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

移动和无线通信系统以及雷达系统需要可重新配置的滤波器、可调谐和自适应天线、自适应耦合器、电控延迟线等，以便在不同的通信标准下灵活和可操作。此外，出于安全原因，轻型野战无线电和雷达系统需要在短时间内重新配置并在不同的频段上运行。一些早期的研究表明，液晶对这些应用特别有吸引力。液晶可以在较低的外加电压控制下实现较大的介电常数变化，因此将液晶作为微波器件的衬底可以为通信和雷达器件带来许多优势，并使自适应/可重构系统发生革命性的变化。有必要彻底探索液晶提供的可能性，并在所涉及的频率和几何形状下对材料进行全面的表征。开发准确的建模技术也很重要，该技术可以预测复杂器件配置中的液晶材料的行为，然后可以在对器件的RF操作进行建模时考虑其介电常数分布的各向异性和非均匀。液晶性质可以通过改变混合物中的成分来控制，但为了配制混合物，必须在相关的频率范围内测量性质。在这项工作中，我们将发展和测量射频液晶的介电常数、损耗、自然共振和非线性，并从理论和实验上获得基于液晶的微带和共面波导从30 GHz到110 GHz的相似参数。简单器件中液晶行为和波传播的模型将与实验测量一起使用，以设计、制造和测试实际器件的原型，包括两个可重新配置的过滤器。

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.