## 4 Flat-Panel Beam-Steering Antennas with Low-Profile Phase Shifters with Fast Response
### Flat-Panel Beam-Steering Antenna Arrays  
This subsection will focus on flat-panel beam-steering antennas, combining state-of-the-art LCD technology with the introduced MLC technology, featuring very low power consumption and enabling the low-cost, large-scale manufacturing of large arrays with a great number of radiating elements, since the LC phase shifter stack is fabricated in standard LCD processes and established LCD production lines. Moreover, those smart antennas can provide combinations of frequency, polarization, and pattern agility as well as steered multiple beams. A first small lab-scale demonstrator was already presented in [170; 175] for a proof-of-concept at 17.5 GHz, using a 100 \(\upmu\)m thick inverted microstrip delay line phase shifter. It is depicted in Figures 31 and 32. It is a \(2\times 2\) microstrip patch array with an element spacing of \(0.65\lambda_{0}\times 0.65\lambda_{0}\). The overall thickness is 1.5 mm only, where the four spiraled 360\({}^{\circ}\)-delay line phase shifters are on the backside together with an SMA connector for the RF signal and a four-pin DC biasing connector to control the four delay line phase shifters. These inverted microstrip line phase shifters have to be spiraled in order to implement them within the limited area beneath the radiating element, since their total length \(\ell\) is about \(4.7\lambda_{0}\) for a 360\({}^{\circ}\)-phase shift. Their FoM is around 105\({}^{\circ}\)/dB without noticeable coupling at the bends. The insertion loss is in a range between 3.5 and 4.25 dB at 17.5 GHz. Figure 31: First realized two-dimensional electronic steerable antenna array using a spiraled IMSL phase shifter with 100 \(\upmu\)m LC layer height, being processed similar to LCD technology [170; 175]. Side view of the array with an overall thickness of 1.5 mm only (**left**). Bottom view with the four spiraled IMSL lines, the feeding network, and four thin bias lines (not visible) on the substrate as well as an SMA connector for the RF signal and a four-pin DC connector to control the four delay line phase shifters (**right**). Courtesy of ALCAN Systems GmbH. The assembly of this lab-scale demonstrator consists of three stacked substrates according to Figure 32, where the LC material layer of 100 \(\upmu\)m thickness is sandwiched in between two 700 \(\upmu\)m thick Borofloat glass (front and back) substrates with relative permittivity and dielectric loss tangents of 4.65 and 0.008, respectively. Both sides of a large glass substrate are processed one by one by using the same lithography processes, which are chromium (Cr)/gold (Au) evaporation, photolithography, Au plating, and wet etching. When the lithography is completed, the substrate is diced precisely within \(\pm 5\upmu\)m accuracy into two pieces [170]. On the front glass substrate, \(2\times 2\) microstrip patches made of gold are processed on top, and on the backside, a ground plane is coated with a thin polyimide film for pre-alignment of the LC. On the back substrate, the spiraled microstrip lines for 360\({}^{\circ}\) phase shifting are processed within the limited area; then, there is the RF feeding network by using standard \(\lambda\)/4-impedance transformers and four thin highly resistive Cr bias lines for controlling each phase shifter individually, which are again coated with a thin polyimide film for pre-aligning the LC molecules on top. As spacers, micro pearls with 100 \(\upmu\)m diameter are used to maintain a cavity of constant thickness. Then, after gluing both glass substrates together and sealing them, the LC TUD-566 is filled in. The signals of the four microstrip delay lines are coupled through the first polyimide film, the LC layer, and the second polyimide film, through the four slots in the ground plane and then through the front glass to the radiating elements. Test measurements with this aperture-coupled array exhibited an antenna matching of around \(-20\) dB for broadside (unbiased), going down to \(-15\) dB when it is steered at 17.5 GHz.