title: Integration topologies for high-temperature sub-THz frequency-coded chipless cooperative radar targets
Creators: name: Sánchez Pastor, Jesús
Publisher: name: UNSPECIFIED
Year: 2025

description: Radio-Frequency Identification (RFID), wireless sensing, and precise localization are key enablers propelling the Industry 4.0 revolution through widespread adoption of the Internet of Things. These technologies provide information about objects, including their state, position, and surrounding environment. Chipless identification and sensing using cooperative radar targets offers a solution for applications where semiconductor technology is impractical or overly complex, such as harsh environments characterized by extreme temperatures, strong vibrations, corrosive gases, or ionizing radiation. Particularly in the case of emergencies such as house fires, frequency-coded chipless sub-THz tags are a promising approach to support indoor localization-and-rescue autonomous systems. These systems are projected to achieve sub-millimeter positioning accuracy, while relying on a simple solution that does not require energy sources or complex infrastructure modifications.

On the one hand, the tags must be manufactured with materials able to withstand harsh conditions in the long term. To achieve so, it is crucial that they can be monolithically manufactured in a single piece so that they are both rugged and capable of resisting harsh environments. On the other hand, the wireless readout of chipless radar targets faces challenges in dynamic reflective environments, where the target backscattered signatures can become indistinguishable from clutter to a remote reader. One solution to address this issue is the use of retroreflective high Q-factor resonating tags, whose backscattered response is received over a long time and outlives the clutter echoes produced in the environment, hence ensuring the radar target’s detectability. 
 
This work systematically investigates cooperative sub-THz chipless tag concepts for extremely harsh environments. It covers the conception, design, manufacturing, and verification in harsh environments of novel monolithic tag topologies made from metallic and ceramic materials. The encoding of metallic tags is based on placing frequency selective surfaces at the aperture of conventional radar targets, such as corner reflectors and spherical lens reflectors, profiting from their long-range and wide Field-of-View response. Furthermore, a different approach is pursued in the form of fully dielectric tags, which are implemented in additively manufactured Alumina due to its low-loss and moderately high dielectric permittivity. Moreover, this material is a ceramic that is suitable for harsh environments. In this regard, tag concepts relying on dielectric corner reflectors present an enhanced Field-of-View compared to their metallic counterparts.

However, the aforementioned tag concepts do not implement inherent clutter suppression, which can be achieved by using high Q-factor resonators. These resonators feature a long ringing tag echo, which outlives clutter reflections and thus allows for the identification of the tag in dynamic and cluttered scenarios. To achieve so, two further fully dielectric tags are developed based on resonators embedded within one-dimensional and three-dimensional photonic crystals. Furthermore, the former features an optimized response for a reader located in front of it, whereas the latter is combined with a ceramic flattened Luneburg lens to achieve both a wide Field-of-View and long-range operation.

The wireless readout of the frequency-coded spherical lens reflector and the two photonic crystal-based tags is analyzed in the context of high-temperature environments, up to the extreme temperature of 1200 °C.
It is found that, while the metallic tag melted, the two tags manufactured in Alumina are able to continuously operate at temperatures above 1000 °C without any significant degradation in their performance. By demonstrating the feasibility of monolithic cooperative sub-THz chipless RFID tags in harsh environments, this work paves the way towards their use in real-world environments, as well as deepens the understanding of the challenges and opportunities present in such devices.