the tag. The read range is then measured for each considered elongation.

**Figure 3.** Modified Voyantic test bench for the measurement of the helical RFID tag's reading range when elongation efforts are applied.

#### **3. Discussion of the Simulation and Measurement Results**

*3.1. Helical RFID Tag's Reflection Coefficient* Γ *and Its Radiation Pattern*

After optimization, the helical RFID tag's geometrical parameters were: *h* = 50 mm; *N* = 42*,* in addition to the fixed ones given in Table 1. The reflection coefficient obtained from simulation is shown in Figure 4. It can be observed that the tag's antenna exhibited a minimum value of the reflection coefficient Γ of −6.27 dB at 865 MHz.

**Figure 4.** Reflection coefficient Γ obtained by simulation at the antenna feed point.

The radiation pattern obtained by simulation is shown in Figure 5, where the antenna is positioned along the z-axis and has a maximum gain of 1.27 dB. It can be seen that the radiation pattern was omnidirectional in the *xoy* plan, which is identical to a half-wave's dipole radiation pattern. Moreover, through the obtained axial ratio (AR) as shown in (Figure 6), defined as *<sup>E</sup><sup>θ</sup> Eϕ* = 35.8 dB for the main lobe (*E<sup>θ</sup>* and *E<sup>ϕ</sup>* being the orthogonal components of the radiated electric field), the antenna is elliptically polarized with a vertical major axis [23].

**Figure 5.** Helical RFID antenna's radiation pattern.

**Figure 6.** Helical antenna's axial ratio for an azimuth angle *ϕ* = 90◦ . The axial ratio is independent of the azimuth angle.

#### *3.2. Helical RFID Tag's Experimental Characterization*

#### 3.2.1. Fabricated Prototypes

Figure 7a presents the fabricated textile yarn obtained from the modified E-Thread® assembling process. The spool of the textile filament is composed of helical RFID tags, which are cascaded. Note that in practice, each tag can be cut at the appropriate length in order to be operational at the desired frequency. One helical RFID tag was isolated from the spool by cutting at the length that allowed it to have a resonance frequency in the UHF RFID band.

**Figure 7.** Fabricated helical RFID tags. (**a**) Spool of cascaded helical RFID tags. (**b**) RFID helical tag after isolation from the spool.

The obtained RFID helical tag is shown is Figure 7b and has the following geometrical parameters: *h* = 47.5 mm; *N* = 40, in addition to the ones given in Table 1. An error of 5% can be observed regarding the height, which is due to the fact that in the simulation, the material properties are known with a certain imprecision and the pitch *s* is not ideal. Thus, the helical tag's length has to be adjusted after fabrication.
