Tunable Microwave Components based on Few Layer Graphene

This doctoral thesis is about the use of graphene for microwave tunable passive components. It opens a new paradigm in the use of innovative and cost-effective methods for producing tunable microwave components based on graphene. Specifically, it paves the way for future key components of microwave and wireless communication systems such as attenuators, phase shifters and antennas. A review of the state-of-the-art microwave passive components based on graphene in terahertz and microwaves is also provided. The integration of a number of components on a transmitter receiver system requires functional materials of nanometric scale. The use of innovative nanomaterials for designing state of the art microwave components is not new. The signature property of monolayer graphene that can be exploited for tunable microwave components is its electronically tunable resistance. This property is valid for dimensions as large as mm/cm to as small as micro and nanometers keeping a constant aspect ratio. The big challenge in research on future communication systems is to cost effectively design, implement and measure such proposed components. To this aim, in this thesis few layer graphene is deployed in the design of tunable attenuators, phase shifter and antenna. The advantage of using FLG is its cost effectiveness, technological simplicity and eco friendliness unlike most nanomaterials. A new design of tunable graphene attenuator was proposed based on shunt grounded vias connected to FLG flakes and a microstrip line. The grounded vias were symmetrically placed on each side of the microstrip line with two ports. The design, even though of not very high structural complexity resulted in superior functionality both in terms of dynamic range of insertion loss and the reflective insertion loss. The number of vias were then increased for improved functionality. With the increase in the number of vias, emerging structural parameters were optimized for higher insertion loss and improved mismatch. Simulations were performed for the optimization while fabrication of prototype and measurements were performed which were in good agreement to the simulated results. For the final case of eight vias connected to FLG, a total of more than 65 dB insertion loss was measured with reflective insertion loss as low as 2dB. Phase Shifter being a vital component of a communication system was also made incorporating FLG flakes. The tunable FLG resistance was converted to tunable reactance by the help of a stub composed of tapered line connected to FLG and a shorted stub. The various lengths and widths of the line were optimized so as to provide maximum shift in reactance when the change in FLG resistance would occur by an applied DC bias voltage. Subsequently, the optimized stub with variable reactance was connected to a two-port 50 Ω transmission line, the transmission on which would cause a phase shift by an applied DC voltage across the FLG. The maximum phase shift obtained was 43 degree with an additional insertion loss of 3 dB. The concept can be applied to a number of such units connected in cascade since the insertion loss is not very high. A combination of the phase shifter and attenuator can be used in the design of a tunable modulator based on a combination of amplitude and phase variation. The concept of the phase shifter was applied to a frequency reconfigurable patch antenna. FLG accompanied by a shorted stub optimized for maximum reactance change were deployed in a microstrip antenna. A total shift in the radiating frequency of 450 MHz was measured at an applied DC bias voltage of 5V with limited gain degradation.