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Microwave sensors (MS) have been the subject of intensive study and development in both academic and industrial circles due to their numerous advantages such as high sensitivity, high selectivity, and ease of fabrication. These sensors operate based on radio frequency (RF) signals, detecting changes in the environment and subsequently shifting the frequency, phase, amplitude, or reflection values. Several proposed MS have only monofunctional capability. In contrast, to support the development of IoT technology, multifunctional sensors are needed. The development of multifunctional MS structures faces several major challenges: 1) They exhibit a high degree of complexity, requiring additional insulator structures between the device and the MS. 2) They require an additional power supply to support the various functionalities. 3) They typically support only a single-band communication system with a fixed band, which limits their versatility and compatibility. 4) Many proposed multifunctional devices offer switchable functionalities, meaning that only one feature can be active at a given time. This constraint prevents the simultaneous operation of multiple functions, potentially reducing efficiency and causing inconvenience. 5) In some cases, the different functions integrated within a combined device may not operate independently. The performance of one function may be affected by the operation of another, leading to compromised performance and unpredictable behavior. 6) The ambiguous and non-independent characteristics of multifunctional devices often introduce uncertainties regarding their performance and limitations. It can be challenging to precisely define their capabilities and ensure consistent performance across all functionalities. Consequently, the challenges in developing more agile and versatile multifunctional MS/devices are numerous, leaving many voids and open problems to address. Therefore, this tutorial will explain several strategies to develop multifunctional sensors, including the cross-sectional effect. The subsections include hybrid MS and Localized Surface Plasmon Resonance (LSPR) sensors, hybrid MS with attenuators, and hybrid MS with dual-band bandpass filters.
