## 5 Conclusions  
In this paper, a miniaturized quad-stopband FSS was designed based on convoluted and interdigitated stripe. The equivalent circuit model was analyzed, and simulation verification and experimental testing were performed. Also of note, it can be easily manufactured by using low-cost PCB technology. It turns out that for TE and TM modes, the response is not sensitive to the incident angle. The structure shows a very stable resonance frequency with an incident angle of up to 60\({}^{\circ}\).The measured results are basically consistent with the simulated results. The results show that the proposed FSS achieves miniaturization and quad-band characteristics, and it has more frequency band choices than the traditional dual band FSS.  
The proposed FSS forms four transmission band rejections of 3 dB in 1-6.65 GHz, 8.35-16.9 GHz, 18.0-24 GHz, and 24.50-27.84 GHz. The center frequencies of the passband are 7.5 GHz, 17.5 GHz, and 24.3 GHz, respectively. According to the working characteristics of the passband and stopband, the proposed structure can be applied to the design of the secondary reflector, reflecting the signal in the stopband and transmitting the signal in the passband. This multiband multiplexing reflector antenna has potential applications in satellite communications, navigation, and deep space exploration. The second option is to design a radome for radar applications to realize the free transmission of its own signal and the stealth of the enemy signal.  
Conceptualization, J.D., Y.M. and Z.L.; methodology, J.D. and Y.M.; software, Y.M. and Z.L.; validation, Y.M. and Z.L.; investigations, Y.M. and Z.L.; resources, J.D.; data curation, Y.M. and Z.L.; writing--original draft preparation, Y.M. and Z.L.; writing--review and editing, J.D. and J.M.; visualization, Y.M. and Z.L.; supervision, J.D. and J.M.; project administration, J.M.; funding acquisition, J.D. and J.M. All authors have read and agreed to the published version of the manuscript.  
This research was funded in part by the National Natural Science Foundation of China, grant number 61801521, 61471368, and 61971450, in part by the Natural Science Foundation of Hunan Province, grant number 2018 JJ2533, and in part by the Fundamental Research Funds for the Central Universities, grant number 2018 gczd014 and 20190038020050.  
Data Availability Statement: Data are contained within the article.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Ref.** & **Unit Cell Size** & **No. of** & **No. of Metal** & **No. of** & **Operating** & **Angle Stability** \\  & **(mm)** & **Stopbands** & **Layers** & **Layers** & **Frequency Band** & **(deg.)** \\ \hline
[11] & \(6.6\times 6.6\times 1\) & 2 & 1 & 1 & L, S, C, X & 60 \\
[17] & \(1.9\times 1.9\times 4\) & 2 & 5 & 4 & Ku, Ka & 45 \\
[21] & \(50\times 50\times 1\) & 3 & 1 & 1 & GSM & 45 \\
[27] & \(16\times 16\times 1.6\) & 2 & 1 & 1 & WiMAX, WLAN & 60 \\
[28] & \(46\times 46\times 1\) & 3 & 1 & 1 & GSM & 45 \\
[29] & \(6\times 6\times 11.5\) & 1 & 2 & 5 & Ku & 60 \\
[30] & \(5\times 50.508\) & 3 & 1 & 1 & X, Ku, Ka & 80/80/60 \\
[31] & \(4\times 4\times 1\) & 2 & 1 & 1 & Ku, Ka & 60 \\
[32] & \(10\times 10\times 1.6\) & 3 & 2 & 2 & WiMAX, WLAN, X & 50 \\
[33] & \(8.4\times 8.4\times 0.8\) & 2 & 1 & 1 & L, S & 60 \\
[34] & \(8.8\times 8.8\times 0.762\) & 2 & 1 & 1 & X & 60 \\
[35] & \(4.0\times 4.0\times 8.78\) & 1 & 4 & 9 & L, S, C, X, Ku & 30 \\ Proposed & \(3.94\times 3.94\times 1.6\) & 4 & 1 & 2 & L, S, C, X, Ku, K, Ka & 60 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Comparative analysis of the proposed FSS and other reported FSSs.  
**Acknowledgments:** We deeply acknowledge Central South University for supporting this research through the Fundamental Research Funds for the Central Universities, grant number 2018 gczd014 and 20190038020050.