# A Low-Profile Dual-Polarized Transmitarray with Enhanced Gain and Beam Steering at Ku Band
## 5 Simulation and Experimental Results of the TA Antenna
The employed LPKF Protolaser U3, ultraviolet laser writing equipment, allows a very good fabrication tolerance, providing a scanning beam resolution SBR = 2 \(\upmu\)m. As result of the conducted investigation, the reproducibility can be increased by considering a prepreg sheet to assemble the substrates and a more precise mechanical positioner of the feed antenna. However, as reported in [49; 50; 51; 52], low profile TAs can suffer from limited aperture efficiency. Moreover, the unmodeled fabrication imperfections can be the factors that contribute to the low measured aperture efficiency. Optimization strategies such as UC geometry refinement for improved amplitude-phase balance can be employed to improve aperture efficiency by reducing loss mechanisms. The beam-steering performance of the TA is investigated by moving the SIW-based 2 \(\times\) 2 patch array feed antenna. The feed antenna is shifted along the \(x\)-axis (axis shown in Figure 9a) by a feed shifting length (FSL) of 65 mm in both positive and negative directions. The farfield gain patterns for the feed coordinates (x, y) = (0 mm, 0 mm); (x, y) = (+26 mm, 0 mm); (x, y) = (+45 mm, 0 mm); (x, y) = (+65 mm, 0 mm); (x, y) = (-26 mm, 0 mm); (x, y) = (-45 mm, 0 mm); (x, y) = (-65 mm, 0 mm) are plotted in Figures 14 and 15 for f\({}_{0}\) = 12 GHz and f\({}_{1}\) = 12.4 GHz, respectively. The qualitative agreement between the simulated and measured patterns is excellent. The beam-steering angles \(\gamma_{\text{max}}\) = \(\pm\)30\({}^{\circ}\), \(\gamma_{2}\) = \(\pm\)20\({}^{\circ}\), and \(\gamma_{1}\) = \(\pm\)11\({}^{\circ}\) are achieved. Figure 13: Comparison between simulated and measured realized gain of TA antenna f\({}_{1}\) = 12.4 GHz. (a) E plane; (b) H plane. However, in every case, the measured gain is slightly lower than the simulated one, as previously observed and discussed for the broadside pointing direction. Again, this discrepancy may be attributed to fabrication errors. As example, the width of the circular ring in some UCs is R\({}_{1}\)-R\({}_{2}\) = 0.1 mm, a width requiring tight tolerances. For \(\gamma_{\rm max}\) = \(\pm\)30\({}^{\circ}\) beam steering the simulated scan loss is \(\triangle\)G\({}_{\rm SSGL0}\) = 3.9 dB at f\({}_{0}\) = 12 GHz and \(\triangle\)G\({}_{\rm SSL1}\) = 3.03 dB at f\({}_{1}\) = 12.4 GHz. In measurement, it is observed that for \(\gamma_{\rm max}\) = \(\pm\)30\({}^{\circ}\) steering, the scan loss is \(\triangle\)G\({}_{\rm MSL0}\) = 4.03 dB at f\({}_{0}\) = 12 GHz and \(\triangle\)G\({}_{\rm MSL1}\) = 2.73 dB at f\({}_{1}\) = 12.4 GHz. Moreover, in the experiment, the actual position of the feed antenna during beam steering operation is slightly different with respect to the simulated one, this resulting in a lower measured gain with respect to the simulation. Also, the air gap between the substrates which is not considered during simulation causes a mismatch between the simulated and the measured  
Figure 14: Simulated (solid lines) and measured (dashed lines) beam-steering performance at f\({}_{0}\) = 12 GHz. Figure 15: Simulated (solid lines) and measured (dashed lines) beam-steering performance at f\({}_{1}\) = 12.4 GHz. scan loss. It can be concluded that the TA at f\({}_{0}\) = 12 GHz outperforms the same at for gain increment but in the case of beam steering it shows better performance at in terms of scan loss \(\triangle\)G. The measurement is done by shifting the feed antenna only along the x axis. Similar results are obtained by shifting the feed antenna along the y axis due to the symmetrical nature of this TA. Though the mechanical feed displacement for beam steering works well in this case, it is not feasible for some applications. Future work will explore the integration of reconfigurable elements, such as varactor diodes or tunable materials, within the UC architecture for beam steering to overcome the limitations associated with mechanical feed displacement.