A temperature and humidity chipless RFID sensor is presented in this paper. The tag ID as well as the two physical parameters are extracted using a traditional chipless RFID tag and interrogation procedure. No specific material, i.e. particularly sensitive to the quantities to be measured, is used for ease of implementation. The method is based on a model which describes precisely the evolution of the scatterer’s resonance frequency as a function of temperature and humidity. The materials permittivity (impacted by these two quantities) and dilatation (impacted only by the temperature) are taken into consideration to estimate these two physical quantities independently and simultaneously using only the measurement of the resonance frequencies of two scatterers. Measurements in a controlled environment and in a real environment are achieved showing the potential of this approach for the sensor application. The sensing limitation as well as the possibility of this approach to characterize the temperature and humidity dependencies of unknown materials are also presented.

Thermal modeling of different transmission lines (TLs) based on resonant scatterers is presented. The coplanar strip (CPS) and microstrip (MS) TLs are used to model resonators and to introduce the thermal dependence. A reflectometry approach is employed to validate the model by detecting the scatterers’ resonance frequency and comparing it with analytical expressions. The observable shift in the resonance frequency of the scatterers with temperature variations is due to the thermal expansion of metals and temperature dependence of the substrate permittivity. Since all the measurements are done remotely with no direct line of sight, it is shown how such a reflectometry approach can be used for remote temperature sensing using a passive label composed of resonators. Unlike previous works in this domain where the thermal dependence is considered empirically, the introduced model is used to take into account all thermal effects affecting the resonant scatterers allowing to link rigorously the variations of the measured resonance frequency with the temperature without any lookup table. Temperature sensing using very simple TLs based on resonant scatterers was demonstrated in a real environment. A temperature error of less than 3 °C is obtained. Once the temperature has been determined, it is possible to go back to the TL parameters, such as the effective permittivity and the physical length.

In this paper, a novel method to characterize the thermal dilatation of metals using a contactless measurement of the frequency-dependant reflection coefficient of a resonator is introduced. This wireless technique allows extracting the thermal expansion coefficient of the material without the need of any calibration. Two antennas, a VNA and a scatterer fabricated with the under-test material are required. All the measurements have been achieved using a climatic chamber with absorbers to control the scatterer’s temperature. Measured values for thermal expansion coefficients are in good agreement with the values provided by the manufacturer which is confirmed by a sensibility study.

In this article, a novel method to improve the readability of chipless tags is presented. When the EM wave falls upon the tag’s surface at normal incidence or with a known incidence, the proposed approach permits to obtain a higher signal-to-noise ratio (SNR) of resonators whose orientation with respect to the normal to the plane in which the tag is positioned is unknown and, thus, to increase the reading distance of the tag. It introduces a technique based on the detection of this tag orientation in relation to the antennas; a projection of the signals will allow correcting this misalignment in order to always be able to decode the tag identifier on a signal that corresponds to an ideal alignment between the tag and the antennas. With this principle, it is also possible to separate the resonance mode of the resonator that is connected to the identifier from other parasitic resonance modes that may appear due to a possible misalignment between the tag and the antenna. This principle is applied to loop resonators commonly used in chipless radio frequency identification (RFID), which can present strong parasitic modes of resonance. Also, this method makes the reading orientation invariant, allowing to read the resonator whatever its orientation with respect to the normal to the plane in which the tag is positioned. Likewise, the proposed method can be also used to sense this resonator orientation. This method requires a dual-polarization antenna and a two-port vector network analyzer (VNA). Results are validated in simulation and with real-environment measurements for a single resonator and multiple resonators, as it is classically used in chipless RFID technology. Measurements in a harsh environment, as well as comparisons with classical techniques, such as time gating (TG), are presented. A study on the improvement of the read range is achieved to highlight the potential of the proposed technique. A reading distance of more than 80 cm was, thus, obtained, which represents 20 cm more than with the classic use of a postprocessor based on time gating. Finally, the principle of the measurement is generalized by applying it to a tag with a ground plane and considering that the antenna is not necessarily placed in normal incidence with respect to the plane of the tag.

In this article, a method to characterize complex permittivity of dielectric is presented. The backscattered signal from a resonant scatterer placed in contact with the dielectric is used to estimate the dielectric properties. The proposed method was tested in simulation and validated in practice using different dielectric samples and different dielectric thicknesses. This method is wireless, nondestructive, with no restriction on the sample thickness and is done using a vector network analyzer (VNA) and an antenna. Discussions on the geometry of the resonator as well as the calibration step is proposed to improve the sensing capability of the approach. Monte-Carlo (MC) simulation has been performed to define a confidence interval for the values extracted with the proposed approach in the range 1–3.5 for permittivity and 0–0.2 for $\tan\delta$ with an SNR of 20 dB. For example, a permittivity $\varepsilon_r = 3.54 \pm 0.06$ and $\tan\delta = 0.0024 \pm 0.003$ has been measured for Rogers RO4003C and $\varepsilon_r = 2.31 \pm0.05$ and $\tan\delta = 0.004 \pm 0.002$ for Duroid RT5880 with different thicknesses. It is also shown in simulation that the approach is compatible with materials having loss tangents up to 0.5, and real permittivity up to 10.

This paper presents an analytical model of the radiation pattern of a loop resonator which is frequently used as chipless RFID tags. The radiation pattern of this resonator can be used in practice as a sensor to determine the orientation between the tag and the antenna. Contrary to already implemented chipless RFID angle sensors, the one introduced here allows to extract rotation of the tag or the antenna position in 3D using a novel approach based on the resonator radiation pattern. An analytical model is developed to measure the tag orientation determining both the inclination and azimuthal angle of the tag in spherical coordinates system for both mono-static and bi-static antenna configurations. Uncertainties are studied with Monte-Carlo simulations. Simulations and measurements are performed to validate the orientation extraction and evaluate the performance of the proposed solution. We show that the accuracy of this totally passive and wireless sensor is always below 5° in the measurable range with a maximum error lower than 3° for the mono-static case.