Abstract
Microwave sensors based on electrically small planar resonant elements are reviewed in this paper. By virtue of the high sensitivity of such resonators to the properties of their surrounding medium, particularly the dielectric constant and the loss factor, these sensors are of special interest (although not exclusive) for dielectric characterization of solids and liquids, and for the measurement of material composition. Several sensing strategies are presented, with special emphasis on differential-mode sensors. The main advantages and limitations of such techniques are discussed, and several prototype examples are reported, mainly including sensors for measuring the dielectric properties of solids, and sensors based on microfluidics (useful for liquid characterization and liquid composition). The proposed sensors have high potential for application in real scenarios (including industrial processes and characterization of biosamples).
Highlights
Within the paradigm of the internet of things (IoT), or more generally the internet of everything (IoE), and the advent of the fourth industrial revolution, the use of sensors has experienced an exponential growth
The most extended technology in modern sensors is probably optics/photonics, significant efforts have been dedicated in recent years to the research and development of microwave sensors, especially for applications related to material characterization and composition
Slot resonators are typically etched ground plane and are sometimes designated as defect ground structure (DGS) resonators. Examples of such resonators include the complementary counterparts of the SRR and the open split-ring resonator (OSRR), that is, the the ground plane and are sometimes designated as defect ground structure (DGS) resonators. Examples of such resonators include the complementary counterparts of the SRR and the OSRR, that complementary split-ring resonator (CSRR) [81]
Summary
Within the paradigm of the internet of things (IoT), or more generally the internet of everything (IoE), and the advent of the fourth industrial revolution ( known as Industry 4.0), the use of sensors has experienced an exponential growth. Despite the fact that non-resonant planar sensors operating at microwave frequencies have been reported [12,13,14], the combination of sensor size and performance (sensitivity) of resonant-type sensor is difficult to achieve with non-resonant methods To end this introductory section, let us mention that probably most planar microwave resonant sensors have been devoted to material characterization (the main interest in this review paper, to be discussed in detail later), there are many reported realizations focused on motion control applications (linear and angular displacement and velocity measurements) [15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30]
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