Wideband Dual-Circularly Polarized Antennas Using Aperture-Coupled Stacked Patches and Single-Section Hybrid Coupler

This paper presents a dual-circularly polarized (dual-CP) antenna with simple configuration, low-profile, and wide bandwidth. Its primary radiating element is an aperture-coupled patch loaded with a stacked patch for a wide operational bandwidth. A single-section hybrid coupler is utilized as a feeding structure of the antenna to generate dual-CP radiation. Different from the priors normally requiring both wideband radiator and wideband feeding structure, this work demonstrates that the wideband dual-CP radiation can be obtained by combining a wideband radiator and a conventional hybrid coupler. A design operating at the center frequency of 5.5 GHz has been fabricated and tested. The measurements result in a −10-dB reflection coefficient bandwidth of 30.0% (4.66 – 6.31 GHz), a 10-dB isolation bandwidth of 30.9% (4.66 – 6.35 GHz), a 3-dB axial ratio (AR) bandwidth of 29.1% (4.70 – 6.30 GHz), and a realized broadside gain of 5.5 – 7.05 dBic. For a more robust dual-CP radiation, a low-sidelobe 4 <inline-formula> <tex-math notation="LaTeX">$\times $ </tex-math></inline-formula> 4 element array of the proposed antenna has been designed, fabricated, and measured. Compared to the single element design, the array prototype yields similar impedance and 3-dB AR bandwidths, but a higher isolation. The measurements on the array prototype result in an isolation of <inline-formula> <tex-math notation="LaTeX">$\geq 14$ </tex-math></inline-formula> dB, a peak gain of 16.3 dBic, a side-lobe level of <inline-formula> <tex-math notation="LaTeX">$\leq -20$ </tex-math></inline-formula> dB, and a cross-polarization of <inline-formula> <tex-math notation="LaTeX">$\leq -16$ </tex-math></inline-formula> dB across the frequency range of 4.7 – 6.3 GHz (29.1%).


I. INTRODUCTION
In modern wireless systems, circularly polarized (CP) wave is used as a powerful technique to mitigate multipath interference, polarization mismatch, and the Faraday effect. As a result, CP antennas [1] have been developed widely for several applications in radio frequency identification [2], global positioning system [3], wireless local area network (WLAN) [4], satellite communications [5], [6], synthetic aperture radar [7], and so on. Moreover, dual-CP antennas, which can generate both right-and left-hand CP (RHCP and LHCP) radiations, are expected for not only polarization diversity but also frequency reuse [8].
The associate editor coordinating the review of this manuscript and approving it for publication was Qi Luo .
Microstrip patch antennas [9] are a preferred choice to construct dual-CP systems because of their features, such as low-profile, simple fabrication, and low cost. In order to generate the dual-CP radiation, the conventional patch antennas [10]- [12] are commonly fed by the single-section hybrid couplers, which provide either RHCP or LHCP depending on the port excitation. Alternatively, both RHCP and LHCP can be generated when a single microstripline [13] traversed a cross-slotted aperture in a serial manner for coupling with the microstrip patch. Furthermore, a dual-CP patch antenna [14] can be realized by exploiting even and odd modes in a coplanar waveguide transmission line. In addition [15], a dual-CP radiation was achieved at WLAN frequency band (2.4 -2.485 GHz) when a truncated-corner square patch was fed by a square ring slot with two orthogonal microstrip-lines. Since these dual- VOLUME 10, 2022 This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/ CP antennas utilized a conventional patch as the primary radiating element, they suffer a common drawback of narrow operational bandwidth. Here, the operational bandwidth is defined by the following specifications, namely reflection coefficients ≤−10-dB, port-to-port isolation ≥10-dB, and axial ratio (AR) ≤3-dB.
In the last decade, several approaches have been devoted to broaden the operational bandwidth of dual-CP patch antennas. By using stacked patches and a two-section hybrid coupler, a dual-CP antenna array [16] enlarges the operational bandwidth up to 14.7% for satellite communications in the X band. An air-substrate patch antenna [17] is fed by a four cross slots via a microstrip-line with multiple matching segments to achieve a dual-CP bandwidth of 16%. In [18], a square patch antenna is surrounded by 12 parasitic metal plates to obtain an operational bandwidth of 16.5%. Another way to improve the operational bandwidth of a dual-CP patch antenna is to combine a broadband radiator and a broadband feeding network to generate orthogonal fields. As an example, a patch array [19] uses a sequentially rotated feeding network to provide a RHCP bandwidth of 12.5% (4.95 -5.61 GHz) and LHCP bandwidth of 14.7% (5.05 -5.85 GHz). Although these approaches have improved the dual-CP radiation bandwidth considerably compared to what the conventional designs provide, their bandwidths are still limited within 17%.
Besides the preferable microstrip patch type, many other types of antenna have been developed for the dual-CP systems. A cavity-backed ring-slot antenna [20] is loaded with an artificial magnetic conducting reflector to produce dual-CP radiation, and to have a compact size, and low profile. In [21], a traveling wave series-fed patch array is fed by a microstrip-line through proximity coupling for a dual-CP radiation at 10 GHz. In particular, the dual-port feeding helps to control the direction of the traveling wave to generate either RHCP or LHCP. In another effort, a dual-CP antenna with an electrically small size of ka = 0.94 [22] is realized by utilizing multilayer stacked near-field resonant parasitic elements. In addition, a highgain dual-CP radiation can be realized by constructing a Fabry-Perot antenna [23], [34], [35], which consists of a dual linearly polarized (dual-LP) patch antenna and a polarization conversion metasurface. Since the aforementioned dual-CP antennas targeted a compact size [20], [22] or employed conventional patch as the primary radiating elements [21], [23], [34], their operational bandwidths are less than 5% only.
Recently, several wideband dual-CP antennas have been presented. In one direction, the wideband and low-profile features of metasurface are exploited to construct wideband dual-CP antennas in [24], [25]. These designs, however, possess a drawback of metasurface antenna, which is a large footprint. In another attempt [26], four printed dipoles with two balun-included feeding networks are incorporated with a three-section quadrature hybrid coupler to achieve a dual-CP radiation with an operational bandwidth of >66%. In addition, crossed dipoles loaded with a circular patch [27] are excited in quadrature for wideband dual-CP at the L-band satellite applications. In [28], a magnetoelectric (ME) dipole antenna with four V-shaped patches and four V-section columns is fed by two -shaped probes for a 41.9% dual-CP bandwidth. For the efficient broadside radiation, the antennas in [26]- [28] require a distance of 0.25λ c between the radiator and reflector, which causes a bulky configuration.
In this paper, we propose a low-profile dual-CP antenna with wideband operation and its low-sidelobe 4 × 4 element array. Differing from the mentioned techniques, our proposed antenna employs a single-section hybrid coupler as the feeding structure to achieve dual-CP radiation. Meanwhile, the aperture-coupled stacked patches are used to broaden the operational bandwidth. The proposed antennas are arrayed in a 4 × 4 layout to strengthen the robustness of dual-CP radiation, i.e., increasing broadside gain, reducing sidelobe level (SLL), and enhancing port-to-port isolation. The antenna features are computationally demonstrated by using ANSYS Electronics Desktop and confirmed experimentally.  The GND is sandwiched between the Subs. 1 and 2 with no air-gap, whereas the Sub. 3 is stacked above the Sub. 2 at a height h a . Patch 1 is coupled with the hybrid coupler via the two orthogonal I-shaped slots embedded in the GND. The proposed antenna is characterized by using the ANSYS Electronic Desktop for a wideband dual-CP radiation at the center frequency of about 5.5 GHz. The design parameters are given in Table 1.

B. HYBRID COUPLER
In order to generate the dual-CP radiation, the proposed antenna utilizes a conventional quadrature hybrid coupler [29] as the feeding network. Based on the basic configuration, the branch-line coupler is compensated for the center frequency of 5.5 GHz and the RO4003C substrate ( r = 3.38, tanδ = 0.0027, h 1 = 0.508 mm), and slightly modified for a more compact deployed area. The feeding network is numerically characterized by the ANSYS Electronic Desktop, and its performances are illustrated in Figs. 2(b) and 2(c). It is observed that the coupler has a good performance in the frequency range from 5 GHz to 6 GHz, i.e., the reflection coefficients (e.g., S 11 ) are <−10 dB; the isolation (S 41 ) is >10 dB with a peak of 35 dB at 5.3 GHz; S 21 and S 31 have a nearly equal magnitude (3.2 ± 0.3 dB) and a 90 • ± 0.5 • phase difference. The performance of the coupler degrades when the examined frequency range increases. S 21 and S 31 have a 90 • ± 5 • phase difference at 4.5 -6.5 GHz.

C. DESIGN EVOLUTION
The proposed antenna is a combination of a conventional hybrid-coupler and aperture-coupled stacked patches to generate a wideband dual-CP radiation. For a better illustration of the antenna operation, Fig. 3 shows the design evolution and corresponding performance for each step. The initial design is a conventional aperture-coupled patch antenna (Ant. 1). To broaden the bandwidth, Ant. 1 is loaded with a stacked patch to create Ant. 2. The aperture position of Ant. 2 is shifted forward one patch corner (Ant. 3) to improve the impedance matching. The proposed design is formed by adding an aperture and the hybrid coupler to Ant. 3. The design parameters of all configurations are same as the final antenna.
Figs. 3(b) and 3(c) show the S 11 values and broadside realized gains of the four configurations. Since Ant. 1 is the conventional aperture-coupled patch antenna and not fully optimized, it yields a resonance at 5.54 GHz with S 11 = −6 dB only and a gain of 0.44 dBi. By loading a stacked patch, Ant. 2 yields two resonances at 5.14 GHz and 5.95 GHz and a realized gain of >6 dBi in the frequency range of 5.04 -6.19 GHz. In the case of Ant. 2, the aperture is located at the center, the patch 1 acts as a feeding patch, and therefore, its resonance is not appeared. By tuning the aperture position, we can excite three resonances generated by the two patches and aperture slot, and consequently, achieving a wideband operation [30]. This is illustrated by the performance of Ant. 3, which yields a −10 dB bandwidth of 27.8% (4.77 -6.31 GHz) with three resonances at 5.0 GHz, 5.5 GHz, and 6.18 GHz. Moreover, Ant. 3 achieves a broadside realized gain of 6.3 -7.55 dB within its bandwidth. Due to presence of the hybrid coupler as feeding structure, the proposed antenna achieves a broader impedance bandwidth and a dual CP radiation. As shown in Figs. 3(b) and 3(c), the proposed design achieves an impedance bandwidth of 34.3% (4.64 -6.56 GHz) and a broadside realized gain of 5.7 -7.3 dBic. The dual-CP performance of the proposed antenna will be presented later in the next subsection.

D. FABRICATION AND MEASUREMENT
For verification, the proposed wideband dual-CP antenna was fabricated and measured. Fig. 4(a) shows an antenna prototype with overall size of 36 mm × 50 mm × 4.7 mm (0.56λ min × 0.77λ min × 0.07λ min , where λ min is the free space wavelength at the lowest operating frequency). All components, including feeding structure and stacked patches, were realized by using the printed circuit board (PCB) technology. The three PCB components were combined together by using plastic posts and screws. Two 3.5-mm sub-miniature version A (SMA) connectors are used as the microstripline-to-coaxial adapters. Note that the size of the fabricated prototype was chosen to be compatible with the available SMA connectors, plastic posts and screws. The simulation results indicate that the antenna size can be reduced to 30 mm × 30 mm × 4.7 mm (0.46λ min ×0.46λ min × 0.07λ min ) with a negligible degradation in performance.
The S-parameters of this antenna prototype were measured by using a vector network analyzer and compared to the simulation results in Fig. 4(b). The measurement results show a −10 dB reflection coefficient bandwidth of 30.0% (4.66 -6.31 GHz) for both ports, while the simulated impedance bandwidth is 33.9% (4.65 -6.55 GHz). The measured bandwidth for isolation >10-dB was 30.9% (4.66 -6.35 GHz), which is comparable with the respective simulation result of 27.2% (4.7 -6.18 GHz). A slight difference between the measurement and simulation results could be caused by the undesired air gaps between the stacked substrates.
The radiation characteristics of the antenna prototype was measured in a tapered anechoic chamber (provided by Microwave Vision Group at ACE Antenna Co., Ltd, Ha Nam Province, Vietnam). Fig. 5 shows the AR and broadside gain values. Due to the structure's symmetry, both the excitations at Port 1, 2 generate nearly identical values of ARs and gains. As shown in Fig. 5(a), the measurement result shows a CP bandwidth of 29.1% from 4.7 to 6.3 GHz (AR ≤3 dB for both ports excited), which is relatively close to the simulated CP bandwidth result of 30% (4.71 -6.37 GHz). As shown in Fig. 5(b), within the CP bandwidth, the measured gain is 5.5 -7.05 dB, while the simulated gain is in the range of 5.7 -7.3 dB. The measured gains are slightly smaller than the simulated values, which could be attributed losses caused by the SMA connectors, plastic posts, and screws. Fig. 6 shows the 5.5 GHz radiation patterns of the proposed antenna. Simulation and measurement results agree that the proposed antenna has achieved a good dual-CP radiation, with Port 1 for RHCP and Port 2 for LHCP. Moreover, the antenna prototype has a highly symmetrical radiation pattern with half-power beamwidth (HPBW) of 65 • ± 5 • , crosspolarization level of <−15 dB at the broadside-direction, and a front-to-back (F-B) ratio of >15 dB. Because the radiation pattern of the prototype is stable across the operational bandwidth, only the radiation pattern at 5.5 GHz is shown for brevity.   Table 2 shows a comparison between the performance of the proposed antenna and those of the existing wideband dual-CP designs. It is observed that the proposed antenna has a simpler configuration and a considerably wider operational bandwidth as compared to most of the dual-CP antennas with low-profile, e.g., [17], [18], [24], [25]. Differing from the prior designs [26]- [28] that require both a wideband radiator and a wideband feeding structure, the proposed antenna has achieved a wideband dual-CP radiation by a simple combination of a wideband radiator and a conventional hybrid coupler. Although a wider bandwidth and higher gain can be obtained in [26]- [28], but these antennas suffer a more complicated feeding structure, and thus they have a larger size, and higher profile as compared to our proposed design.

III. DUAL-CIRCULARLY POLARIZED ARRAY
For a more robust dual-CP radiation in terms of highgain, broad bandwidth, as well as high isolation, the traditional method is to make an array with multiple dual-CP elements [16], [24], [31]- [33]. In our case, the proposed antennas are arrayed in a 4 × 4 layout, as shown in Fig. 7, in which the center-to-center spacing are D x = 36 mm (0.6λ 0 at 5-GHz) and D y = 35.5 mm (0.59λ 0 at 5-GHz) in the x and y directions, respectively. A feeding network is designed to provide excitation with in-phase and tapered power distribution to achieve a low sidelobe level (SLL). The power distribution ratios are approximately 1 -4.5 -4.5 -1 for exciting the elements along both x and y directions. The feeding network employed quarter-wavelength transformers for impedance matching. The center frequency of 5.5 GHz was chosen for the impedance transformers. The array was first characterized by using the ANSYS Electronics Desktop, and then realized and tested. The array prototype has an overall size of 160 mm × 160 mm × 4.7 mm (2.47λ min × 2.47λ min × 0.07λ min ). The feeding network and two patch VOLUME 10, 2022  arrays were realized by using the PCB technology. Being similar to the single-element prototype, the three PCBs of the array were combined by using plastic posts and screws. The simulation and measurement results of the array prototype are compared in Figs. 8−11. Fig. 8 shows the measured and simulated S-parameters of the array prototype. The impedance-matching bandwidth of the array is similar to that of the single element, which is 32.3% (4.56 -6.32 GHz) for reflection coefficients (S 11 and S 22 ) <−10 dB. The port-to-port isolation of the array is greater than that of the single element. The measured isolation was ≥14 dB from 4.7 to 6.3 GHz (29.1%), whereas the simulated bandwidth for isolation ≥14 dB was 26.6% (4.66 -6.09 GHz). As mentioned in the case of a single element, a slight discrepancy between the simulated and measured S-parameters of the array could be attributed to some undesired air-gaps between the Sub. 1 and Sub. 2. Fig. 9 shows the ARs and broadside realized gains of the array prototype. The measurement results agreed rather closely with the ANSYS Electronics Desktop simulations. From Fig. 9(a), the array prototype has achieved a measured 3-dB AR bandwidth of 32.73% (4.6 -6.4 GHz) for both RHCP and LHCP ports, whereas the simulated value was 32.7% (4.58 -6.37 GHz). As shown in Fig. 9(b), the array prototype has achieved a measured gain of 12.0 -16.3 dBic, Figs. 10 and 11 show the radiation patterns of the array prototype at 4.8, 5.4, and 6.0 GHz, when the RHCP and LHCP ports are excited, respectively. Again, there is a good agreement between the measurement and simulation results. Across the operational bandwidth, the array prototype has achieved an excellent dual-CP radiation with HPBWs of 24 • ± 2 • in both x − z and y − z planes, a cross-polarization level of ≤−16 dB, SLLs <−20 dB, and a F-B ratio of ≥25 dB.
The simulated radiation efficiencies (REs) of the array prototype are from 0.6 to 0.7 in the frequency range of 4.7 -6.0 GHz. These RE values are due to the fact that the patches are built on the low-cost FR4 substrates and the losses are also caused by the feeding network. Higher efficiency can be obtained by using a higher-quality substrate, but the feed losses are still a common shortcoming of the feeding networks based on microstrip-lines [16], [17], [19], [21], [33]. The RE measurements have not been conducted due to the function limitation of the anechoic chamber. Nevertheless, the good agreement between measurement and simulation gains promises a relatively high value of practical RE.
A performance comparison between the proposed design and the previous high-gain dual-CP antennas is given in Table 3. Since the gain is proportional to the number of elements, the larger-size arrays yields higher-gain values.  As compared to the antenna arrays that are also realized by the PCB technology [24], [31], [32], the proposed antenna has a comparable low-profile, but has a significantly broader bandwidth. For a 50% bandwidth, the stacked-patch array in [16] employed sequential rotation technique with a multilayer power distribution network, which increases the profile and complexity significantly. A ME dipole array fed by a 2-section hybrid coupler [33] yields a broader bandwidth than our proposed design does, but it has larger-profile and a more-complicated configuration. The F-P antennas [23], [34], [35] can achieve a high-gain dual-CP radiation with a simple configuration, but they suffer a large-profile and narrow bandwidth. Finally, due to the compact radiator and a fully-optimized feeding network, the proposed antenna has achieved the lowest SLL in comparison with the priors.

IV. CONCLUSION
A low-profile wideband dual-CP antenna and its 4 × 4 element array have been presented. The proposed antenna use a aperture-coupled, stacked patches and a single-section hybrid coupler to achieve the wideband dual-CP radiation. The optimized design has an impedance bandwidth of 30.0% (4.66 -6.31 GHz), 10-dB isolation bandwidth of 30.9% (4.66 -6.35 GHz), 3-dB AR bandwidth of 29.1% (4.7 -6.3 GHz), and a broadside realized gain of 5.5 -7.05 dBic. The array prototype has achieved a gain of 16.3 dBic, a SLL of ≤−20 dB, a cross-polarization of ≤−16 dB, a F-B ratio of ≥25 dB, and a port-to-port isolation ≥14 dB from 4.7 to 6.3 GHz. With the advantages of low-profile, simple configuration, wideband, dual-CP radiation, and easy fabrication, the proposed antenna design is a good candidate for applications in many wireless communication systems, such as WLAN, satellite communications, synthetic aperture radar, etc.  Dr. Dao-Ngoc has been a Reviewer for several journals/transactions of Optical Society of America (OSA), IEEE, Elsevier, and American Geophysical Union (AGU) as well as numerous of technical and science conferences. VOLUME 10, 2022