Circularly polarized (CP) antennas offer advantages over conventional linearly polarized antennas as they are more resilient against multipath fading and mitigate polarization-related issues. Therefore, CP antennas are widely utilized in modern wireless communication systems [1, 2]. However, the drawbacks of traditional CP antennas are their narrow axial ratio (AR) bandwidth and low gain. This has spurred a need to design a wideband high-gain CP antenna.

Single-layer CP patch antennas have a narrow AR bandwidth because of their high Q factor [3]. To overcome this limitation, techniques involving stacked patches and parasitic structures have been employed to extend the AR bandwidth [4–7]. For example, the introduction of notched stacked patches has enabled a single capacitive-fed patch antenna to achieve an AR bandwidth of 10% [4]. In [5], two-layer stacked patches coupled with a driven patch featuring a windmill slot configuration achieved wideband CP radiation. In addition, CP cross-dipole antennas have gained attention for their wide AR bandwidth and high gain characteristics [8–10]. Notably, a CP cross-dipole antenna designed for the Navigation Satellite System achieved an AR bandwidth of 41.3% [8]. In [9], an antenna composed of a cross-dipole and dual-cavity structure achieved unidirectional radiation and wide CP radiation. In particular, an excellent AR bandwidth of 90.9% was realized in [10]. However, cross-dipole antennas typically have a relatively high profile. Another type of single-fed patch CP antenna exhibits a wide 3-dB AR beamwidth (ARBW) and thus has a wide range of applications [11–14]. The common methods to realize CP radiation and a wide 3-dB ARBW are modifications to rectangular patches, including the addition of four unequal circular patches [11], four bending stubs [12], or rectangular patches with four angled narrow slots [13]. In [14], a 3-dB ARWB of 190° was realized by four three-dimensional radiating patch elements connected by a four-port feeding network with a 90° phase difference. These wide ARBW CP antennas can be utilized for a variety of applications, such as global positioning systems; however, traditional wide ARBW CP patch antennas still often suffer from a narrow 3-dB ARBW.

Multi-feed is another conventional method for widening AR bandwidths. Two feeds with a 90° phase shift [15], three feeds with a 120° phase shift [16], and four feeds with a 90° phase shift [17] have been used to excite the same radiation patch to realize wideband CP radiation. In [18, 19], a wideband four-fed CP antenna array composed of four sequential rotated radiation patches and a four-way feeding network with a phase difference of 90° was designed. Multi-feed CP antennas can yield a broad CP bandwidth and high gain, but they have drawbacks such as a large structure size and complex feeding networks.

To further widen the AR bandwidth, metasurface antennas have been introduced and designed [20–26]. In general, CP metasurface antennas can be excited by two methods: aperture-coupled feed [20–23] and patch-coupled feed [24–26]. In [20], a novel metasurface design utilized unit cells with two pin-loads in the diagonal direction to realize wide CP radiation. The feed system of the CP metasurface antenna in [23] was composed of a cross-slot and a bending microstrip feeding line. In [26], a metasurface-based CP antenna was coupled by a square patch with a tilted slot. Notably, the backward radiation of metasurface-based antennas with aperture-coupled feed is stronger, while the AR bandwidth of patch-coupled metasurface-based antennas is narrow.

Recently, some differentially fed antennas have been proposed in [27, 28]. These antennas, in contrast to conventional single-ended antennas, do not necessitate the use of redundant baluns or 180° hybrids. As a result, they are widely utilized in differential microwave circuits. In [29], the presented differentially fed CP antenna, which was designed to achieve a wide AR bandwidth, consisted of two cross-dipoles and phase shifters. In [30], a series-fed CP differential antenna, which utilized wideband phase shifters and crossed open slot pairs, realized a wide 3-dB ARBW.

In this paper, a wideband differentially fed CP slotted patch antenna is presented, and the linearly polarized differentially fed dipole antenna is bent for achieving CP radiation. Two centrosymmetric coupling patches and slotted patches are added in the simulation to the surface of the bending dipole antennas to further extend the AR bandwidth and impedance bandwidth. When the slotted patch is used, the final differentially fed CP antenna achieves high gain and wideband CP radiation. The measured result exhibits broadband performance with a −10-dB impedance bandwidth of 78.8% (3.81–8.76 GHz), a 3-dB ARBW of 36.7% (4.78–6.93 GHz), and a maximum gain of 10.2 dBic.

The prototype, as depicted in Fig. 7, was fabricated to validate the simulation results of the presented CP antenna. The Agilent N5062A network analyzer was utilized to measure the S-parameters. The anechoic chamber was used to test radiation characteristics, including gains and ARs. A power divider with a 180° phase difference was used in the tests. Fig. 8(a) shows that the simulated −10-dB impedance bandwidth is 56.9% (4.10–7.36 GHz) and the measured −10-dB impedance bandwidth is 78.8% (3.81–8.76 GHz). The difference between the measured and simulated reflection coefficient can be attributed to the idealized differential feed used in the simulation, where the amplitude and phase differences of each port are assumed to be equal across all frequency ranges, while, in reality, the power divider exhibits variations. Fig. 8(b) shows a simulated 3-dB ARBW of 32.3% (4.93–6.83 GHz) compared to a measured ARBW of 36.7% (4.78–6.93 GHz).

The differentially fed antenna demonstrates the characteristics of a large ARBW, as verified by simulation and experiment. As shown in Fig. 9(a), the ARBWs of 132.3° in the xoz plane and 116.2° in the yoz plane are realized at 5.5 GHz. The simulated and measured normalized radiation patterns at 5.5 GHz are shown in Fig. 9(b). Fig. 10 demonstrates the normalized radiation patterns at significant frequencies of 5.2, 6, and 6.8 GHz. The normalized right-hand CP is clearly greater than the normalized left-hand CP in the z-axis direction. The measured radiation patterns closely resemble those obtained from simulation, indicating a high degree of similarity.

Table 1 provides a comparison of various parameters, including thickness, bandwidth, and peak gain, for different antennas. The crossed-dipole antennas in [8–10] showed a wide −10-dB impedance bandwidth and 3-dB ARBW, but they had a high profile. The CP antennas in [11–14] exhibited a wide 3-dB ARBW but suffered from a limited 3-dB ARBW. The antennas in [20, 23, 26] were low-profile metasurface-based designs. In [20, 23], incomplete ground planes were used to achieve 3-dB ARBWs of 14.9% and 20.9%, respectively. A 3-dB ARBW of 17.5% was achieved with the antenna in [26] fed by a slotted patch. The differentially fed CP antennas in [29, 30] with a high profile showed a wide 3-dB ARBW, but the peak gain was less than 8.5 dBic.

This paper presents a novel differentially fed antenna design with a wide 3-dB ARBW and a wide ARBW. We compared the resonant frequencies and mode currents of a conventional dipole antenna and a bending dipole antenna to identify the resonant modes responsible for CP radiation and determine the source of the phase difference in the orthogonal mode current. Our findings reveal that placing the bending dipole excitation structure in the same plane as the unit cell radiation structure enables the generation of a broad 3-dB ARBW. Experimental results demonstrate that our proposed differentially fed CP antenna exhibits an impressive ARBW of 132.3° in the xoz plane and 116.2° in the yoz plane at 5.5 GHz.

Furthermore, our results show a measured (10 dB) impedance bandwidth of 78.8% (3.81–8.76 GHz) and a measured (3-dB) ARBW of 36.7% (4.78–6.93 GHz). It is worth noting that the overlapping bandwidth indicates excellent AR performance for this antenna design.