Weickhmann, Christian (2017)
Liquid Crystals Towards Terahertz: Characterisation and Tunable Waveguide Phase Shifters for Millimetre-Wave and Terahertz Beamsteering Antennas.
Technische Universität Darmstadt
Dissertation, Erstveröffentlichung

Kurzbeschreibung (Abstract)

This thesis treats methods of characterisation of the anisotropic complex permittivity tensor of Liquid Crystals (LCs) from the microwave range up to the terahertz range, and the use thereof in a tunable waveguide phase shifter for a Ka band phased array antenna for space application. An overview of the relevant properties of LCs is presented and relevant concepts are introduced. The characterisation is carried out using two methods: a resonant cavity technique at 30 and 60 GHz, and transmission measurements at frequencies from 100 GHz to 8 THz. A numerical method based on the Finite Element Method (FEM) is developed in order to extract permittivity and loss angle from the measurement of resonance frequencies of a given cavity containing a small amount of LC in a quartz tube. The method can be used on any geometry exhibiting cylindrical symmetry and extends earlier approaches significantly. The software library used to this end is FEniCS, which provides high performance and a great degree of flexibility. Separation of the anisotropic properties is obtained by magnetically biasing the setup. The data obtained for commercial and non-commercial microwave-optimised LCs provided by Merck KGaA, Darmstadt (Merck) are presented. Transmissivity data is presented for several LC mixtures obtained by Time-Domain Spectroscopy (TDS) and Fourier Transform Interferometry (FTIR) comparing material loss angles over a large frequency range. The measurement method is described and the obtained data is shown. This thesis furthermore presents the design, implementation and verification of a waveguide phase shifter using LC as functional material and light-weight construction techniques, namely the Light-weight Intersatellite Antenna - Electronical Steering (LISA-ES). The phase shifter developed within the frame of this thesis functions in the Ka band, covers two bands at 23 GHz and at 27 GHz. The biasing of this otherwise fully passive device is implemented by embedding an all-electric biasing network into the waveguide structure. The fully integrated phase shifter exhibits a maximum differential phase shift of 540° a figure of merit of up to 120°/dB, and an overall weight of less than 19 g per device. Its transient performance is assessed and switching times of less than 45 s in the fast direction and 210 s in the slow direction are verified. It can be shown that all performance criteria derived from the project’s specifications can be met using LC technology. Furthermore, it is the first time, such a high figure of merit is reported for a highly integrated, light-weight microwave phase shifter. In conclusion, an outlook on future technologies based on LC waveguide phase shifters at mm-wave frequencies for extremely large datarates is discussed. First experiments show the potential of LC technology in waveguide topology at 250 GHz, where the dimensions are challenging for fabrication but favourable for switching speed.

Frage zum Eintrag

Redaktionelle Details anzeigen