Design and Evaluation of Tunable Microstrip Liquid Crystal Devices

Liquid crystals (LCs) are a promising microwave material due to their advantages of low voltage tuning, low cost, low power consumption. The first objective of this thesis is to develop a general design method for tunable microstrip devices using LCs working at microwave frequencies for wireless communications. As a second objective, the method was then applied to design, simulate and fabricate three types of filter to achieve different bandwidths and to maximise the tuning range. A novel general design method for tunable microstrip devices based LCs is first proposed. The design process consists of two stages, the first uses a lumped element modelling and the second, full wave simulation to optimise the dimensions. The design process was tested and verified by designing tunable bandpass filters using ELC (Electric, inductive, capacitive) resonators. Three types of ELC resonators have been designed, simulated and fabricated. The comparison between the simulation and measured results shows that the proposed design method is effective, providing the electric field of the microwave signal is considered and is compatible with the LC switching. For the second objective, interdigital capacitor (IDC) filters and ring filters were investigated. The IDC filters were designed for 2.4GHz and 5GHz. Compared with LC devices working at 5GHz in the literature, the designed IDC filter using LCs has the largest tuning range. The tunable ring resonators were designed to have compact size and narrow bandwidth so that high frequency selectivity can be realized