## 4 Emerging Microwave NDT Techniques  
The advancements in microwave NDT techniques have significantly enhanced material inspection capabilities. These emerging methods utilize microwave technology to inspect, characterize, and detect defects in numerous materials with improved precision and sensitivity. Such advancements broaden the scope of applications and offer nonin  
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\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline  & **Far-Field** & **Near-Field** & **Resonant Method** & **Microwave Imaging** & **Reflectometry** \\ \hline \multirow{8}{*}{**Working principle**} & \multirow{4}{*}{Based on the propagation of electromagnetic waves in the far-field in the near-field in the region. & Relies on the electromagnetic waves in the far-field in the near-field region. & Printed circuit board & Metal characterization [82, 83] & \
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\begin{tabular}{} \end{tabular} \\  & & & & & & \\ \hline \multirow{8}{*}{**Probe type**} & \multirow{4}{*}{Horn antennas [56, 58, 59]} & \multirow{4}{*}{Dual-polarized circular aperture} & \multirow{4}{*}{\(\bullet\)} & \multirow{4}{*}{\(\bullet\)} & \multirow{4}{*}{\(\bullet\)} & \multirow{4}{*}{\(\bullet\)} & \multirow{4}{*}{\(\bullet\)} & \multirow{4}{*}{\(\bullet\)} & \multirow{4}{*}{\(\bullet\)} & \multirow{4}{*}{\(\bullet\)} \\  & & & & & & & \\ \cline{1-1}  & & & & & & & & \\ \cline{1-1}  & & & & & & & & \\ \cline{1-1}  & & & & & & & & \\ \hline \multirow{8}{*}{**Advantages**} & \multirow{4}{*}{Non-contact method} & \multirow{4}{*}{\(\bullet\)} & \multirow{4}{*}{\(\bullet\)} & \multirow{4}{*}{\(\bullet\)} & \multirow{4}{*}{\(\bullet\)} & \multirow{4}{*}{\(\bullet\)} & \multirow{4}{*}{\(\bullet\)} & \multirow{4}{*}{\(\bullet\)} & \multirow{4}{*}{\(\bullet\)} \\  & & & & & & & & \\ \cline{1-1}  & & & & & & & & \\ \cline{1-1}  & & & & & & & & \\ \cline{1-1}  & & & & & & & & \\ \cline{1-1}  & & & & & & & & \\ \cline{1-1}  & & & & & & & & \\ \cline{1-1}  & & & & & & & & \\ \cline{1-1}  & & & & & & & & \\ \cline{1-1}  & & & & & & & & \\ \cline{1-1}  & & & & & & & & \\ \cline{1-1}  & & & & & & & & \\ \cline{1-1}  & & & & & & & & \\ \cline{1-1}  & & & & & & & & \\ \cline{1-1}  & & & & & & & & \\ \cline{1-1}  & & & & & & & & \\ \cline{1-1}  & & & & & & & & \\ \cline{1-1}  & & & & & & & & \\ \cline{1-1}  & & & & & & & & \\ \cline{1-1}  & & & & & & & & \\ \cline{1-1}  & & & & & & & & \\ \cline{1-1}  & & & & & & & & \\ \cline{1-1}  & & & & & & & & \\ \hline \multirow{8}{*}{**Limitations**} & \multirow{4}{*}{Lower resolution compared to near-field} & \multirow{4}{*}{\(\bullet\)} & \multirow{4}{*}{\(\bullet\)} & \multirow{4}{*}{\(\bullet\)} & \multirow{4}{*}{\(\bullet\)} & \multirow{4}{*}{\(\bullet\)} & \multirow{4}{*}{\(\bullet\)} & \multirow{4}{*}{\(\bullet\)} & \multirow{4}{*}{\(\bullet\)} & \multirow{4}{*}{\(\bullet\)} \\  & & & & & & & & \\ \cline{1-1}  & & & & & & & & \\ \cline{1-1}  & & & & & & & & \\ \cline{1-1}  & & & & & & & & \\ \cline{1-1}  & & & & & & & & \\ \cline{1-1}  & & & & & & & & \\ \cline{1-1}  & & & & & & & & \\ \hline \multirow{8}{*}{**Applications**} & \multirow{4}{*}{Material inspection using Wi-Fi [62, 63]} & \multirow{4}{*}{\(\bullet\)} & \multirow{4}{*}{\(\bullet\)} & \multirow{4}{*}{\(\bullet\)} & \multirow{4}{*}{\(\bullet\)} & \multirow{4}{*}{\(\bullet\)} & \multirow{4}{*}{\(\bullet\)} & \multirow{4}{*}{\(\bullet\)} & \multirow{4}{*}{\(\bullet\)} & \multirow{4}{*}{\(\bullet\)} & \multirow{4}{*}{\(\bullet\)} \\  & &  
-vasive, rapid, and cost-effective solutions for quality assurance and structural integrity assessments across multiple industries. Moreover, ongoing developments in emerging techniques aim to address existing limitations, such as pre- and post-inspection cleaning requirements, sensitivity to magnetic permeability and object coating, and the ability of some MNDT techniques only to examine non-porous surfaces [110]. Some of such emerging methods are discussed in this section.