## 2 Non-FO Smart Materials: Metamaterials
### Positive Perceptive of Metamaterials in SHM
There is no clear distinction between the near and far fields, but in the literature the differences between these fields are decided based on (a) the wavelength of the scattering (transmitted) wave, (b) distance from the antenna, and (c) the largest dimension of the antenna [28; 29]. The near field setup consists of a pair of tiny dipoles or antennas, where one is a source emitting antenna and the other is a transmission receiving antenna, placed nearer to the metamaterial sensor. However, source antenna can function as receiver if it is required to capture reflection signals as well. Thus, we can measure both the transmission and reflection signals in this setup. On the other hand, a far field setup consists of a pair of large antennas, which are used separately as a transmitter and a receiver, placed far from the metamaterial specimen but inside an enclosed anechoic chamber. This chamber consists of polarization positioner and a large source antenna for streamlining input EM radiations aimed to transmit along propagation direction, which eventually scatters around, and a part of it gets reflected and refracted. The percentage of reflection, transmission, and absorption depends on the properties of the metamaterial sample, the suspended medium or the engineering structure under investigation. However, only the transmission gets captured by the receiver antenna while the reflection does not go (or feed) back to the transmitter unlike the near field. The reflection plays an important role, especially for large surface structures. Thus, an engineer/scientist working in near field may collect more EM data as compared to the far field, a desirable feature for SHM. Additionally, set-up of a far field laboratory is very expensive as scattering radiation needs to be completely absorbed. Furthermore, the electric and magnetic fields can exist independent of each other in the near-field region [30; 31], and we can measure electric fields using electric analysers and magnetic fields using magnetic probes or a common analyzer for both fields. One such electric and magnetic field measuring analyser is a VNA, a commercially available product for EM input supply and output measurements. This VNA can be used in near field and far field applications by changing or tuning it accordingly [32; 33; 34]. In either case, the output results in Z or S parameters that change/shift in the presence of any discrepancy/fault in the structure or medium. The impedance output is measured at lower frequencies in the order of Hz or K Hz [35], in which a total current or voltage measurement in sinusoidal frequency distribution is acquired at the input (reflected wave) port 1 or the output (transmitted wave) port 2 of a device or nodes of a VNA. However, at high frequencies (Giga Hz or Tera Hz) it is difficult to measure the total current or voltage because high energy is associated at higher wave frequencies. Energy increases as we move from left to right in the EM spectrum, which will damage the circuits of analyser if Z-parameters are measured. Hence S-parameters are generally measured instead of the impedance. The choice of adopting near field or far field depends on many factors such as EM radiation wavelengths, application, expertise, funding, etc. For example, kitchen oven can be a very simple model of the near field EM appliance where EM radiations in the microwave range heats the food due to the tiny dipole antennas closer to the food items, which may take longer time in the far fields with large antenna placed at a farther distance from the food. Thus, after studying various such pros and cons of EM fields, we have adopted near field experimental setup for our SHM applications in the microwave range as it avoided expensive anechoic chamber and can provide on-site monitoring.