Future communication systems at W-band are demanding highly directive antenna systems with beam-steering capability. For the hardware implementation of analogue beam-steering at millimetre waves, the microwave liquid crystal (LC) technology is ideally suited. It takes advantage of specifically synthesised LCs for microwaves in combination with appropriate device and biasing concepts, where the orientation of the LC, and therefore, its effective permittivity can be continuously tuned. It has low dielectric losses above 10GHz with a decreasing trend with increasing frequency. To exploit these unique characteristics, the focus of this scientific work is set for the first time on the investigation of an LC-based network with mixed discrete beam-switching and continuous beam-steering capability between the switching states for high-gain antennas at W-band. It consists of a Butler matrix combined with continuously tuneable phase shifters and a novel type of RF switch, an interference-based Single-Pole n-Throw (SPnT). The interference principle of the SPnT allows a continuously adjustable power splitting ratio, and hence, the
generation of multiple beams.
Different technologies are investigated for the realisation of this mixed network. Due to its high level of integrability and compact designs, the standard low temperature co-fired ceramic technology is examined, however, for a first proof-of-concept at Ka-band only. For W-band, two low-loss technologies are investigated: tuneable metallic and dielectric waveguides. While metallic waveguides are well suited for the realisation of low-loss non-tuneable feeding networks, dielectric waveguides are better suited for the realisation of tuneable LC components at (sub)millimetre waves, since no metallic boundaries are limiting the integration of an electrical biasing network. As non-tuneable core part, a Butler matrix with an average insertion loss of 3.5 dB at 102 GHz is realised, which is based on a novel multifunctional crossover design, allowing a miniaturised in-plane realisation of the overall mixed network. As key component for tuning of the mixed beam-switching and beam-steering network, a step-index dielectric waveguide phase shifter is presented. With a phase
shifter figure-of-merit of 100 °/dB at 102 GHz, this fully electrically biased phase shifter is going far beyond the state-of-the-art for electrically tuneableW-band phase shifter.
To stay on the same technology platform and to allow an in-plane realisation from the input port up to the radiating elements, the interference-based SPnTs are additionally investigated by a hybrid implementation of metallic and dielectric waveguides. It exhibits an insertion loss of 3dB, while providing an isolation of 27 dB. Hence, this hybrid metallic and dielectric waveguide technology reveals a
high potential not only for the presented LC-based mixed beam-switching and beam-steering network, but also for LC-tuned continuous beam-steering networks at frequencies above 100 GHz, since low-loss metallic waveguide feeding networks can be generally combined with high-performance tuneable dielectric waveguides
