## 3 Microwave NDT Techniques
### Far-Field Method  
Microwave NDT techniques can be segmented into two main sub-methods: far-field and near-field. The terms far-field and near-field represent regions of the electromagnetic field (EM) radiation surrounding a microwave source. The boundary between these two regions relies on the predominant wavelength (\(\lambda\)) of the electromagnetic signal produced by the source and the largest dimension (\(D\)) of the source's antenna [56]. The wavelength (\(\lambda\)) is calculated as the ratio of the speed of the electromagnetic signal in a medium (\(u\)) to its frequency (\(f\)), that is,  
\[\lambda=\frac{u}{f}.\]  
Furthermore, the far-field region occurs when the sample is \(2D^{2}/\lambda\) away from the source. Hence, the far-field NDT method, as the name suggests, utilizes the propagation characteristics of microwave signals in the far-field region. Since the object under test is \"far\" from the source, it enables the detection of flaws, structural anomalies, or material properties in a noninvasive manner, with a large inspection area. Generally, researchers commonly use the far-field method to detect defects and cracks when contact or near-contact measurements pose challenges. Figure 6 depicts a far-field system. An antenna radiates a specimen with an electromagnetic signal. This antenna is located at the far-field region with respect to the specimen. Furthermore, the same antenna measures the reflected signal from the specimen with a defect. This is repeated for different locations of the antenna surrounding the specimen. Using complex algorithms, the received signals from the different antenna locations are used to create an image of the specimen to locate the defect. With its popularity, the far-field method has been investigated heavily in the literature, with one notable research path being incorporating far-field technology into synthetic aperture radars (SARs) for imaging and detecting exposed or covered defects [57]. The following are some examples of research utilizing the SAR far-field method [PERSON] et al. [58] investigated the potential use of a far-field MNDT technique for imaging concrete through experimental tests and numerical simulations on cement-based samples. Their method employed far-field microwave reflection coefficients, and it achieved promising results. Another application example is that of [PERSON] et al. [59], who utilized a far-field antenna to detect damage and defects in glass-fiber-reinforced polymer (GFRP) wrapped in concrete columns. The noninvasive measurements indicated that there was an increase in the concrete strength of the GFRP and bar deformation coefficient, leading to a likely shift in the bond failure mode of the composite structure [59]. Despite the results, this study has limitations, as one significant constraint is the requirement for extensive spatial and frequency bandwidths to achieve accurate imaging results [60]. This demand can lead to challenges in the operational efficiency of data processing. Using polarimetric SAR, [PERSON] et al. [61] explored the detection of randomly oriented cracks hidden beneath a thick dielectric layer by studying notches concealed under a 6.25 mm thick Teflon insulator. They utilized six 0.25 mm wide electrical discharge machining (EDM) notches and a single dual-polarized circular aperture antenna combined with synthetic aperture radar imaging [61]. Another far-field method used in material characterization is the processing of commodity Wi-Fi signals transmitted through a specimen [62; 63]. In this method, Wi-Fi signals are radiated through a sample, and the received Wi-Fi signals' channel state information (CSI) is analyzed. The collected CSI is utilized to identify the type of materials in the specimen. This method was successful in distinguishing between different constructive materials [62] and in identifying composites in various concrete mixtures [62]. In addition to SAR and Wi-Fi, a common application in far-field MNDT is ground-penetrating radar (GPR) [64].