This paper highlights interest in the implementation of microwave sensors based on resonant elements, the subject of a special issue in the journal. A classification of these sensors on the basis of the operating principle is presented, and the advantages and limitations of the different sensor types are pointed out. Finally, the paper summarizes the different contributions to the special issue.
Keywords: coupling-modulation sensors; differential-mode sensors; electrically small resonators; frequency-splitting sensors; frequency-variation sensors; humidity sensors; internet of things (IoT); microfluidics; microwave sensors; permittivity sensors; phase-variation sensors; planar sensors; resonant sensors; substrate integrated waveguides (SIW).