# A Wideband Circularly Polarized Filtering Dipole Antenna
## 3 Antenna Implementation  
The proposed CP filtering dipole antenna was designed, fabricated, and measured, as shown in Figure 9a. The optimization was performed using a high-frequency structural simulator (HFSS), and the measurement was accomplished by an Agilent (Santa Clara, CA, USA) N5227A network analyzer and anechoic chamber. The results, including the S-parameters, realized gain, and axial ratio (AR), are shown in Figure 9b,c. The simulated and measured results are in good agreement, except for slight frequency shifts and reduced measured gain due to fabrication and testing tolerances. Figure 8: Frequency control of the radiation nulls at (**a**) \(1.1\) GHz, (**b**) \(4.20\) GHz, (**c**) \(4.20\) GHz, and (**d**) \(5.0\) GHz. Figure 7: Simulated radiation nulls radiation patterns at (**a**) \(1.11\) GHz, (**b**) \(4.2\) GHz, (**c**) \(5.0\) GHz. Both simulated and measured \(S_{11}\) (below \(-10\) dB) cover frequency ranges of 1.65-3.66 GHz and 1.825-3.925 GHz, respectively. The antenna simulation achieves a stable LHCP gain of approximately 8 dBic, while the realized gain measures about 7 dBic. As expected, three radiation nulls are observed at approximately 1.42 GHz, 4.5 GHz, and 5 GHz, ensuring high out-of-band radiation suppression. The simulated and measured bandwidths are approximately 47% (2.25-3.63 GHz) and 46.2% (2.33-3.73 GHz), respectively. As shown in Figure 10, the antenna exhibits radiation efficiencies above 80% across the entire operating band, demonstrating good radiative performance. Figure 11 presents  
Figure 9: (**a**) The proposed antenna prototype, (**b**) measured reflection coefficients and measured realized gains, and (**c**) measured AR. To illustrate the advantages of the antenna designed in this study, a comparison with recent antennas is conducted, and the results are summarized in Table 1.  
In the literature [5; 6; 9; 10; 11; 12; 13; 14; 19], filtering CP antenna functions were implemented based on circuit structures such as filtering phase shifters [5; 6; 9]. While these approaches effectively achieve frequency-selective characteristics, the introduction of external filtering networks leads to insertion losses, along with limitations in axial ratio (AR) bandwidth and reduced radiation gain. In references [10; 11; 12; 13; 14], stacked patches were employed to broaden the antenna bandwidth, enhance gain, and improve out-of-band suppression levels, thereby introducing filtering radiation responses. Although the stacked-patch configuration \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Ref.**} & \multicolumn{2}{c}{**Size**} & \multirow{2}{*}{**Filtering Method**} & \multicolumn{2}{c}{**Imp/AR.**} & \multicolumn{2}{c}{**Ave Gain**} & \multirow{2}{*}{**Sup.**} & \multirow{2}{*}{**GSI**} \\  & (\(\lambda_{0}\times\lambda_{0}\times\lambda_{0}\)) & & & BW (\%) & & (dBic) & (dB) \\ \hline
[5] & \(1.11\times 1\times 0.039\) & Filtering phase shifter & 7.10/7.09 & 5.4 & 28.7/26.9 & 0.892 \\ \hline
[6] & \(1.55\times 0.9\times 0.058\) & Filtering phase shifter & 11.40/14.29 & 7.8 & 24.1/25.9 & 0.694 \\ \hline
[9] & \(0.97\times 0.97\times 0.07\) & Filtering phase shifter & 21.6/23.1 & 6.5 & 23/14 & 0.626 \\ \hline
[10] & \(0.52\times 0.52\times 0.06\) & Stacked patch + Slot & 15.2/8.2 & 7 & 26.8/28.2 & 0.73 \\ \hline
[11] & \(0.51\times 0.51\times 0.26\) & Stacked patch & 59.00/51.8 & 7 & 18/15.8 & 0.934 \\ \hline
[12] & \(0.71\times 0.71\times 0.31\) & Stacked patch + Slot & 58.3/46.9 & 7.6 & 13.6/10.1 & 0.855 \\ \hline
[13] & \(0.59\times 0.47\times 0.1\) & Stacked patch & 17.72/9.32 & 6.3 & 19/20 & 0.645 \\ \hline
[14] & \(1.53\times 1.53\times 0.19\) & Stacked patch + Slot & 15/16.4 & 13 & 14/16 & 0.821 \\ \hline
[19] & \(0.7\times 0.7\times 0.024\) & Stacked resonator & 4.7/3.2 & 5.5 & 31/31 & 0.804 \\ \hline
[20] & \(0.8\times 0.8\times 0.17\) & Ring structure + Slot & 25.8/13.20 & 7.2 & 26.2/36 & 0.744 \\ \hline
**Pro.** & \(0.49\times 0.49\times 0.16\) & Patch + Slot & 75/46.9 & 7 & 20/20 & 0.888 \\ \hline \hline \end{tabular}
\end{table}