There is significant interest in metamaterials (MTMs) for the design of novel microwave circuits and sensors. Metamaterials with their unique properties allow for the design of circuits and sensors that are compact and provide new functionalities that are difficult to achieve using conventional design approaches. Split ring resonators (SRRs) and complimentary split ring resonators (CSRRs) have been studied in great detail over the last decade as metamaterial structures. However, so far these designs have largely been implemented at low-frequencies (1-3 GHz) and require complex fabrication. To design active microwave circuits, planar metamaterial unit cell structures that readily allow the integration of active devices as an integral part of the structure are necessary. This thesis investigates the use of composite right/left handed microstrip metamaterial transmission lines in the design of microwave planar circuits and sensors. Microstrip based designs are compatible with wafer-level integration and lead to the integration of active device elements as an integral part of the metamaterial unit cell. Microfluidic channels can also be integrated with these planar structures to form sensors. Here, high frequency metamaterial transmission lines integrated with active devices to design microwave circuits are studied. Such metamaterial structures that are sensitive to their environments in the near-field are also investigated for sensor design applications. Metamaterial structures can be designed to achieve high field strengths at local spots within the unit cell structure. Dielectric (or capacitive) loading at these local spots is investigated in detail. Actively changing the capacitance at these spots using varactor diodes leads to reconfigurable circuits and allows for the design of new functions that are difficult to achieve using conventional circuit designs. In contrast, observing a change in the performance of microwave circuits by loading with biological or chemical samples allows for the design of novel microwave sensors. The dispersion diagram of these structures shows composite right/left handed properties. These properties change upon loading with capacitive elements and are analyzed to demonstrate the working principle of the sensor circuits. In order to accommodate active device elements as an integral part of the MTM structure, a new metamaterial unit cell is proposed in the X-band (7.5-12 GHz) frequency range that utilizes single-side metallization. Detailed analysis of the unit cell is carried out to incorporate varactor diodes at optimum locations for the design of reconfigurable or tunable microwave circuits. A novel reconfigurable power splitter with unequal power function, a wide-band reconfigurable X-band phase shifter with high linearity of phase shift, and a miniaturized reconfigurable antenna are designed and demonstrated. Apart from the design of high frequency microwave circuits, metamaterial structures have been exploited in the designs of novel microwave sensors. A metamaterial-inspired microfluidic sensor and a novel high-Q compact volatile molecular sensor are designed and demonstrated. Furthermore, a near-field RF probe array for material characterization and simultaneous sub-wavelength imaging of structures is demonstrated. Sensors built using metamaterials-based microstrip transmission show high sensitivity compared to conventional designs.
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