## 4 Sensing-Oriented Issues and Solutions
### Defect Types and Antenna Topologies
#### 4.1.3 Corrosion Detection and Characterization
\begin{table}
\begin{tabular}{l l l l l l l} \hline \hline
**Measurand** & \begin{tabular}{l} **Sensing Principle** \\ **(Antenna Type)** \\ \end{tabular} & **Sensing Variable** & **Feature** & **Pros.** & **Cons.** & **Refs.** \\ \hline Crack depth & Inductive (coil) & Potential drop & Voltage ratio & \begin{tabular}{l} Resolution: 0.5 mm \\ in depth \\ \end{tabular} & \begin{tabular}{l} Location dependent; \\ VNA is required \\ \end{tabular} & [108] \\ \hline Crack (length) growth and orientation detection & 2D grid (meander line dipole) & Reflectometry & \begin{tabular}{l} Time difference \\ of arrival \\ \end{tabular} & \begin{tabular}{l} Chipless; Large \\ dynamic range \\ \end{tabular} & \begin{tabular}{l} VNA is required \\ \end{tabular} & [32] \\ \hline Crack (length) growth and orientation detection & \begin{tabular}{l} Mode orthogonality \\ (dual-resonant patch) \\ \end{tabular} & S11 & RFS & \begin{tabular}{l} Resolution: sub-mm; \\ Large dynamic range \\ \end{tabular} & \begin{tabular}{l} VNA is required; not \\ compatible with \\ Gen2 regulation \\ \end{tabular} & [33] \\ \hline Crack (length) growth and orientation detection & \begin{tabular}{l} Spatial division \\ (dual-resonant patch) \\ \end{tabular} & Backscattered power & Power variation & Multi-site crack & Dedicated receiver & [34] \\ \hline Crack (length) growth and orientation detection & 2D Grid (dipole) & Backscattered power & Power variation & Read range: 1 m & \begin{tabular}{l} Power is susceptible \\ to wireless channel \\ \end{tabular} & [35] \\ \hline Fatigue crack & Deformation (patch) & Turn on power & RFS & Read range: 2.1 m & \begin{tabular}{l} Large antenna size \\ \end{tabular} & [36] \\ \hline Crack (width) growth & \begin{tabular}{l} Mutual coupling \\ (patch antenna array) \\ \end{tabular} & Backscattered phase & Phase shift & \
\begin{tabular}{l} Sub-mm resolution; \\ Platform tolerance \\ \end{tabular} &
\begin{tabular}{l} Crack position should \\ be known prior; \\ \end{tabular} & [37,100] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: List of RFID tag antenna sensors for crack detection and characterization. \begin{table}
\begin{tabular}{l l l l l l} \hline \hline
**Sensing Principle (Antenna Type)** & **Sensing Variable** & **Feature** & **Pros** & **Cons** & **Refs.** \\ \hline Inductive coupling (coil) & Envelope & Peak value & Fast & \begin{tabular}{l} Read range: 3 cm; \\ Lift-off dependent \\ \end{tabular} & [47,49] \\ \hline Inductive coupling (coil) & Complex impedance & \begin{tabular}{l} principal component \\ analysis (PCA) \\ \end{tabular} & Lift-off independent & \begin{tabular}{l} Read range: 2.5 cm; \\ VNA is required \\ \end{tabular} & [50] \\ \hline Capacitive coupling (3D antenna) & Analogue identifier (AID) & PCA & \
\begin{tabular}{l} Read range: 1 m; Wireless \\ channel independent \\ \end{tabular} & Antenna profile: 1.6 cm & [51] \\ \hline Stub resonator (patch antenna) & Transmission coefficient (\(\mathbb{S}_{21}\)) & RFS & Chipless; Read range: 2 m &
\begin{tabular}{l} Influence from immersed \\ water; Not compatible \\ with Gen2 regulation; \\ VNA is required \\ \end{tabular} & [48] \\ \hline \hline \end{tabular}
\end{table} 
Table 3: List of RFID tag antenna sensors for corrosion detection and characterization.