## 3 Microwave NDT Techniques
### Resonator Method  
Another microwave NDT technique is the resonant method. This technique utilizes the resonant behavior of microwave signals to assess an object's structural integrity and composition qualities without destroying its serviceability. It is based on the principle that when a material is exposed to microwave signals at specific resonant frequencies, the electromagnetic response is influenced by the material's characteristics, such as its electrical properties and the presence of defects. By analyzing shifts in the resonant frequency, quality factor, and signal amplitude, information about the material can be obtained [38]. An example of a resonant probe in an MNDT experimental setup is shown in Figure 8 [70]. In this system, the resonant probe consists of a spiral resonator, an electrically small loop, and a matching network. The probe is connected to a vector network analyzer (VNA) to measure its frequency response. Furthermore, the probe is integrated into a scanning table to detect defects in a sample. When there is a defect, the frequency response from the probe changes, with the results processed and visualized as an image. The resonator method has high sensitivity and can be susceptible to small variations in the material structure or properties. Consequently, discrepancies or shifts in the resonant frequencies can indicate the occurrence of cracks, voids, or changes within a material [42]. The technique for detecting cracks based on resonant frequency has shown promising  
Figure 8: An MNDT system that uses the resonator method: (**a**) the simulation model of the developed probe consisting of the spiral resonator, an electrically small loop, and the matching network; (**b**) the circuitry of the probe; (**c**) the measurement system; (**d**) a sample with various defects; and (**e**) the results for this sample [70]. potential in determining crack sizes by leveraging the correlation between the crack dimensions and variations in resonant frequency [42], making it the source of significant research in the literature. The resonator method has been utilized in various MNDT research applications due to its shared characteristics with the near-field method [51]. Different studies have presented sensitive sensors for defect inspection on metallic surfaces, such as those in [82; 83] that used complementary split-ring resonators (CSRRs). The sensor presented in [82] was shown to detect notches with a width of 0.2 mm and a depth of 2 mm by shifting the resonance frequency by 1.5 GHz. The sensor introduced in [83] detected cracks with a width of 0.1 mm by shifting the resonance frequency by more than 240 MHz. The two sensors in [82; 83] were shown to operate while either in contact with the object under test or with a very small stand-off from it; in practice, it is difficult to control and maintain a small stand-off due to dirt, corrosion, and the roughness of the surface. It is also worth noting that, in these methods, the sensitivity to depth is reduced after a certain crack depth, making their use challenging in certain applications. In a different study, a sensor based on a microstrip line coupled with a split-ring resonator (SRR) was developed [84]. The optimization of the unit cell design was achieved through a comprehensive parametric analysis of the critical design elements of the ring. The design offered multiple benefits, including eliminating the need for calibration, exceptional high-resolution capabilities down to 0.7 mm, a broad operational frequency spectrum spanning 6 to 12 GHz, and flexible integration into an array configuration for expanded functionality [84]. MNDT has undergone rapid advancements, yet challenges persist in detecting cracks on metal surfaces, characterized by difficulties in measurement accuracy and low detection sensitivity. To address these issues, [PERSON] et al. [85] introduced a novel approach involving a dual-cell waveguide MNDT probe equipped with an SRR array, which functions at a frequency of 12.5 GHz.