## 4 Flat-Panel Beam-Steering Antennas with Low-Profile Phase Shifters with Fast Response
### Fast Tuning Low-Profile Planar Delay Line Phase Shifter
The results of the S-parameter measurements of this loaded-line phase shifter in MEMS technology filled with GT3-23001 in the frequency range from 65 up to 95 GHz have been [267; 268]: Matching is better than \(-\)15 dB, the maximum differential phase shift \(\Delta\varphi_{\mathrm{max}}\) goes up from 100\({}^{\circ}\) to 240\({}^{\circ}\), while the maximum \(IL_{\mathrm{max}}\) varies between \(-\)2 and \(-\)4 dB. This results in a FoM between 40\({}^{\circ}\)/dB and 60\({}^{\circ}\)/dB. The maximum biasing voltage was set to \(V_{b}=40\) V.  
Investigations of the transient behavior of this loaded-line phase shifter in MEMS technology is shown in Figure 29, where the rise and decay of the measured differential phase shift with respect to its maximum \(\frac{\Delta\varphi}{\Delta\varphi_{\mathrm{max}}}\) in % at 77 GHz is plotted versus time \(t\) in ms for different temperatures. Meanwhile, the measured rise time (for 10 to 90% in \(\frac{\Delta\varphi}{\Delta\varphi_{\mathrm{max}}}\)) is sparsely dependent on temperature and less than 2 ms; except for 6.8 ms for 10 \({}^{\circ}\)C, the measured decay time (for 90 to 10% in \(\frac{\Delta\varphi}{\Delta\varphi_{\mathrm{max}}}\)) changes significantly with temperature: \(T_{10}^{90}=\) 136, 69, 37, 17, 11, 7 and 5.2 ms for 10, 23, 30, 40, 50, 70, and 90 \({}^{\circ}\)C, respectively. The value at room temperature agrees well with \(\tau_{\mathrm{off}}=\) 62 to 77 ms for GT3-23001 in Figure 8 for \(h_{LC}\) in between 4.5 and 5 \(\upmu\)m.  
Figure 23.4 Principle of the reflection-type phase shifter with a 90\({}^{\circ}\)-loaded line phase shifter and a MicroElectroMechanical Systems (MEMS) switch at the end [267; 268]. Figure 23.5 Transient behavior of the loaded-line phase shifter in MEMS technology with the rise and decay of the measured differential phase shift with respect to its maximum in percentage at 77 GHz versus time for different temperatures [268]. Based on measurements of the reflection-type phase shifter with an MEMS switch at the end, the rise time and decay time were modeled in [268] by taking the following formulas: \(T_{90}^{10}=\frac{t\;(ms)}{t\;(ms)+0.2}\) and \(T_{10}^{90}=\frac{296}{t\;(ms)+2.96}\), respectively. The last one reveals 0.33 ms and 26.64 ms for the 90% and 10% values, respectively. Both formulas were taken for the phase change rate of the phase shifter to simulate the beam-scanning duration of a phased array and reflectarray at 77 GHz, consisting of 38 \(\times\) 38 radiating elements 0.51\(\lambda\) apart, for different scanning scenarios [268]. To scan the beam from \(-45^{\circ}\) to \(+45^{\circ}\), it took 1 ms for the reflectarray; however, this was accompanied with some higher side lobe levels and 9 ms (a beam-scanning rate of 10\({}^{\circ}\)/ms) for the phased array with lower side lobe levels. The difference is mainly because the path length between the feed and the unit cells on the reflectarray are different, and hence, so is the initial phase distribution, which has to be compensated first in the phase shifters of the phased array. Hence, for scanning the beam in a certain angle range, e.g., \(\pm 45^{\circ}\), not all phase shifters have to change its phase completely, i.e., 360\({}^{\circ}\) within \(T_{90}^{10}\) or \(T_{10}^{90}\), depending on the start or aim position of the phase shifters for the scan. This means that the overall performance is governed by the distribution of the phase change rate rather than a minimum or maximum value. This means that the beam-scanning duration is always less than the decay time \(T_{10}^{90}\) (maximum, worst case) or switch-off response time \(\tau_{\text{off}}\) of the LC phase shifter. Another option aims for miniaturized and fast tunable slow-wave phase shifters with thin LC layers \(h_{LC}\leq\) 4 \(\upmu\)m, which can be realized either by a 0.35 \(\upmu\)m CMOS process in connection with a CPW topology [71] or by using a nanowire membrane (NaM) [269] on top of the ground plane on the backside of an inverted microstrip line [81].