## 1 Introduction
The sensing information is directly encoded into the antenna backscatter behavior. For this reason, the sensor may function in an extreme environment, e.g., high temperature. * _Passive operation_: The tag chip has its own energy harvesting module, as such, no onboard battery is needed. * _Medium read range_: The read range for a general passive tag can be up to 10 m, however, the read range largely depends on the frequency, antenna gain, and tag chip's sensitivity. * _Low cost_: The cost for each dipole tag is ~$0.10-0.20 for mass production. The antenna sensors can be fabricated on inexpensive substrate materials, such as paper, PVC, using low-cost fabrication techniques, such as inkjet printing. * _Unique identification_: Each tag has its own UID, which is used to identify the location of the defect as well as connect the things into internet. This sensor multiplexing capability enables densely distributed passive WSNs and parallel interrogation of multiple sensors with anti-collision algorithms. * _Multimodality_: The antenna can be designed to be sensitive to various physical/mechanical/chemical things in a real-time or periodic. * _Planar or flexible_: The antenna sensors can be fabricated on low-profile, flexible substrates that completely conform to the surface they attached to. * _Cover Penetration_: The surface of the metal may be covered with paint, cladding, or a similar compound, and the defect may still be detected because microwaves can penetrate dielectric materials. The future IoT will consist of heterogeneously connected devices that further extends the borders of the world with physical entities and virtual components [28]. The middleware is designed for the potential integration of a heterogeneous IoT sensor network while the internal data is for seamless access to a Cloud Computing system [29]. The defect information can be extracted by detecting the change in antenna sensors, such as the resonant frequency shift (RFS) from RCS. In addition to mechanical actions, e.g., fatigue, structure (concrete and steel) can develop cracks because of various physical and chemical processes (stress corrosion). Various antenna sensors have been developed for this purpose [30; 31; 32; 33; 34; 35; 36; 37; 38]. Strain [36; 38; 39; 40; 41; 42; 43; 44; 45; 46] and corrosion [47; 48; 49; 50; 51] can also be monitored by antenna sensors, enabling early warnings about structural health. In addition, the liquid level [52] and displacement [53; 54] can be monitored as well. Beyond the passive sensors developed for monitoring mechanical/physical parameters, the passive sensors and systems can be expanded for monitoring chemical parameters in the environment with proper electrochemical materials. This is out of scope of this work. Antenna sensors that are sensitive to environmental conditions, e.g., moisture [55; 56; 57; 58], gas [59; 60; 61], temperature [62; 63; 64; 65; 66; 67; 68], have been demonstrated. More information about this topic can be found in [69; 70]. Recent emerging work on passive antenna sensors illustrate the great potential for future SHM in terms of integration of passive sensing, communication, location and identification. Permanent installation removes problems such as signal variability because of texture and geometry changes with position and can provide better damage growth rate estimation by taking data more frequently--at the cost of reduced area coverage; the benefits, however, can only be realized if the systems are _reliable_ over long periods, the data obtained can be reliably related to the health of the structure, and any defects are reliably detected with low false alarm rates [71]. Because of the limited scavenging power and fading effect of radio frequency (RF) signal, the challenges for _accurately and reliably_ detecting and characterizing defects based on passive antenna sensors in a remote distance are of special concern and need to be systematically studied. In this paper, issues for this type of sensor are outlined.