# Microwave Liquid Crystal Technology
## 7 Conclusions  
In this paper, a comprehensive review of the latest developments in microwave liquid crystal technology has been given. Starting from the material properties and their characterization, requirements for mmWave communication systems have been introduced. From these basic considerations, the necessity for electronic steerable antennas systems is derived. An energy efficient and high-performance realization uses liquid crystals as tunable dielectric. Above 60 GHz metallic rectangular waveguide components can usually provide highest performance. Due to the implementation of biasing electrodes, the performance degrades substantially. To overcome this problem, the dielectric waveguide technology, commonly known from optics, is adapted to millimeter  
Figure 30: Field plot of the fully dielectric antenna array. Figure 29: Fully dielectric antenna array presented in [44]. (**Left**) It consists of four rod antennas which are fed by a multimode interference power divider; (**Right**) Further development could be the integration of the subwavelength fiber phase shifter. waves. With this, an easy integration of biasing electrodes is possible. The first demonstrators presented in this paper show promising results. Table 3 shows an overview on different phase shifter realizations at W-band. The high potential of LC-based devices is obvious. All given phase shifters are line phase shifter, which make them operate in a wide frequency band. Usually, MEMS and semiconductor phase shifters have response times in the range of \(\upmu\)s, as exemplarily 15 us in [45]. LC devices are suffering from comparatively high response times, as for example 340 ms in the best case for a loaded CPW line [46] and approximately 1 min for the presented dielectric waveguide phase shifters. Further work includes optimization of components based on dielectric waveguides. Specific focus is on the fabrication of the devices and on LC integration. Here, modern additive manufacturing techniques can be implemented. The tunable dielectric waveguide technology is one example for research beside LC material engineering. Material optimization is ongoing [9] and will stay a major task in future. The development of new tunable structures is the second major field of research. Recently, planar structures utilizing plasmonic or slow wave effects have been presented [53; 54; 55]. These structures show a huge potential, as they facilitate integration. Unfortunately, these types of components usually show high insertion losses. The main advantages compared to metallic rectangular and dielectric waveguides are much faster response times of the components. This is due to a much thinner LC layer which can be tuned faster with lower voltages. Also, substrate integrated waveguides (SIW) [56; 57] will probably play a significant role in the future. The right technology has always to been chosen based on a tradeoff. Researchers in academia and industry are working hard iso that LC-based devices can manifest their large potential in many applications. Conceptualization: H.M., M.J., R.R. and E.P. ; Investigation: M.J., R.R. and E.P. ; Writing: H.M., M.J. and R.R. ; Review & Editing, M.N., E.P. and R.J.; Supervision, H.M. and R.J. Funding Acquisition, H.M. and M.J. This research was partially funded by DFG grants JA921/68-1, MA6281/4-1, JA921/29-2 and BMWi/DLR under contract 50 YB1113. The authors gratefully acknowledge Merck KGaA and CST AG for supplying Liquid Crystal mixtures and CST Microwave Studio, respectively. We acknowledge support by the German Research Foundation and the Open Access Publishing Fund of Technische Universitat Darmstadt. The authors declare no conflict of interest.