Investigation of Planar Transmission Lines on Liquid Crystal Substrates at mm-Wave Frequencies

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Mobile and wireless communication systems as well as radar systems require reconfigurable filters, tuneable and adaptive antennas, adaptive couplers, electronically controlled delay-lines, etc, in order to be flexible and operational under different communication standards. Also, for security reasons, lightweight field radios and radar systems are required to reconfigure and operate over different frequency bands in a short time span. Some earlier research have shown that liquid crystals would be particularly attractive for these applications. Liquid crystals offer the possibility of large permittivity changes, controlled by low, externally applied voltages, so their incorporation as substrates in microwave devices can bring about many advantages into communication and radar devices and revolutionise adaptive/reconfigurable systems. It is necessary to explore the possibilities offered by liquid crystals thoroughly, with a full characterisation of the materials at the frequencies and geometries involved. It is also important to develop accurate modelling techniques that can predict the behaviour of liquid crystal materials in complex device configurations and can then take into account the anisotropy and non-uniformity of its permittivity distribution in the modelling of the RF operation of the device. The liquid crystal properties can be controlled by changing the components in the mixtures, but in order to formulate the mixtures the properties must be measured over the relevant frequency ranges. In this work we will develop methods and measure dielectric constants, losses, natural resonances, nonlinearities of RF liquid crystals and we will obtain theoretically and experimentally similar parameters for liquid crystal based microstrip and coplanar waveguide from 30 GHz to 110 GHz. Modelling of the liquid crystal behaviour and of the wave propagation in simple devices will be used alongside experimental measurements to design, fabricate and test prototypes of practical devices including two reconfigurable filters.