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In the upcoming era of ubiquitous cognition, the capability of gathering, processing, and presenting vast amounts of data captured by spatially distributed sensors in an efficient and low-cost way is as essential as ever. Although a plethora of ultra-low cost but yet powerful and physically small sensors have been demonstrated, only a few of these research efforts have managed to integrate these sensing modules with massive wireless sensor network (WSN) platforms: a sine-qua-non step.
The necessity of deploying WSN capabilities in sensing systems is highlighted by: (i) the increased physical range and the ability to overcome non-line-of-sight and path loss effects through multi-hopping; (ii) the enhanced reliability due to multipath communication even if some intermediate WSN nodes on the path toward the destination or a gateway fail; and (iii) the decreased power consumption of the radio transmission with multi-hopping, since it is more power efficient to relay packets over lower-strength shorter links than transmitting high strength signals over fewer long single-hop wireless links .
Toward that goal, demonstration of the prototypes presented in this chapter that realize real-time and distributed remote sensing within the scope of different applications serves as a proof of concept; it is possible to deploy WSN nodes in-between sensors and internet gateways. As a result of this approach, the primary task of the WSN nodes is not as data generators, as this is almost exclusively carried out by the sensors. Instead, the WSN nodes here serve simply as data routers that relay the sensed information through wireless multi-hop links to one or multiple gateways. It is important to note that the aforementioned prototyped wireless sensors cover the “last mile,” or more precisely the last wireless hop the length of which can range from a couple of meters to a few hundreds of meters.
Selecting proper materials for substrates depending on applications is one of the most important design steps in the field of microwave designs because substrate material determines relative dielectric constant (εr), loss (tan δ), and flexibility. The impedance of transmission lines such as microstrip lines and co-planar waveguides (CPW) is a function of the relative dielectric constant (εr) and thickness of the substrate as well as its physical dimensions. Therefore, the dimensions of transmission lines, like width of conductors or gap, are decided by the substrate material. For compact system integration, a substrate which has high relative dielectric constant (εr) is preferable since it increases capacitances of the overall microwave circuit components, resulting in small feature sizes and low radiation losses. Otherwise, a material with low relative dielectric constant (εr) is a good substrate for structures for radiation like antennas or RFIDs. The thickness of the substrate is also an important design parameter. It is directly related to effective relative dielectric constant (εeff), which has effects on resonance frequency as well as on feature sizes of the structures on the substrate. It is obvious that the resonance frequencies or poles of RF components like antennas or filters change depending on effective relative dielectric constant (εeff), which is a function of substrate thickness. In addition, directivity of antennas is affected by the thickness of the substrate because the radiated wave tends to propagate to a material with high dielectric constant (εr), which results in an uneven radiation pattern as the substrate thickness increases. The loss of the substrate should be considered when the substrate is chosen too. The loss of the substrate may limit microwave circuit designs since some designs are not compatible with high loss substrates like a cavity filler or an antenna which has high-Q factor. The loss of the substrate can be reduced by utilizing a thin substrate but sometimes it may also be acting as a fabrication limitation for stability of fabrication reasons. For flexibility, the thickness and the natural properties of the substrate material are critical. Flexible substrates are preferred for use in certain applications, such as biomedical. The material and the thickness should be carefully chosen based on application requirements.
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