# A Circularly Polarized Antenna Array with Gain Enhancement for Long-Range UHF RFID Systems
## 4 Conclusions  
This paper has described the design of a high-gain CP \(2\times 2\) antenna array for long-range UHF RFID applications. A two-level SR feed network was been developed to guarantee the required bandwidth without gain degradation, and SICL technology was used for minimizing transmission line  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Ref.**} & \(f_{0}\) & **S**\(\mid\)**\(\leq\)**-10** & **AR**\(\leq\)**3** & **Gain at/\(\theta\)** & **3 dB gain** & \multirow{2}{*}{**Ant. Size (\(\lambda^{3}\))**} & \multirow{2}{*}{**min Q**} & \multirow{2}{*}{**\(\epsilon_{A}\)**} \\  & **(GHz)** & **dB** & **BW** & **dB** & **BW** & & & & **(theoret)** & \\ \hline
[7] & 0.9 & n.a. & 16.4\% & 9.3 & n.a. & \(0.75\times 0.75\times 0.105\) & 0.3136 & 6.0976 & n.a. \\
[9] & 0.915 & 10.6\% & 6\% & 8.9 & n.a. & \(0.67\times 0.67\times 0.043\) & 0.3549 & 9.4340 & n.a. \\
[11] & 0.92 & \(>\)2.7\% & 2.7\% & 12.3 & 1.7\% & \(1.8\times 1.8\times 0.005\) & 0.1260 & 3.0370 & 41.7\% \\
[12] & 5 & 20.8\% & 17.6\% & 11.5 & 13\% & \(1.5\times 1.5\times 0.006\) & 0.1517 & 4.8077 & 49.96\% \\
[13] & 5.9 & 55.6\% & 41.6\% & 12.08 & 37.3\% & \(1.26\times 1.26\times 0.046\) & 0.1815 & 1.7986 & 80.92\% \\
[14] & 0.915 & 36.5\% & 28.8\% & 12.9 & 26.1\% & \(1.32\times 0.026\) & 0.1730 & 2.7397 & 89.05\% \\
[20] & 0.92 & \(>\)2.7\% & 2.2\% & 12.6 & 1.8\% & \(1.8\times 1.8\times 0.005\) & 0.1260 & 37.037 & 44.09\% \\ Prop. & 0.9 & 29.9\% & 18.2\% & 12.5 & \(>\)26.3\% & \(1.32\times 0.068\) & 0.1730 & 3.3445 & 81.22\% \\ \hline \hline \end{tabular}
\end{table}
Table 2: Measured performance comparison of the proposed design and other mentioned \(2\times 2\) circular polarization (CP) antenna arrays.  
Figure 10: Experimental read range results with the proposed antenna and the antenna in [28] as a function of the frequency for \(P_{EIRP}=36\) dBm.  
losses. Furthermore, the effects of microstrip patch height and interelement distance were studied for both optimizing broadside peak gain and minimizing gain degradation within the whole working bandwidth. Based on this analysis, an antenna array prototype of size \(1.32\times 1.32\times 0.068\)\(\lambda^{3}\) was manufactured, and the measurement results showed a peak gain of 12.5 dBic, and an overlapped bandwidth from 828 to 994 MHz (18.2%) which covers the required universal UHF RFID bandwidth. A comparison with state-of-the-art solutions demonstrated that the proposed design process is valid for achieving a good trade-off between gain and size, providing excellent results in terms of read range.  
Conceptualization, W.H., D.I., and G.W.; methodology, W.H., and D.I.; formal analysis, D.I.; investigation, W.H., and Y.H.; resources, W.H., and J.L.; data curation, W.H.; writing--original draft preparation, W.H.; writing--review and editing, D.I., G.W., and Z.C.; supervision, D.I., G.W., Y.H., J.L., and Z.C.; funding acquisition, G.W., Y.H., J.L., and Z.C.  
This work was supported in part by the National Natural Science Foundation of China under project contracts no. 61601093, 61701082, 61701116, 61371047, and 61471107, in part by Sichuan Provincial Science and Technology Planning Program of China under project contracts no. 2018 HH0034, 2019 YFG0418, and 2019 YFG0120, in part by the Fundamental Research Funds for the Central Universities under project contract no. ZYGX2016Z011, and in part by Science and Technology on Electronic Information Control Laboratory.  
The authors declare no conflict of interest.