# Wideband Circularly Polarized Filtering Hybrid Antenna
## 4 Antenna Performance  
To validate the presented design concept, a prototype antenna is fabricated and measured. The photographs of the prototype and the measured environment are shown in Figure 6. Figure 7a shows the simulated and measured results of the proposed antenna. The impedance bandwidth with \(\,1\mathrm{S}_{11}\,\mathrm{<-10}\) dB is 38.8% from 1.36 to 2.02 GHz, and the 3-dB AR bandwidth is 39.4% from 1.37 to 2.04 GHz as shown in Figure 7b. The usable overlap bandwidth of the antenna is 38.2% (1.37 GHz-2.02 GHz). Figure 8 exhibits the simulated and measured realized gains. When the U-slot and C-stub are not loaded, the gain of the antenna changes smoothly and has no obvious filtering response. The gain of the final antenna in the whole operating band is stable around 7.8 dBi, while two radiation nulls will be generated at the lower and upper edges of the working band, respectively. The smaller frequency offset is due to the tiny air bubbles in the liquid causing the actual size to be different from the simulated size.  
Figure 5: Simulated realized gains for different length of (**a**) the C\(-\)stub. (**b**) the U\(-\)slot.  
Figure 6: The proposed antenna. (**a**) Fabricated prototype. (**b**) Measured environment.  
Figure 9a-c depicts the simulated and measured radiation patterns of the prototype in the xz and yz planes at 1.5, 1.74, and 2 GHz, respectively. The measured results agree well with the simulation results. It is observed that radiation patterns of the antenna remain stable throughout the frequency band.  
Figure 8: Simulated and measured realized gain.  
Figure 7: Simulated and measured results. (**a**) Reflection coefficient. (**b**) AR.  
Figure 9: _Cont._  
The performance comparisons of the proposed antenna and other CP antennas are conducted, as shown in Table 1. Although the design in [16] owns a wider bandwidth, but the proposed antenna has a higher gain and owns the filtering properties. Compared with the CP antenna in [17; 18; 19; 20], the proposed antenna has a wider bandwidth. Compared with the above design, the proposed work has advantage of wide bandwidth, high gain, and filtering property.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Ref.**} & **Center Frequency** & **AR** & **Impedance** & **Element** & **Gain (dBi)** & **Filtering** & **Size (\(\mathbf{\lambda_{6}}\)\({}^{\mathbf{3}}\))** \\ \hline
[13] & 2.44 & 4.1\% & 4.5\% & DRA & 5.1 & Yes & 0.8 \(\times\) 0.8 \(\times\) 0.15 \\ \hline
[16] & 2.97 & 49.5\% & 46.9\% & DRA & 4.73 & No & 0.72 \(\times\) 0.72 \(\times\) 0.18 \\ \hline
[17] & 1.835 & 35\% & 27.8\% & DRA & 6.5 & Yes & 0.68 \(\times\) 0.68 \(\times\) 0.138 \\ \hline
[18] & 2.435 & 3.8\% & 3.8\% & patch & 6.1 & Yes & 0.72 \(\times\) 0.72 \(\times\) 0.019 \\ \hline
[19] & 5.03 & 3.9\% & 7.2\% & patch & 8 & Yes & 1.01 \(\times\) 1.01 \(\times\) 0.036 \\ \hline
[20] & 2.44 & 4.5\% & 5.3\% & patch & 8.3 & Yes & 0.98 \(\times\) 0.98 \(\times\) 0.02 \\ \hline This work & 1.695 & 39.4\% & 38.8\% & DRA & 7.8 & Yes & 0.85 \(\times\) 0.85 \(\times\) 0.22 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Comparison between the proposed antenna and previously published works.  
Figure 9: Simulated and measured normalized radiation patterns. (**a**) 1.5 GHz. (**b**) 1.74 GHz. (**c**) 2 GHz.