## 4 Emerging Microwave NDT Techniques
### Chipless Radio Frequency Identification  
Chipless radio frequency identification (RFID) technology has emerged as a pioneering solution, distinguishing itself from conventional RFID systems by eliminating the need for electronic chips or power sources within tags. Incorporating chipless RFID into MNDT provides a promising avenue for advanced structural health monitoring, enabling the detection and characterizing of defects and anomalies in various materials without direct contact or the necessity for onboard power. Chipless RFID operates without the need for a chip or communication protocol. The tag's physical signal or signature exclusively encodes the data and information [125; 126]; therefore, data are acquired through passive backscattering and are unmodulated, following the principle of radar. Researchers are increasingly gravitating toward chipless RFID technology and diverging from chipped RFID, driven by its cost-effectiveness, inherent simplicity, enhanced printability, and robust performance in high-temperature settings [127; 128; 129], making it a focus of intense research. Figure 13 shows a block diagram demonstrating the working principle of a chipless RFID sensor system [126]. The sensor reader transmits a signal through an antenna to interrogate a sensor tag mounted on the specimen. The reflections from the RFID tag and the specimen result in a backscattered signal measured by the receive antenna. Next, signal processing and feature extraction are performed on the backscattered signal to identify the tag ID and read the sensor information. Furthermore, changes to the specimen due to defects will be interpreted as changes in the sensor information. An innovative chipless RFID sensor tag was developed by [126] for metal defect detection and characterization, utilizing frequency signature-based identification. The design incorporates dipole resonators for ID encoding and a circular microstrip patch antenna (CMPA) to serve as the crack sensor. The experimental results demonstrate the sensor's ability to detect cracks of widths 1 mm, 2 mm, and 3 mm on a metallic surface of 80 cm \(\times\)80 cm, positioned 30 cm below from the antenna [126]. The horizontal cracks in the specimen resulted in a resonant frequency shift of approximately \(-\)13.43 MHz for every 0.1 mm expansion in crack width, proving the tag's high sensitivity and potential for structural health monitoring applications. Figure 13: A diagram demonstrating the use of a chipless RFID in MNDT. An RFID tag is placed on the specimen and is interrogated by a sensor reader. The variations to the backscattered signal from the tag indicate the presence of a defect. In a different study, [PERSON] and [PERSON] [130] presented a novel methodology for material characterization using embedded chipless RFID in materials. Through simulations and measurements, this study investigates how changes in material properties affect the RFID tag's frequency response, which is binary-encoded. Notable results include the demonstration that embedding the tag in different materials shifts its frequency response, providing a significant understanding of the material's characteristics. For instance, when embedded in a lossless dielectric material with a relative permittivity of 4, the frequency response of the tag decreased, illustrating the method's potential for NDT and material characterization [130]. Another research paper presented a 7-bit chipless RFID multisensory device for temperature sensing and crack monitoring within an Internet-of-Things (IoT) framework [125]. The device was designed on a Rogers RT/Duroid 6010.2 LM material with six metallic resonators and a circular microstrip patch antenna [125]. The sensor exhibited a sensitive response to temperature variations, drifting the frequency response by approximately 26.2 MHz for every 10\({}^{\circ}\)C increase in temperature.