# A Wideband Circularly Polarized Filtering Dipole Antenna
## 1 Introduction  
With the development of wireless communication technologies, various wireless communication systems are demanding increasingly higher data and capacity rates. Filtering antennas can reduce the insertion loss caused by filters and connecting lines in communication systems. CP antennas possess unique advantages such as reducing polarization mismatch, suppressing multipath interference, and mitigating Faraday rotation effects. CP antennas have been widely applied in communication system transceivers, including Global Positioning Systems (GPS), satellite communications, and wireless local area networks (WLAN).  
In recent years, an increasing number of antenna researchers have devoted their efforts to studying high-performance CP filtering antennas [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]. Common methods for achieving CP filtering include using filtering feed networks to excite CP antennas [1, 2, 3, 4] or employing filtering phase shifters as feed networks to drive linearly polarized antennas [5, 6, 7, 8, 9]. For example, some designs have integrated both phase shifters and filters in the feed network [5], utilized phased feed networks to excite dual-element linearly polarized filtering antenna arrays [6], and implemented filtering phase-shifting feed networksloaded with radiating patches to achieve CP filtering functionality [7; 8; 9]. However, these approaches based on the feed network introduce additional losses and increase the size of the antenna. Alternatively, stacked structures can be used to introduce radiation nulls and broaden the operating bandwidth. Reference [10] employed stacked patch structures with U-shaped slots to realize CP filtering effects, while Refs. [11; 12] both used stacked patches with etched slots to achieve high-performance CP filtering antennas. Although the stacked patch approach can achieve good broadband CP filtering performance, it increases the antenna's profile height and overall size, thereby limiting its application scenarios. Specifically, the antenna in [13] utilized a stacked patch structure with a crossed slot to implement CP filtering functionality, and [14] achieved CP filtering by loading a phase feed network with stacked patch structures. Other approaches include: loading filtering structures onto CP antennas [15; 16], realizing CP filtering effects using metasurfaces [17], lens antennas integrated with CP radiators [18], separately exciting two orthogonal resonators to achieve CP filtering [19], and [20] implementation of a good bandpass CP antenna using crossed dipoles loaded with two square-ring patches. However, the AR bandwidths achieved by these CP filtering antennas remain relatively narrow.  
This paper presents a wideband CP filtering antenna design based on a conventional single-fed CP dipole radiator with direct coaxial feeding. To achieve enhanced frequency selectivity, four parasitic patches are introduced--extending vertically from the ends of the four crossed dipole arms and aligned parallel to the arm terminals--thereby generating two radiation nulls in the upper band. Additionally, a square-ring slot etched into the ground plane creates a third radiation null in the lower band. Ultimately, the proposed antenna achieves a 46.9% axial ratio bandwidth. Without requiring any additional feeding network, it delivers a stable in-band gain of 7 dB with three radiation nulls. The design exhibits stopband suppression levels achieves 20 dB for both lower and upper stopbands, demonstrating good bandpass filtering performance.