I. Introduction
The new application of microwave circuits for sensing has been attracting the attention of academic and industrial researchers is the past decade. Microwave sensors have demonstrated promising functionalities in many applications such as concentration study [1], agricultural, microfluidic systems [2], biomolecule detection [3], and gas sensing [4]–[7]. Microwave sensors operate based on interaction of electromagnetic waves at microwave regimes with the chemical/material under test [8]–[10]. The fact that makes microwave sensing devices shine among the available sensing platform is their ability to perform non-contact real-time detection and monitoring of small variations [11], [12]. These resonator-based structures have several inherent superior characters such as simple fabrication process, easy integration with CMOS, and flexibility in design [5], [13].