# Small-Size Square Ring 1-Bit Reconfigurable Transmitarray Unit Cell for C-Band Applications
## 4 Conclusions  
The reconfigurable unit cell with two-phase states suitable for C-band transmitarray applications is presented. The structure of the unit cell consists of two square ring patches of reduced size, which are connected by the coupling element. The switching states of the p-i-n diodes allow changing the direction of current flow in the antennas providing 180\({}^{\circ}\) phase difference with negligible phase error. The proposed square ring patch antenna unit-cell has reduced lateral dimensions equal to \(0.44\lambda_{\text{g}}\times 0.44\lambda_{\text{g}}\) with respect to the standard \(0.5\lambda_{\text{g}}\times 0.5\lambda_{\text{g}}\) patch. The paper provides a detailed analysis of the unit-cell insertion loss contribution, which allows optimizing the structure design from the point of view of insertion loss, the bandwidth, and the antennas area. Additionally, the p-i-n diodes bring the loss associated with ohmic resistance and parasitic parameters. The possibility of compensating the loss caused by the p-i-n diodes due to mismatching is demonstrated. As a result of EM simulation, the scattering characteristics reveal a passband insertion loss of less than 1.6 dB, while ensuring the flat 180\({}^{\circ}\) phase difference with phase error \(\pm 1^{\circ}\). The simulation results have been validated by measurements. The measured transmission coefficient module reveals higher insertion loss (2.3 dB), which is associated with higher metallic and dielectric losses and more narrowband response. Nevertheless, the flat 180\({}^{\circ}\) phase difference with 1\({}^{\circ}\) phase error and perfect matching makes  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Ref.** & **Type** & **Size** & \(f_{\text{0}}\) **(\(\lambda\)/\(f_{\text{0}}\))** & **Loss** & **Phase** & **Placement of p-i-n** \\  & & & & & & & **Error** & **Diodes** \\ \hline
[9] & O-slot rectangular patch & \(0.5\lambda\times 0.5\lambda\) & 10 GHz -(-15\%) & 1.9 dB & \(<\)13\({}^{\circ}\) & radiating element \\
[11] & circular patch with slot & \(0.54\lambda\times 0.54\lambda\) & 11.5 GHz, n/a & 1.4 dB & n/a & coupling elements \\
[13] & square ring patch & \(0.5\lambda\times 0.5\lambda\) & 10 GHz, -4\% & 1.75 dB & \(+\)6\({}^{\circ}\) & radiating element \\
[14] & magnetic dipole & \(0.5\lambda\times 0.5\lambda\) & 13.5 GHz (-13\%) & 2 dB & n/a & radiating element \\
[15] & H-shaped slots & \(\lambda\)/\(3\times\lambda\) & 12.5 GHz (-10\%) & 3 dB & n/a & phase delay line \\
[This work] & square ring patch & \(0.46\lambda\times 0.46\lambda\) & 5.9 GHz (3.4\%) & 2.3 dB & \(\pm 1^{\circ}\) & coupling elements \\ \hline \hline \end{tabular}
\end{table}
Table 1: Comparison of reconfigurable unit cell realization using p-i-n-diodes.  
Figure 6: Measured scattering parameters and differential phase shift of the proposed reconfigurable unit cell structure and insets fabricated prototype of the reconfigurable unit cell structure inside a rectangular waveguide.  
this structure an attractive candidate for reconfigurable transmitarray with beam-steering capability for sub-6 GHz 5G communication systems application.  
Software, validation, investigation, data analysis, visualization, V.K.; original draft preparation, writing--review and editing, I.M. and P.T.; conceptualization and supervision, P.T.; All authors have read and agreed to the published version of the manuscript.  
This work has been supported by the Russian Science Foundation (project 17-79-20374).  
The authors would like to thank [PERSON] for fruitful discussions of the work.  
The authors declare no conflict of interest.