## 3 Microwave NDT Techniques
### Near-Field Method  
Near-field refers to testing in a region close to the antenna/probe. This reactive near-field region is commonly found at a distance less than one-sixth of a wavelength from the nearest surface of the antenna. Thus, unlike the far-field method, the near-field method involves testing an object close to the transducer. This minimizes signal attenuation and maximizes sensitivity to minor defects, allowing for higher spatial resolution [51]. A near-field testing setup is demonstrated in Figure 7. A probe is positioned at a near-field stand-off distance from a specimen. This probe moves along the specimen during a measurement scan. At each scan location, a microwave source transmits electromagnetic signals to the specimen via the probe. The reflected signals from the specimen are captured by the probe and processed by a workstation to obtain information about the defect, such as its location and characteristics. Near-field technologies for defect detection make use of different transducers or probes such as open-ended coaxial probes, rectangular waveguide (RWG) apertures, circular waveguide (CWG) apertures, and flanged parallel-plate waveguides [51]. Coaxial probes have been demonstrated to have the ability to detect cracks irrespective of orientation. For example, [PERSON] et al. [69] designed an open-ended coaxial sensor for surface defect detection in conducting and dielectric materials. This sensor was proficient in identifying small defects in the order of micrometers and accurately measured surface depths in an aluminum plate across various depths and diverse shapes. Additionally, using this probe, the method excelled in measuring various electromagnetic attributes such as  
Figure 7: A demonstration of near-field testing. Along the scan path, a probe at a stand-off distance from the specimen transmits electromagnetic signals (green) and receives the reflected signals (red). The reflected signals from different probe positions are processed to locate the defect and can be used to create a colormap showing the defect position. complex permeability, complex permittivity, and reflection coefficients, among other critical parameters, thereby offering a comprehensive evaluation of material properties. Despite their success, coaxial probes face a significant limitation. As the gap between the end of the probe and the material being tested (stand-off or lift-off distance) widens, the sensitivity of the coaxial probe decreases drastically [70]. Consequently, their sensitivity is low compared to other probes due to the substantial reflection at the probe's open end. In addition to coaxial probes, rectangular waveguides (RWGs) have served as near-field probes for capturing images of subsurface inclusions and cracks in various dielectric materials. Open-ended RWG probes have become the foremost instrument for applying near-field techniques for accurately inspecting and assessing cracks [71]. This probe stands out due to its adaptability to multilayered materials, making it ideal for analyzing complex structures like laminar composites and thermal heat shields. Its ability to handle layered configurations enhances the performance, safety, and efficiency in various applications, including aerospace, construction, and sports equipment [72]. Using RWG transducers, [PERSON] et al. [73] developed an innovative method employing MNDT for improved inspection and evaluation of corrosion under paint. The findings were encouraging, showing that substituting the stand-off layer (air) with a dielectric layer of known properties led to notable enhancements in measurement sensitivity and resolution in specific MNDT applications. In a different investigation by the same researchers, the standard RWG was tapered to achieve better resolution while preserving sensitivity [74]. The outcomes indicate that tapering the RWG improves the spatial resolution of the captured images compared to standard rectangular waveguides. Another common probe in near-field imaging is the circular waveguides (CWGs).