Wideband Circularly Polarized Microstrip-Slot Antenna with Parasitic Ground Planes
In this study, a new stacked microstrip slot antenna (MSA) with wideband circular polarization (CP) characteristics is proposed. The antenna consists of a square-loop feed configuration, four parasitic square patches, four parasitic vertical planes, and a square ground plane etched with four parasitic square slots. The corner-cut square-loop can excite a stable 270° phase difference by loading an arc-shaped strip into the square-loop. Square-patches, vertical planes, and square slots as parasitic elements are placed together at the side of the square-loop to stimulate two CP resonant points. Simulation and measurement are performed on the designed antenna prototype to demonstrate the design’s rationality. The measured results depict that the measured impedance bandwidth (IBW) for |S11| < −10 dB is 36.4% (4.65 to 6.70 GHz) and the measured axial ratio bandwidth (ARBW) for AR < 3 dB is 25.1% (5.01 to 6.45 GHz). Compared with other reported stacked CP antennas, the proposed antenna has significant advantages in CP bandwidth, which could occupy the wireless local area network ITS (5.8 GHz), (5.725–5.85 GHz), and WIFI (5.85–5.925 GHz) bands.
Although linear polarization wideband antennas [1–7] are used in various wireless communication systems, circular polarization (CP) antennas have a stronger attraction in some communication systems due to the merits of suppressing multipath interference and reducing polarization misalignment [8–11]. In addition, with the development of high-rate transmission and processing systems, CP antennas with wide bandwidth, high gains, and low-profile characteristics are also in great demand [12–15].
Various CP antennas have been extensively designed to achieve wideband operation, such as microstrip patch antennas [13–15], dielectric resonator antennas , and cross-dipole antennas [17–19]. Currently, a pair of orthogonal vacant-quarter fabricated rings as sequentially-rotated feeding structures has been extensively used in the design of crossed-dipole CP antennas, which can provide a stable 90° phase difference to achieve CP operation. Based on the feeding structure, many different shapes of crossed-dipole antennas are presented to obtain broadband CP radiation [20–26]. These shapes of crossed-dipole involve liner , rectangular , stepped rectangular , metallic cuboids , L-shaped , elliptical , and asymmetric bowtie  cross-dipoles, which are designed in turn to obtain wide 15.6%, 27%, 55.1%, 86.4%, 67.5%, and 96.6% of 3-dB ARBW, respectively. However, the profile of the cross-dipole antenna is too high due to the presence of a quarter wavelength grounded reflector, which is not suitable for low-profile applications. CP microstrip patch antennas (MPAs) have a lower profile than cross-dipole antennas and have also been studied in recent years [27–34].
Recently, many sequential phase feeding structures, which could provide a stable sequential phase difference, have been often used in the design of CP MPAs. These sequential phase feeding structures could be summarized as: an aperture-coupled feeding structure , a coplanar-waveguide feeding structure , series-parallel feeding structures [13, 16], and square-loop feeding structures [15, 27–44]. Besides, the stacked technique [42–46] is also an effective method for enhancing the CP bandwidth on the basis of the sequential phase feeding structures. Circular patch , rectangular patch , L-shaped patch , and square patch  are stacked above the sequential phase feeding structure as parasitic elements, which could realize 6%, 28.1%, 16.7%, and 19.4% CP bandwidth, respectively.
In contrast to the conventional parasitic patches, square slots and vertical grounded patches as parasitic elements are introduced into the design of the CP MPA in this study. The proposed antenna is comprised of a square-loop feed configuration, four parasitic square patches, four parasitic vertical planes, and a square ground plane etched with four parasitic square slots. By using these parasitic elements, two CP resonant points could be excited at the same time. Simulation and measurement are performed on the designed antenna prototype to demonstrate the design’s rationality. The measured impedance bandwidth (IBW) for |S11| < −10 dB is 36.4% (4.65 to 6.70 GHz) and the measured axial ratio bandwidth (ARBW) for AR < 3 dB is 25.1% (5.01 to 6.45 GHz).
2. Antenna Design
2.1. Antenna Configuration
Figure 1 shows the entire geometry and details of the wideband stacked CP MPA. As seen, the proposed MPA consists of a coaxially fed square-loop structure, four rotated square patches (L1 × W1 × G1), four rotated square slots (L2 × W2), and four vertically grounded patches (L3 × W3). The proposed MPA is fabricated on the Rogers 5880 substrate (εr = 2.2, h = 1.524 mm, and tanδ = 0.001). Note that the square-loop feeding structure is composed of an arc-shaped strip (R0 × R1) with the angle of 270° and a length of λg/4 (λg is the waveguide length at 5.5 GHz) and a square-loop (D2 × D3) that is used to stimulate the fundamental one-wavelength mode . By using these parasitic patches and slots, two CP resonant points could be stimulated at the same time, achieving a wide CP bandwidth. Moreover, four parasitic vertical planes are used to improve the CP bandwidth of the proposed antenna. At last, the optimized dimensions of the proposed MPA are summarized in Table 1 after using ANSYS HFSS to tune and optimize.
2.2. Antenna Mechanism
To demonstrate the design mechanism, four different step-by-step prototypes are investigated in Figure 2. For a clear comparison, the relevant |S11| and AR results are depicted in Figures 3(a) and 3(b). First, a conventional square-loop feeding structure with four parasitic square patches (Ant. 1); second, the square-loop feeding structure with four parasitic square slots (Ant. 2); third, the square-loop feeding structure with these parasitic square patches and slots (Ant. 3); Finally, based on Ant. 3, four vertical planes are introduced into the proposed antenna. As seen in Figure 3, when four parasitic square patches and slots are etched in the side of the square-loop feeding structure, two AR resonant points are excited at 5.6 and 5.8 GHz, respectively. When combing the square patches with the square slots, the Ant. 3 has good impedance bandwidth and ARBW. However, the IBW and ARBW of the Ant. 3 are not wide enough for wide communication. To improve the IBW and ARBW of the Ant. 3, four vertical planes are introduced into the presented antenna. As seen, the IBW and ARBW of the presented antenna are significantly enhanced compared with Ant. 3. This is because these parasitic patches, slots, and vertical planes could stimulate multiple current flow patches, which excite two CP resonant points. At last, the presented antenna could realize a broad IBW of 4.75–6.75 GHz (34.8%) for |S11| < −10 dB and a broad ARBW of 5.04–6.48 GHz (25.0%) for AR < 3 dB.
To study the effects of the antenna sizes on the CP performance, multiple antennas’ parameters are scanned, compared, and analyzed in this part. These various parameters consisted of the side length (L1) of the square patches, the side length (L2) of the square slots, the height (h1) of the vertical planes, and the distance (D1) of the feeding point. Figure 4 gives the corresponding |S11| and AR results for these parameters. First, the distance (D1) of the feeding point has an obvious impact on the AR values because it determines the initial 90° phase difference. Second, the side lengths (L2) and height (h1) have significant influences on the |S11| and AR results in the whole frequency band. This is because these CP resonant points come from the joint action of the square slots and vertical planes rather than a single action. Moreover, compared with the variations in the other parameters, the length (L1) has a dramatic impact on the AR values at high frequencies. The reason is that the role of the square patches is to stimulate an AR resonant point at high frequencies, which is consistent with the results of the step-by-step prototypes in Figure 3. When setting D1 = 3.0, L1 = 12, L2 = 12, and h1 = 4 mm, the optimal CP performance could be obtained.
The vector current distributions of the CP MSA on upper and lower surfaces are simulated and plotted in Figure 5, to clarify the CP operating mechanism. As seen, at two AR resonant points (5.35 and 6.35 GHz), the main current and part of the current component with different phases (0° and 90°) are characterized by red and black arrows. First, it is observed that the surface currents are mainly focused on the square patches, the edges of the square slots, and the vertical planes. Second, it can be seen that the total surface currents in the square slots and metal columns are opposite those of the square patches at 5.35 GHz, which would reduce the electrical length and move the operating frequency of the antenna to the higher frequency. Third, it is observed that the total surface currents of square patches and vertical planes are in the same direction and opposite those of the square slots at 6.35 GHz, which would increase the electrical length and move the operating frequency of the antenna to a lower frequency, compared with the results at 5.35 GHz. Finally, the total surface currents of the antenna are orthogonal at 0° and 90° at lower and higher frequencies, respectively. With the variety of phases, the main currents on the antenna rotate anticlockwise at two AR resonant points, which suggests the designed MSA would stimulate the right-hand CP wave in the +z direction.
3. Experimental Results
To prove the accuracy of the design, the designed antenna was simulated, fabricated, and measured in this section. The |S11| value was measured by a vector network analyzer (N5224A), while the radiation pattern, gain, and AR values were measured by the Satimo Starlab Measurement System. Figure 6 gives the simulated and measured |S11| and AR values. As seen, the simulated and measured −10-dB IBW are 34.8% (4.75–6.75 GHz) and 36.4% (4.65 to 6.70 GHz), whereas the 3-dB ARBW are 25.0% (5.04–6.48 GHz) and 25.1% (5.01 to 6.45 GHz), respectively. Further, the simulated and measured CP bandwidths (|S11| < −10-dB and AR < 3-dB) are 25.0% (5.04–6.48 GHz) and 25.1% (5.01 to 6.45 GHz). In addition, Figure 7 depicts the measured and simulated gain curves with frequency changes and a photograph of the fabricated antenna. As shown, the measured and simulated peak gains are 10.1 and 9.8 dBi, and the gains gradually increase from low-frequencies to high-frequencies. This is because square slots mainly participate in CP radiation at low frequencies, while square patches participate in that at high frequencies. At last, the simulated and measured radiation pattern values at AR resonant points (4.35, 5.8, and 6.35 GHz) are also shown in Figure 8. It can be seen that the left-hand CP waves are at least 15 dB less than the right-hand CP parts in the xz- and yz-plane at these AR resonant points, indicating the antenna could excite right-hand CP waves. There is good consistency between the simulation and measurement results. A comparison between the proposed antenna and the recently published CP antennas is shown in Table 2, to demonstrate the advantages of the proposed antenna. The compared results reveal that the proposed antenna has a wider CP bandwidth.
In this study, a new stacked microstrip slot antenna (MSA) with wideband circular polarization characteristics is proposed. The antenna consists of a square-loop feed configuration, four parasitic square patches and vertical planes, and a square ground plane etched with four parasitic square slots. These square patches, vertical planes, and square slots as parasitic elements are used to stimulate two CP resonant points. Simulation and measurement are performed on the designed antenna prototype to demonstrate the design’s rationality. The measured results depict that the measured impedance bandwidth for |S11| < −10 dB is 36.4% (4.65 to 6.70 GHz) and the measured axial ratio bandwidth for AR < 3 dB is 25.1% (5.01 to 6.45 GHz). Compared with other reported stacked CP antennas, the proposed antenna has significant advantages in CP bandwidth, which could occupy the wireless local area network ITS (5.8 GHz), (5.725–5.85 GHz), and WIFI (5.85–5.925 GHz) bands.
The data used to support the findings of this study are available from the corresponding author upon request.
Conflicts of Interest
The author declares that there are no conflicts of interest.
This study was supported by GDAS’ Project of Science and Technology Development, No.2020GDASYL-20200103120 Special fund project for Enterprise Science and Technology Commissioners of Guangdong Province, No.GDKTP2021020300.
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