# A Wideband Circularly Polarized Filtering Dipole Antenna
## 2 Antenna Design
### Analysis of Radiation Nulls  
To elucidate the working mechanism of the three radiation nulls, Figure 6 illustrates the surface current distributions at these frequencies. As shown in Figure 6, at 1.1 GHz, the etched square-ring slot demonstrates carefully balanced antiparallel current distributions along its opposing edges. These equal-amplitude, yet phase-opposed currents establish a destructive interference condition that efficiently suppresses far-field radiation and a characteristic behavior of slot-line resonators where symmetric current modes mutually cancel their radiative components. The radiation null at 4.2 GHz stems from the electromagnetic interaction between adjacent parasitic patches. When these patches are excited, they generate currents with reverse phases, leading to mutual energy cancellation that effectively eliminates radiation. The higher band radiation null at 5 GHz demonstrates an interplay between the square-ring structure formed by the extended orthogonal lines adjacent to the crossed dipole arms. The geometric arrangement ensures these current systems produce opposing electromagnetic fields that interfere destructively in the far field. The radiation patterns at these radiation null frequencies are presented in Figure 7. As depicted, both right-hand circular polarization (RHCP) and left-hand circular polarization (LHCP) components remain minimal at these frequencies, indicating satisfactory  
Figure 4: Surface current distribution of the proposed antenna when t = 0 and t = T/4 at the CP radiation frequencies. (a) 2.5 GHz. (b) 3.4 GHz.  
Figure 5: Frequency control of the dual CP moods at (a) 2.5 GHz and (b) 3.4 GHz.  
radiation suppression performance. These results further confirm the non-radiative nature of these three radiation nulls. Since the lower band radiation null is introduced by the square-ring etched slot, its frequency can be adjusted by modifying the slot dimensions. In contrast, the upper band radiation null is primarily generated by the loop currents formed between the crossed dipole arms. Thus, its frequency can be tuned by altering the size of the square-loop structure enclosed by the dipole arms, as demonstrated in Figure 8. When \(w_{2}\) decreases from 13.5 mm to 12.5 mm, the radiation null at 4.2 GHz shifts from 4 GHz to 4.5 GHz. When \(l_{2}\) reduces from 28 mm to 24 mm, the upper band radiation null increases from approximately 4.90 GHz to 5.00 GHz, while the radiation null in the lower band remains nearly unchanged. When the length of the rectangular etched slot \(G_{2}\) decreases from 70 mm to 62 mm, the radiation null shifts from 1.1 GHz to 1.25 GHz, whereas the radiation null in the upper band remains almost unaffected. These characteristics are essential for filtering antenna design. Figure 8c illustrates the parametric effects of the gap between parasitic elements on the proposed antenna. It can be concluded that, when the gap (\(g_{3}\)) increases, the depth of the radiation null at the higher band decreases, and vice versa. This also verifies that the higher band radiation null is generated by the reverse currents between the parasitic elements.  
Figure 6: Current distribution of the proposed antenna at three radiation nulls. (**a**) 1.1 GHz. (**b**) 4.2 GHz. (**c**) 5.0 GHz.