## 2 Microwave Liquid Crystal Technology
### Orientation Mechanisms and Biasing Schemes
Therefore, the switch-off response time is the most critical parameter for tunable LC devices. It is plotted in Figure 8 for most LCs in Table 1 versus the effective LC layer height \(h_{LC}\), where it is getting obvious that the simple hybrid biasing concept is only feasible for low profile (thin, flat), planar structures with \(h_{LC}\leq 5\)\(\mu\)m to achieve switch-off response times \(\tau_{\text{off}}<100\) ms for some LCs, and at the same time, control voltages \(V_{b}<10\) V. With increasing effective LC layer height \(h_{LC}\), the control voltage will increase up to a few tens of volts, and particularly, the switch-off response time will increase, e.g., up to about 30 s for \(h_{LC}\approx 100\)\(\mu\)m, which might be still feasible for portable applications, just to reconfigure the characteristics such as the antenna beam or for slow-moving applications such as for a relay GEO satellite to track LEO satellites, but this will be impractical for on-the-move applications, e.g., to track the signal from a satellite by a moving car, ship, train, or airplane. Hence, the effective LC layer thickness \(h_{LC}\) is a decisive design parameter to reduce the control voltage and the switch-off response time. However, the decreasing LC layer thickness is limited in planar line topologies, because of the increasing metallic losses and 50 \(\Omega\) impedance mismatch; e.g., a typical 50 \(\Omega\) design of a microstrip delay line (see Section 4.1) would be of an effective LC layer height of 60 to 100 \(\upmu\)m and a width of the signal electrode of 100 to 200 \(\upmu\)m to avoid increasing metallic losses, which exceeds the dielectric one [74]. Hence, high performances combined with fast switch-off response times is not possible to achieve at the same time for standard planar line topologies. Here, innovative concepts are required to overcome these problems. A promising solution is the loaded-line concept, where lines are periodically loaded by very thin, fast-tuned LC varactors with \(h_{LC}<5\)\(\upmu\)m [175, 185, 225, 226, 227, 228]. Figure 11 exhibits the side view of such a LC varactor, presenting two capacitors in series. Floating top and bottom electrodes are processed on 700 \(\upmu\)m thick fused silica substrate, respectively. After applying a polyimide film to both substrates to pre-align the LC molecules, they are glued together. Two such varactors with different geometry, both filled with GT3-23001, had been realized and characterized in [225, 226]:  
\(\gg\): Var1 with \(w=1000\)\(\upmu\)m, \(\ell=450\)\(\upmu\)m, \(h_{LC}\approx 5\)\(\upmu\)m, C = 0.95 pF, \(f_{R}=7\) GHz, \(\tau_{\text{on}}=22\) ms, \(\tau_{\text{off}}=92\) ms and \(\gg\): Var2 with \(w=300\)\(\upmu\)m, \(\ell=225\)\(\upmu\)m, \(h_{LC}\approx 1\)\(\upmu\)m, C = 0.8 pF, \(f_{R}=7.5\) GHz, \(\tau_{\text{on}}=1\) ms, \(\tau_{\text{off}}=4\) ms, where the response times measured for \(h_{LC}\) of about 1 \(\upmu\)m and 5 \(\upmu\)m, respectively, clearly follow the square law. Taking into account some uncertainty in the determination of the exact rotational viscosity \(\gamma_{\text{rot}}\), elastic constant \(K_{11}\), and in particular, LC layer height \(h_{LC}\), the values for \(\tau_{\text{off}}\) above agree well with the one from Equation (6) of 3.2 ms and 80 ms for a thin LC capacitor of 1 \(\upmu\)m and 5 \(\upmu\)m height for GT3. Beyond \(h_{LC}>100\)\(\upmu\)m, the hybrid biasing concept is not feasible at all, not only because of the dramatic increase in response times, but also because tunability decreases with increasing \(h_{LC}\), since more and more LC molecules are not perfectly pre-aligned in parallel to the surface due to lower anchoring forces. Hence, for bulky or \"thick\" voluminous metallic or dielectric waveguide-based components with LC layer heights of several hundred of \(\upmu\)m or even of few mm, only a fully- or all-electrical biasing scheme is sensible, using multiple pairs of electrodes without any pre-alignment layer.