3.2.2. Measured Read Range of the Helical RFID Tag

Considering that the RFID reader has an EIRP of 3.28 W and the IC has a threshold power *Pth* = −20 dBm, the measured read range and the one deduced from the simulation using Equation (2) are shown in Figure 8. It was shown that the helical RFID tag exhibited a maximum measured read range of 10.6 m at the frequency of 865 MHz. Moreover, the RFID helical tag exhibited a wide band behavior as it can be operational in the U.S. UHF RFID band (902 − 928) MHz with a read range of 9 m. The measured result is coherent with respect to the simulation as the maximum read range obtained by the simulation was 11.3 m at 865 MHz. It is also worth remarking that the gain value of the antenna helped to compensate for the transmission coefficient and allowed a read range to be obtained closer to that of the current E-Thread solution (12 m).

**Figure 8.** Helical RFID tag's read range obtained by simulation and by experimental measurement.

Moreover, it can be remarked that compared to the simulation, a wider frequency bandwidth was obtained in the experiment, which is very advantageous for an applicative scenario. The difference in the results may be explained by the manufacturing process (some inaccuracies in the dimensions and the permittivity values of materials), which does not allow for an exact fit with the dimensions employed in the simulation.

#### *3.3. Evaluation of the Helical RFID Tag's Robustness in Terms of Stretching*

The impact of the tag's elongation on the read range was measured and the results are presented in Figure 9. At the initial state (without elongation) for an antenna having the total length of 9.5 cm, the maximum read range was 11 m at 865 MHz, which is higher than the previously shown result. This small difference may be attributed to the fact that in the previous measurement, the antenna was slightly bent; this also shows the impact that the curvature will have for a tag in wire form. It can also be observed that up to a length of 10 cm, the helical RFID tag's read range is maintained at the frequency of interest. However, beyond this length, the resonance frequency is shifted to lower frequencies, which is coherent with the increase in the length of an antenna. At the maximum considered length of 10.6 cm, the tag was still readable at a range of 9 m (18% of loss) at the frequency of interest.

**Figure 9.** Impact of the stretching on the helical RFID tag's read range, obtained by experimental measurements.

From these measurements, the robustness of the proposed antenna was confirmed in terms of the read range performance as well as the structural aspect of the textile material.

#### **4. Conclusions**

In this paper, a helical RFID tag was designed to be integrated into a textile yarn using the E-Thread® technology. The simulation results showed that fixing the parameters such as the pitch *s* and the diameter *D* made a complex impedance matching process due to the strong impact these parameters have on the helical antenna input impedance. However, the tag's read range maybe improved to reach a value close to the one obtained in the current solution by ensuring an antenna gain that enables compensating the reflection coefficient Γ. Another improvement solution might be adding lumped elements to achieve an impedance matching with the inconvenience of a complex manufacturing process. From the experimental measurements, the helical RFID tag exhibited a read range of 10.6 m, which is an improvement considering the previous work [15]. Compared to the current solution of the RFID yarn using a half-wave dipole that has a read range of 12 m, the helical RFID tag offers a close read range with the advantage of being robust to elongation. Indeed, as the experiments have demonstrated, up to an elongation of 10% from the initial length, the helical RFID tag is still readable at 9 m.

The presented helical RFID tag may be used in a wide range of applications. The capabilities of the helical RFID tag could also be expanded beyond the classical identification purposes to some other functionalities, for example, using the antenna elasticity in order to measure strain deformation and thus the textile helical RFID tag becomes a sensor.

**Author Contributions:** Conceptualization, S.B.; methodology, S.B., F.D.H. and Y.D.; software, S.B.; validation, S.B., F.D.H. and Y.D.; formal analysis, S.B.; investigation, S.B., F.D.H. and Y.D.; writing—original draft preparation, S.B.; writing—review and editing, F.D.H. and Y.D.; supervision, F.D.H. and Y.D.; funding acquisition, Y.D. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the "La Région Auvergne Rhône-Alpes" Lyon, France, through the "Pack Ambition Recherche 2017" program.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors would like to thank the company Primo1D for their help during the manufacturing process and the experimental characterization.

**Conflicts of Interest:** The authors declare no conflict of interest.
