Reconfigurable tunable microwave devices using liquid crystal.

Abstract

During the past decades, the applications of communication devices have extended widely, from AM radio receivers initially to newly developed GPS, smart mobile phones, radars, wireless LANs, satellite communications and implantable medical devices. The shortage in the available frequency spectrum for radio communications, the demand for portable wireless devices, and the requirement for more functionality in an even smaller volume, requires the development of new concepts in RF technology. One ideal pathway towards development of such new concepts is reconfiguration. Today, due to the rapid progress in material science and electronic technology, there is great possibility in designing reconfigurable portable wireless devices which are frequency tunable, flexible and consume low energy. In this thesis, the anisotropic properties of liquid crystals in their nematic phase are exploited as a low-voltage (< 35 V) mechanism for designing tunable wireless devices at a low microwave frequency (L to C-band). To demonstrate the possibility of using liquid crystal technology, three different design approaches were pursued: a liquid crystal tunable resonator, a tunable band-pass liquid crystal filter, both at S-band, and liquid crystal tunable frequency selective surfaces operating at C-band. The results from full-wave electromagnetic simulations, lumped-element circuit models and prototype measurements in all cases indicate around 3.1 to 8.2% of continuous frequency tuning with low insertion loss (< 1 dB). Given that liquid crystals material are transparent, commercially obtainable and are the only liquid material with tunable characteristics at microwave frequency, they could be ideal, in conjunction with flexible electronics, for designing either external or internal implantable microwave devices where flexibility is of great concern.