## 4 Flat-Panel Beam-Steering Antennas with Low-Profile Phase Shifters with Fast Response
### Flat-Panel Beam-Steering Antenna Arrays
Figure 33 exhibits the far-field radiation pattern in an anechoic chamber, where the antenna is mounted on top of a turn table and electronically scanned within \(\pm 25^{\circ}\) in both planes. Since it is a small array with \(2\times 2\) elements only and with a relatively large element spacing of \(0.65\uplambda_{0}\), the antenna gain reduces by about 2 dB and the side lobe level (SLL) increases to \(-4\) dB at wide-scanning angles \(\Theta_{m}=\pm 25^{\circ}\). Both are confirmed by the simulations [170, 175]. When the main beam is steered to \(\Theta_{m}=-45^{\circ}\), a grating lobe occurs at \(+45^{\circ}\). The measured beam-scanning duration is about 150 s to tilt the main beam from \(\Theta_{m}=0^{\circ}\) (broadside) to \(\Theta_{m}=45^{\circ}\) and vice versa. This empirical result verifies that the beam-scanning duration is lower than the response time of the delay line, which is about 180 s for the LC TUD-566, as discussed above and found out in [268]. Figure 32: Schematic of the \(2\times 2\) microstrip patch array with an element spacing of \(0.65\uplambda_{0}\times 0.65\uplambda_{0}\)[170, 175]. Front view with the four patch elements (**above, left**). Back view with the spiraled IMSL lines (**above, right**). Cross-section of the array (**below**). The larger size of the proposed array and a larger spacing between the radiating elements \(d=0.65\lambda_{0}\) provides higher gain, while a smaller spacing \(d=0.55\lambda_{0}\) allows a wider scanning range, in which the antenna can be steered with SLL less than \(-11\) dB, as can be seen from the simulations in Figure 34.  
Based on these scientific results, ALCAN's first-generation antenna was fabricated with an IMSL phase shifter stack, using a 100 \(\upmu\)m LCD process and a modular concept, where several basic modules of \(8\times 8\) radiating elements build up the ESA with an appropriate size to meet the application's specific requirements. The feasibility was first demonstrated in 2018 with a flat-panel antenna array in Figure 35 that allowed steering the antenna beam between different Ku-band satellites in geostationary orbit for television reception with a bandwidth of about 2.5 GHz [253]. To improve the beam-scanning rate, a new third-generation LC was used with much lower viscosity, with which the beam could be scanned within \(\Theta_{m}=\pm 60^{\circ}\) in less than 20 s, i.e., a beam-scanning rate of \(6^{\circ}\)/s. Figure 34: Antenna gain \(G\) and side lobe level (SLL) versus main beam direction \(\Theta_{m}\) of a \(16\times 16\) and \(32\times 32\) array of the proposed topology with two spacings—\(d=0.65\lambda_{0}\) and \(d=0.55\lambda_{0}\)—between the radiating elements at 17.5 GHz [175]. Reprinted from [2], Cambridge University Press, 2020. Figure 33: Measured antenna gain patterns in the E (**left**) and H (**right**) principal planes [170; 175]. Reprinted from [2], Cambridge University Press, 2020. To speed up the response times significantly, and hence, the beam-scanning duration to single-digit milliseconds, ALCAN's second-generation antenna for Ka-band satellite communications made use of a phase shifter stack again, but this time, it was being filled into loaded-delay lines similar to the one in Figure 27 with LC varactors of \(h_{LC}\leq 4\) um, with some modifications in the design for simpler implementation and improvements in the FoM. However, details cannot be published yet because of IP issues. However, this concept enables a lower loss and more compact size. Experimental investigations indicate an insertion loss of a phase shifter of about 3 dB at 30 GHz for a phase shift of 400\({}^{\circ}\), and hence, a FoM \(>120^{\circ}\)/dB over a bandwidth of more than 2.5 GHz.