## 4 Flat-Panel Beam-Steering Antennas with Low-Profile Phase Shifters with Fast Response
### Low-Profile Planar Inverted Microstrip Line and Grounded Coplanar Waveguide Phase Shifter
These plots exhibit curves of constant phase shifter figure-of-merit (FoM, full red lines), orientation effectivity \(\tau_{\varphi}\) (dashed blue lines), and line impedance \(Z_{Line}\) (dotted gray lines), depending on the signal line width \(w\) (vertical axis) and the thickness \(s\approx h_{LC}\) (horizontal axis). Figure 20: Realized broadband inverted microstrip line (IMSL) phase shifter for frequencies from 1.125 up to 35 GHz. Different parts (**left**) and assembled phase shifter with filling channels (**right**) [47; 186]. For another IMSL phase shifter with a thinner LC layer height of \(h_{LC}\) = 127 \(\upmu\)m and line width of \(w\) = 300 \(\upmu\)m for nearly 50 \(\Omega\) design, using a polyimide film AL-3046, and two LCs, MDA-03-2838 and MDA-03-2844, the FoMs reduce to about 67\({}^{\circ}\)/dB and 75\({}^{\circ}\)/dB, respectively, because of increasing ohmic losses with decreasing electrode distances. However, for both, the FoM was nearly constant over a frequency range from 20 to 33 GHz. This time, fused silica was used instead of TMM3 [47; 186]. Compared to GT3 with a FoM of around 100\({}^{\circ}\)/dB at 30 GHz according to Figure 18, it is less, due to the lower material's figure-of-merit. At 30 GHz, the measured differential phase shift was \(\Delta\varphi_{\text{max}}\approx 330^{\circ}\) and 340\({}^{\circ}\), which was accompanied with insertion losses of \(IL_{\text{max}}\approx 5\) and 4.5 dB for MDA-03-2838 and MDA-03-2844, respectively, while the reflection coefficient \(S_{11}\) was less than \(-10\) dB over the whole frequency range [47; 186]. The measured 90 to 10% decay time in the differential phase shift was in the range of roughly 30 to 50 seconds, which agrees well with the switch-off response times for GT3. To assess the linearity of the phase shifter, first large-signal measurements at \(f_{0}\) = 7.4 GHz were carried out for an IMSL section of 6 cm length; however, seven times meandered. The LC layer height was \(h_{LC}\) = 100 \(\upmu\)m filled with the LC MDA-03-2844. The measured reflection coefficient was \(|S_{11}|<-21.5\) dB and the insertion losses were less than 3.3 dB, leading to a FoM = 40\({}^{\circ}\)/dB [184; 185; 186]. The setup used in [184] allows a resolution of IP\({}_{3}\) measurements of at least 60 dBm at a total available power level of approximately 40 dBm to the device under test or \(P_{0}\) = 37 dBm per tone, respectively. Figure 21 shows the measured IP\({}_{3}\) versus the frequency difference \(\Delta f\) of the two tones at room temperature and the maximum input power for three device configurations: empty (air), filled with K15, and MDA-03-2844. The noise floor generated by the sources is at -55 dBc in close proximity to the tone at 100 Hz and decreases to -80 dBc for distances \(\Delta f>\)10 kHz. Hence, the IP\({}_{3}\) values for \(\Delta f<1\) kHz are only an estimate. For K15 with respect to air, a slightly increased IP\({}_{3}\) for a small \(\Delta f\) is visible, whereas for MDA-03-2844, it is not visible. This is because the reorientation (response times) is much slower in reaction to an external stimulation such as the RF beat, despite higher anisotropy. Generally, the IP\({}_{3}\) measurements exhibit excellent large signal characteristics, i.e., an extreme high linearity for the phase shifter device with both LCs. One way of realizing a reliable, hermetically sealed cavity for LC is by using the standard low-cost multi-layer low-temperature co-fired ceramic (LTCC) technology, which is space qualified. LTCC is mechanically stable and allows a three-dimensional structuring, using conductivity, resistance, or capacitive pastes between the layers. Structures on different layers can be connected by vias, and those vias can also be used to form metallic cavities inside the LTCC structure.