**4. Conclusions**

This paper demonstrated the e fficiency and simplicity of the resonator method to accurately characterize flexible substrates, such as textiles, under environmental conditions. It also shows the cost-e fficiency of the technique proposed. This enables a remote sensing scheme for services within harsh environments where equipment can be damaged or it cannot be placed.

As far as the author's knowledge goes, this paper presents for the first time measured results of dielectric properties variation of fabrics over temperature. First, di fferent fabric substrates (cotton, jeans, viscose and lycra) were measured at 2.45 GHz over a temperature range from 20 to 60 ◦C, at 10 ◦C steps. As no essential di fference was observed among the four textiles, a finer temperature characterization was carried out to focus on organic cotton. Temperature steps were reduced from 10 ◦C to 5 ◦C at 9.5 GHz and to 5 ◦C/1 ◦C at 38 GHz respectively.

From the test campaigns, it was observed that a linear relationship between the change in temperature and the change in dielectric constant exists and it is frequency independent. It was quantified to be Δ<sup>ε</sup>r 1.67 × 10−<sup>3</sup> per degree Celsius. This relation can be linearly extrapolated to any temperature value. For all the cases, within the four substrates and at the three di fferent frequencies a saturation status behavior can be seen when heating up the substrate above 50 ◦C. Limiting the use of this technique up to that temperature range.

The textile antenna working at mmW (38 GHz) presents a substantial potential as a passive temperature sensor. Several applications such as food logistic or on-body sensing could benefit from its sensitivity up to one degree Celsius and its characteristics of a fabric-based device.

For future work would be to improve the test setup to remove some uncertainties in the measurements caused by environmental factors. See the impact of going even higher in frequency in terms of impact on physical properties. Looking into the thermal threshold, actual value and plausible cause, like rarefaction of the air trapped within the resonator structure.

**Author Contributions:** Conceptualization, I.I.-L. and A.A.; methodology, I.I.-L.; software, I.I.-L.; validation, I.I.-L. and A.A.; formal analysis, I.I.-L.; investigation, I.I.-L.; resources, I.I.-L. and A.A.; data curation, I.I.-L.; writing—original draft preparation, I.I.-L.; writing—review and editing, I.I.-L. and A.A.; visualization, I.I.-L. and A.A.; supervision, A.A.; project administration, I.I.-L. and A.A.; funding acquisition, I.I.-L. and A.A. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** This work was possible due to support from the School of Electronic Engineering and Computer Science, Queen Mary University of London.

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