## 4 Sensing-Oriented Issues and Solutions
### Defect Types and Antenna Topologies  
The design of passive antenna sensors is an interdisciplinary research subject. The antenna sensor can use standing (or evanescent) waves for detection, and this information is transferred to the reader via a propagation wave in a form of RCS but influenced by nearby objects. In fact, an antenna lying parallel (and horizontally polarized) to a conductor will see the impedance of free space on one side, and the (surface) impedance of the conductor on the other side, the latter of which can be written as [92]:  
\[Z_{\mathrm{s}}=\frac{E_{\mathrm{z}}}{H_{\mathrm{y}}}=\frac{1+j}{\sigma\delta}=( 1+j)\sqrt{\frac{\omega\mu_{0}}{2\delta}}, \tag{6}\]  
where \(\sigma\) and \(\delta\) are the conductivity and skin depth of the conductor, respectively. Here, \(\omega\) is the angular frequency and \(\mu_{0}\) is the permeability in the air (for non-ferrite conductive material). Consequently, the antenna is shorted out by the conductor underneath, leading to a standing wave formed between the antenna and the conductor. Using the method of images and the concept of self- and mutual-impedances, the input impedance of a half-wavelength dipole placed at a height \(d\) above an infinite conductor is given by [93]:  
\[Z_{\mathrm{in}}=Z_{11}-Z_{12}(d). \tag{7}\]  
Meanwhile, the electromagnetic radiation in the far-field is due to the superposition of the antenna current and the image current. Any disturbance in the area between the antenna and the conductor will cause a variation of stored energy, in turn, the resistance and reactance of the antenna's input impedance. However, the reflection from the conductor and seen by the antenna is polarization, incident angle, and material property dependent [94]. On the one side, this can benefit the defect detection; on the other side, the field distribution in the antenna structure is therefore determined by the antenna mode. This is also the major difference between the antenna sensor and pulsed eddy current (uniform magnetic distribution inside the coil). To improve the sensitivity as well as the spatial resolution of the antenna sensor, the power scavenged by the tag should be confined into a small region, e.g., using a superlens [95], and re-distributed to properly interact with the defect on the tagged object [96]. The design issues can be listed as follows [97; 98]:  
* _Metal mountable_: The design of antennas for metal-mountable RFID tags is challenged by a set of limitations: low-profile and conformal structures, to provide good (gain and impedance matching) and reliable operations on conductive platforms of various shapes and sizes. * _Sensing oriented_: To be successfully turned into sensors, this class of devices should be able to properly detect and characterize the things (e.g., defects on metallic surface), being, for example monotonic, single-valued, and sensitive enough at least in the most critical ranges. As such, the multi-scale, multi-physics of defect phenomena should be properly modeled before the design of antenna sensor in order to guide the selections of antenna topology and operating mode. * _Balanced performance_: RFID communication and sensing capabilities properly demand for opposite requirements: The tag's antenna is usually designed to be perfectly matched to the tag chip in a reference condition, e.g., at healthy state, and it undergoes mismatching along with the continuous variation (propagation) of measurand. Therefore, a trade-off between sensing and communication is a challenging task to be tackled. A remarkable result is that one effective way for an RFID antenna to \"sense\" the physical status of an object, with negligible degradation of communication, is to convert the change of the external phenomenon into a variation of the input resistance only, while preserving the reactance as stable as possible [99]. The antenna size needs to be reduced down to the scale comparable with defect patch to maximize the sensitivity and resolution.