## 6 Conclusions  
In this manuscript, a novel sequential 90\({}^{\circ}\) rotation approach of the antenna elements in a linear array designed for airborne SAR applications is presented. This rotation topology leads to improvements in the cross-polarization level within the antenna main beam in  
Figure 18: Measured cross-polar suppression @ \(f_{0}\) = 1.325 GHz. Beamforming. Vertical polarization.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Ref.**} & **Frequency** & **Array** & **Antenna Unit** & **Cross-Polar** \\  & **Band** & **Topology** & & **Suppression** \\ \hline
[18] & L-Band & \(8\times 2\) & Probe-fed patch & 38 dB \\
[19] & L-Band & \(8\times 2\) & Probe-fed stacked patch & 39 dB \\
[20] & S-Band & \(6\times 6\) & Hybrid-fed patch & 36 dB \\
[21] & S-Band & \(12\times 12\) & Aperture coupled stacked patch & 30 dB \\
[23] & X-Band & \(20\times 20\) & Orthogonal set of dipoles & 40 dB (simulated) \\
[24] & X-Band & \(8\times 8\) & Single-polarized probe-fed U-Slot & Improvement \\ This work & L-Band & \(5\times 1\) & Cavity-box aperture coupled stacked patch & 20 dB \\ Extension of & L-Band & \(5\times 5\) & Cavity-box aperture coupled stacked patch & 39.7 dB \\ this work & & & & 53.8 dB \\  & & & & (simulated) \\ \hline \hline \end{tabular}
\end{table}
Table 9: Comparison cross-polarization performance of the proposed work with some of the given references.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multicolumn{4}{c}{**Polarization H**} & \multicolumn{2}{c}{**Polarization V**} \\ \hline  & **Simulation** & **Measurement** & **Simulation** & **Measurement** \\ \hline Normal Array & 28.75 dB & 22.82 dB & 30.14 dB & 26.04 dB \\ Rotation 90\({}^{\circ}\) & 40.85 dB & 31.95 dB & 44.23 dB & 39.71 dB \\ \hline \hline \end{tabular}
\end{table}
Table 8: Comparison of cross-polarization suppression @ \(f_{0}\) = 1.325 GHz. Beamforming.  
Figure 18: Measured cross-polar suppression @ \(f_{0}\) = 1.325 GHz. Beamforming. Vertical polarization.  
comparison with the standard array configuration of up to 14 dB. Measured values show a cross-polarization suppression of almost 40 dB. The validation of this rotation approach is confirmed by considering uniform phase and amplitude distribution and beamforming with amplitude tapering. Despite the fact that the measurements match the simulated values really well, the required accuracy and sensitivity of cross-polarization measurements make a demanding task to achieve cross-polarization suppression values better than 40 dB, as it is accomplished in the simulation.  
To the best of author's knowledge, no similar sequential 90 deg rotation approach has been implemented in dual polarized linear array applications. This technique can be extended to planar arrays, achieving higher cross-polarization suppression values. The findings of the proposed work are expected to be applied in the design of the new L-Band antenna for the next generation airborne SAR system of the German Aerospace Center (DLR).  
Conceptualization, D.L., M.L., H.E. and V.E.B.; methodology, D.L., M.L., H.E. and V.E.B.; software, D.L., M.L. and B.G.; validation, D.L. and B.G.; formal analysis, D.L.; investigation, D.L.; resources, D.L., M.L. and B.G.; data curation, D.L. and B.G.; writing--original draft preparation, D.L.; writing--review and editing, D.L.; visualization, D.L.; supervision, M.L., H.E. and V.E.B.; project administration, M.L.; funding acquisition, M.L. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The authors would like to thank all the colleagues of the Microwaves and Radar Institute's workshop of the German Aerospace Center for the manufacturing of the antenna elements and array mechanical parts. The authors would also like to thank Evonik Industries for the delivery of sample Rohacell HF 51 foam layers for the antenna construction.  
The authors declare no conflict of interest.