For the past two decades, planar microwave resonators have been used for sensing and monitoring applications due to their unique characteristics such as non-contact and real-time sensing capability, simple and low-cost fabrication process, which make them a high performance alternative to traditional bulky waveguide sensors. However, the limitation of electronic readout circuitry in harsh environment sensing, low-sensitivity, and distance limited sensing performance are the important challenges and limitations for their successful implementation into a wireless sensing system and the Internet of Thing (IoT) ecosystem. The work presented in this thesis focuses on distance and resolution enhancement of coupled split-ring resonator (SRR) based structures, and several highly sensitive distant sensing systems and their applications are presented.

To address the distance limited sensing performance in the conventional SRR-based sensors, a reader-tag structure based on coupled SRR structures is designed. The presence of the second resonator, the tag, enhances the design's sensitivity significantly and enables the sample under the test to be placed at further distances from the reader. Applications of the proposed technology in non-contact real-time hazardous gas sensing, humidity percentage monitoring, and non-invasive glucose concentration sensing is investigated.

In the second step, sensing distance in the reader-tag structure is enhanced using a locally strong coupled microwave resonator. Backed with theory, simulations, optimization, and experimental results, this concept demonstrates the ability to significantly increase the distance between the tag and the reader. The flexible ultra-thin tag resonator empowers the proposed design to perform real-time noncontact sensing using an inexpensive sensing element that could be easily mounted on any material container. The design applications in high-temperature bitumen sensing, humidity monitoring, and disposable microfluidic biomedical sensors are explored.

Furthermore, to enhance the resolution of the sensing, an active feedback loop has been added to the conventional SRR-based planar structure to compensate for different sources of loss and create a sharp high-Q response, capable of high-resolution sensing performance. The application of high-resolution active sensors is investigated for pH level sensing in biomedical and pipeline integrity.

Finally, in order to enable microwave sensing systems to be capable of high-resolution measurement and protect them is harsh environment applications, the active feedback loop and the tag-reader coupled structure are combined in one structure and a flexible low-cost, RF chipless tag-reader sensor is developed, capable of ultra high-quality factor performance. The chipless RF tag is a great candidate for harsh environment sensing applications since the main sensing element in the design is a passive structure. The high level of sensitivity offered by design, empowers it for concentration measurement in nano-liter samples. The presented technique provides a practical solution for highly sensitive, non-invasive, and real-time sensing applications.