A general design method for tunable microstrip devices at microwave frequency based on liquid crystal technology

摘要

This paper proposes a general design method for microstrip devices using liquid crystal (LC) at microwave frequencies. The method employs lumped element modelling to first optimise the filter at the working frequency. Then EM simulation is used for the second optimisation and the full-wave simulation. LC is used as the tunable material since the anisotropy can be changed with low voltages. The approach of modelling LC as a homogeneous anisotropy demonstrated with Computer Simulation Technology (CST) Microwave Studio is an acceptable approximation, which accurately predicts the tuning range. A new tunable inverted microstrip filter using nematic LCs at millimetre-wave frequencies is designed to verify the method. The proposed design utilises interdigital capacitors in parallel with two inductive loops to form a bandpass filter. It is fabricated and measured, and shows that the centre frequency varies from 5.01 to 5.51 GHz (10% tunability) and achieves a 3-dB bandwidth of 450 MHz, which is in good agreement with simulation results. A finite element simulation is used to investigate the behaviour of LC directors in the fully switched state; the effective permittivity is extracted from finite-element simulation and used in CST to achieve an even more accurate agreement with experimental results.