## 5 Conclusions  
Based on the excitation switching reconfigurable mechanism, a compact four-port coplanar antenna design for CR applications is proposed in this paper. The proposed antenna combines a CPW-fed wide slot UWB antenna and three ground-coupled loop type NB antennas. The UWB antenna fully covers the UWB spectrum from 3.1 to 10.6 GHz and can detect unoccupied spectrum bands. NB Antenna #1, NB Antenna #2, and NB Antenna #3 cover the low-frequency band (3 GHz-5 GHz), mid-frequency band (5 GHz-8 GHz), and high-frequency band (8 GHz-11 GHz), respectively. As a result, only three NB antennas are required to cover the full UWB spectrum band. High isolation (greater than 17.3 dB) is maintained between the UWB antenna and the NB antennas over the entire UWB band. The realized gains of the UWB antenna and NB antennas are greater than 2.7 dBi and 1.38 dBi, respectively. Additionally, the proposed CR antenna can operate in eight modes, including a UWB spectrum sensing mode and seven communication modes which include a 5G sub-6 GHz frequency band. Finally, the proposed antenna design has increased spectrum efficiency and wide bandwidth over the entire UWB band. Therefore, the proposed antenna is a good candidate for not only CR systems but also 5G sub-6 GHz applications.  
**Author Contributions:** The presented work was carried out in collaboration between all the authors. [PERSON] wrote the paper. [PERSON] participated in the conception. [PERSON] supervised the research.  
**Funding:** This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (No. 2017R1A2B4002811). This work was supported by Advanced Track for the  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Frequency**} & **UWB Antenna** & **NB Antenna** & **NB Antenna** & **NB Antenna** \\  & **Simulated/Measured** & **\#1 Simulated/Measured** & **\#2 Simulated/Measured** & **\#3 Simulated/Measured** \\ \hline
3.5 GHz & 4.1/4.95 dBi & 2.39/3.69 dBi & - & - \\
4.9 GHz & 3.12/8.58 dBi & 3.4/5.85 dBi & - & - \\
6.52 GHz & 3.94/3.46 dBi & - & 4.8/1.51 dBi & - \\
7.65 GHz & 6.27/7.19 dBi & - & 4.36/2.62 dBi & - \\
9.5 GHz & 6.7/6.53 dBi & - & - & 4.54/2.15 dBi \\ \hline \hline \end{tabular}
\end{table}
Table 2: The comparison of the simulated and measured peak gains of the proposed antennas at center resonant frequencies of the NB antennas.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Ref.**} & **Dimensions [mm** & **Number of NB** & **Communication** & **Minimum** \\  & \(\times\) mm \(\times\) mm] & **Antennas** & **in the Entire** & **Method of Isolation** & **Isolation** \\  & & & **UWB Band** & & \\ \hline
16 & \(70\times 50\times 1.6\) & 5 & Yes & Separated ground & 20 dB \\
17 & \(68\times 54\times 0.97\) & 1 & No & Separated ground & 18 dB \\
18 & \(50\times 45\times 5.16\) & 1 & No & Separated ground & 20 dB \\
19 & \(30\times 30\times 1.6\) & 2 & No & Pattern diversity & 20 dB \\
20 & \(40\times 36\times 1.6\) & 4 & Yes & Pattern diversity \& separated ground & 16 dB \\
21 & \(26\times 50\times 50\) & 1 & No & Separated ground & 10 dB \\
22 & \(75.5\times 58.35\times 0.762\) & 1 & No & Separated ground & 10 dB \\ This work & \(42\times 50\times 0.8\) & 3 & Yes & Orthogonal characteristic mode & 17.3 dB \\ \hline \hline \end{tabular}
\end{table}
Table 3: Comparison of various integrated UWB and NB antenna systems.  
State-of-the-art Technology of an Earth Resources Industry with an ICT Application of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (20174010201170).