Liquid Crystal Based Microwave Components with Fast Response Times: Material, Technology, Power Handling Capability (2010)

The present thesis is concerned with tunable microwave components based on Liquid Crystals (LC). Here, the fact is utilised that LC feature at GHz frequencies as well as at optical frequencies a dielectric anisotropy along a preferential direction. By means of an electric or magnetic control field, which superimposes the RF field, this preferential direction can be reoriented and thus the permittivity effective for the RF field can be controlled. After in a preceding doctoral work the applicability of LC was demonstrated in principle, the motivation of the present work is to advance the usage of LC for microwave applications, particularly for tunable phase shifters for reconfigurable phased arrays, further toward commercial applicability. To this end, both material aspects as well as component and fabrication aspects are considered. In order to improve the dielectric properties, i.e. loss and anisotropy and hence the tunability, numerous nematic mixtures were investigated with respect to their microwave performance. Certain components could be identified which feature both high tunability and low loss. Particularly these compounds based on Diphenylacetylene or on Quarterphenyles. By employing high precision measurements it is shown that the dielectric loss strongly depends on molecular relaxations. If these relaxations are reduced, materials with loss tangents down to 0.006 and relative tunabilities of up to 25 % at 30 GHz can be synthesised. For passively tunable materials, these are exceptional values. Two different approaches were pursued in order to reduce the so far for LC-based microwave devices with layer thicknesses of several 100 µm usual response times of several seconds. On the one hand, this is the stabilisation of the LC by means of a PTFE matrix which reduced the response times considerably down to 90 ms. On the other hand, a method resembling LC display assembly technology has been developed which allows realising thin LC layers also for microwave devices. Using the fabricated tunable capacitors it could be demonstrated that with a LC layer thickness of approx. 5 µm response times faster than 100 ms and with a layer thickness of approx. 1 µm response times faster than 5 ms can be achieved. Based on these tunable capacitors a tunable phase shifter is developed which features a phase shifter performance of up to 60 degrees per dB insertion loss at 20 GHz and a response time of faster than 340 ms. Additionally, for these devices resistive bias electrodes made of Indium-Tin-Oxide were employed. The power handling capability of such devices was investigated by means of single and two tone measurements. With the single tone measurements it could be shown that the device characteristic is influenced at high power levels by self-actuation effects, however only above a certain threshold power. With the two tone measurements it could be shown that these self-actuation effects cause increased intermodulation at small tone distances (<1 kHz). However, for tone distances larger than 100 kHz IP3 values of about 60 dBm can be achieved.