Research Article  Open Access
Lei Chang, LingLu Chen, JianQiang Zhang, Dan Li, "A Wideband Circularly Polarized Antenna with Characteristic Mode Analysis", International Journal of Antennas and Propagation, vol. 2020, Article ID 5379892, 13 pages, 2020. https://doi.org/10.1155/2020/5379892
A Wideband Circularly Polarized Antenna with Characteristic Mode Analysis
Abstract
A wideband circularly polarized (CP) antenna is presented to achieve enhanced impedance, axial ratio (AR), and gain bandwidths. The antenna consists of two circular patches, a splitring microstrip line with six probes, and a circular ground plane. By using these six probes which are placed in sequence on the splitring microstrip line, the operating bandwidth of the proposed antenna is increased. The characteristic mode method is used to analyze different modes of the antenna and reveal the mechanism of extending the 3dB AR bandwidth. Measured results show that the proposed antenna obtains an impedance bandwidth of 1.486–2.236 GHz (40.3%) for S_{11} ≤ −18 dB, a 3dB AR bandwidth of 1.6–2.2 GHz (31.6%), and a boresight gain of 8.89 ± 0.87 dBic.
1. Introduction
With the development of many wireless systems, such as radar, satellite communication, remote control, and telemetry systems, there are more and more applications of circularly polarized (CP) antennas because CP antennas allow for reduction of multipath fading, avoiding polarization mismatching, and better weather adaptability. A CP annularring microstrip antenna using an equalsplit power divider is proposed in [1], which has a 3dB axial ratio (AR) bandwidth of 6%. A CP antenna fed by an Lprobe is presented with a 3dB AR bandwidth of 9%, which uses two stacked folded patches [2]. A square ring slot including four branch slots is applied to produce circular polarization, and a 3dB AR bandwidth of 8.7% is achieved [3].
However, there is a growing demand for antennas with a wideband AR bandwidth. Therefore, it is necessary to investigate methods of extending AR bandwidth. In [4], an air gap is used to a stacked patch with a single probe feed to enhance the AR bandwidth (20.2%). A coplanar parasitic ring slot patch is introduced to achieve good circular polarization performance with a 3dB AR bandwidth of 16% [5]. By using an Hshaped patch and a probe in conjunction with a printed monopole, a 3dB AR bandwidth of 19.4% is obtained [6]. Based on the meandering feed structures, patch antennas are studied to achieve wide 3dB AR bandwidths [7–9]. 3dB AR bandwidths of 13.5% [7], 22.4% [8], and 16.8% [9] are achieved by using a horizontally meandered strip, a 3D meandering strip, and a printed meandering probe, respectively. In [10], a driven patch with a square ring, a 270° loop of microstrip line, and an Lshaped parasitic patch is used to excite four square parasitic patches to achieve a 3dB AR bandwidth of 28.1%. By using a differentially fed method, a 3dB AR bandwidth of 31% is obtained [11]. The CP antenna using an nshaped proximity coupling probe configuration has the impedance and 3dB AR bandwidths of 25% [12]. In [13], a quasimagneticelectric CP patch antenna with a single feed is studied, which can achieve a 3dB AR bandwidth of 15.3%. An Hshaped microstrip patch and a reactive impedance surface are applied to improve the 3dB AR bandwidth to 27.5% [14]. Sequential feed methods are also proposed to achieve good circular polarization performance [15–20]. In [15], four cross slots via a microstrip feed line with multiple matching segments are used to excite a square patch, and a 3dB AR bandwidth of 16% is achieved. Four asymmetric cross slots and a microstrip ring with eight matching segments are applied to obtain an enhanced 3dB AR bandwidth of 26% [16]. Two coinshaped patches and a sequential feed structure using four probes are presented in [17], and a 3dB AR bandwidth of 15.9% is achieved. The 3dB AR bandwidth is enhanced by using a ringshaped strip inlaid along the edge of the parasitic patch and two square holes in the center of the main patch and the parasitic one. In [18], a sequential feed structure using four probes which are connected to a microstrip feed line is applied to achieve a wide 3dB AR bandwidth (16.4%). Two probes connected to a horizontal Lshaped strip are as a sequential feed structure, and a shorting pin is loaded to enhance the 3dB AR bandwidth (17.9%) [19]. In [20], four slot elements fed by a 4way sequentialphase feeding network is proposed to achieve a 3dB AR bandwidth of 15.6%.
In this paper, a wideband righthand circular polarization (RHCP) antenna with two circular patches and a sequential feed structure is presented. By increasing the number of probes of the sequential feed structure from 4 to 6, the impedance and 3dB AR bandwidths are improved. The effects of the number of probes on the antenna performance are analyzed by the characteristic mode (CM) method. The proposed antenna exhibits a 3dB AR bandwidth of 1.6–2.2 GHz (31.6%) and a good impedance matching performance in the band of 1.486–2.236 GHz (40.3%) for S_{11} ≤ −18 dB.
2. Antenna Design
The geometry of the proposed antenna is depicted in Figure 1, and the simulated results are obtained by using HFSS. The diameters of patch 1 and patch 2 are D_{1} and D_{2}, respectively. The distance between the two patches is h_{3}. The inner and outer radii of the splitring microstrip line are r_{1} and r_{2}, respectively. The beginning of the splitring microstrip line contains a rectangular section with length l_{1}. The rectangular section is introduced to enhance the impedance bandwidth. The distance between patch 1 and the splitring microstrip line is h_{2}. The splitring microstrip line, patch 1, and patch 2 are printed on the substrate F_{4}BMX220 with ε_{r} = 2.2 and tanδ = 0.0007. Substrate 1, substrate 2, and substrate 3 have a thickness of 0.5 mm, 1 mm, and 1 mm, respectively. The diameter of the three substrates is rsub. Six probes with a diameter of 2.8 mm are used to feed patch 1, which are placed in sequence on the splitring microstrip line. The probes are distributed along an arc line with radius rp. The angle between the two adjacent probes is α. The number of probes and the angle α affect the CP performance and impedance bandwidth greatly. The diameter of the ground is rg. The distance between the ground and the splitring microstrip line is h_{1}. The detailed dimensions of the proposed antenna are shown in Table 1.
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Three prototypes of the CP antennas are created to exhibit the design process of the proposed antenna, as shown in Figure 2. Antenna 1 has four probes and no parasitic patch. Antenna 2 includes a circular parasitic patch. The proposed antenna contains six probes and a circular parasitic patch. The simulated S_{11} and boresight AR of the three CP antennas are shown in Figure 3.
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For antenna 1, by using the sequential feed structure with four probes, the RHCP wave is obtained. Antenna 1 achieves an impedance bandwidth of 19.28% for S_{11} ≤ −18 dB and 3dB AR bandwidth of 9.36%. The impedance and 3dB AR bandwidths are still narrow.
By adding a parasitic patch, the impedance bandwidth of antenna 2 is increased to 21.41% for S_{11} ≤ −18 dB. The 3dB AR bandwidth is only 8.19%. However, the AR of antenna 2 in the band of 1.8–2.3 GHz is better than that of antenna 1.
For the proposed antenna, the number of the probes is increased from 4 to 6, and the impedance and 3dB AR bandwidths are improved to 41% (1.483–2.247 GHz) for S_{11} ≤ −18 dB and 29.6% (1.628–2.193 GHz). It can be observed that the number of probes has a significant effect on the impedance and 3dB AR bandwidths.
2.1. Effects of the Number of Probes
We study the performance of the antenna when the number of probes is 4, 5, 6, and 7 with α = 90°, 72°, 60°, and 51.429°, respectively, as shown in Figure 4. For traditional four probes with α = 90°, the 3dB AR bandwidth is only 7.8% and the impedance bandwidth for S_{11} ≤ −18 dB is 21.2%. To extend the 3dB AR bandwidth, we proposed a new method to achieve an enhanced AR bandwidth by increasing the number of probes.
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When five probes with α = 72° are used, the impedance bandwidth for S_{11} ≤ −21 dB is 34.6% (1.547–2.194 GHz). However, the 3dB AR bandwidth is only 18.3%. The impedance bandwidth of 41% for S_{11} ≤ −18 dB and the 3dB AR bandwidth of 29.6% are achieved when six probes with α = 60° are introduced. As the number of probes increases to seven (α = 51.429°), the impedance matching becomes worse and the 3dB AR bandwidth is only 15.9%.
2.2. Effects of Patch 1 and Patch 2
Patch 1 is a main radiation patch, and patch 2 is a parasitic patch. The simulated results of S_{11} and AR by varying D_{1} are shown in Figure 5. As D_{1} increases, the operating frequency band of the antenna is shifted to a lower frequency.
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Figure 6 shows the effects of patch 2 on the performance of S_{11} and boresight AR. It is shown that patch 2 has a significant effect on the AR, especially in the highfrequency band.
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2.3. Effects of the Sequential Feed Structure
The parameters of the angle α, the length l_{1}, and the radius rp are investigated to further illustrate the antenna design process.
The effects of the angle α on the performance of S_{11} and boresight AR are shown in Figure 7. It is shown that the maximum value of S_{11} in the band of 1.55–2.25 GHz is increased and the low cutoff frequency is shifted to a lower frequency as the angle α increases. Decreasing α makes the AR become worse in the highfrequency band. Thus, α = 60° was chosen.
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As shown in Figure 8(a), the radius rp has a slight effect on the impedance matching. However, the AR in the band of 1.8–2.2 GHz becomes worse as rp decreases, as shown in Figure 8(b).
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The effects of the length l_{1} of the rectangular section on the splitring microstrip line are shown in Figure 9. The length l_{1} has a slight effect on the AR. Compared with l_{1} = 8.5 mm, the impedance bandwidth is shifted down and the impedance matching in the band of 1.6–2.2 GHz becomes worse with l_{1} = 4.5 mm; while with l_{1} = 12.5 mm, the impedance matching in the band of 1.7–2.2 GHz becomes worse.
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2.4. Effects of the Height h_{2} and h_{3}
The effects of varying height of patch 1 and patch 2 on the performance of S_{11} and boresight AR are shown in Figures 10 and 11, respectively.
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As the height h_{2} decreases, the impedance matching and boresight AR in the operating band get worse. Increasing h_{2} makes the impedance matching and boresight AR better, but narrows the operating bandwidth.
A slight effect of the height h_{3} on the impedance matching has been observed. However, the height h_{3} has a significant effect on the boresight AR. Increasing h_{3} greatly degrades the 3dB AR bandwidth. When h_{3} is reduced to 10 mm, the boresight AR deteriorates in the operating band.
Figure 12 shows the surface current distribution on patch 1 and patch 2 at 1.91 GHz. It is obvious that the vector current rotates counterclockwise in a circular path, which depicts RHCP.
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2.5. Characteristic Mode Analysis (CMA)
In order to illustrate the operating principle of the antenna, CMA is carried out using FEKO.
When four probes with α = 90° are used, the mode current distribution at 1.72 and 1.96 GHz is shown in Figure 13. Figure 13(a) shows that mode 1 is orthogonal to modes 3 and 4 at 1.72 GHz. Thus, the CP radiation characteristics are obtained by modes 1, 3, and 4. However, modes 1 and 5 cannot be excited, and there is no mode that is orthogonal to mode 3 at 1.96 GHz, as shown in Figure 13(b). This leads to the deterioration of AR in the highfrequency band.
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Figure 14 shows the modal current distribution of the proposed antenna with six probes at 1.72 and 1.96 GHz. We can see that mode 2 is orthogonal to modes 3 and 5 at 1.72 GHz, as well as mode 3 is orthogonal to mode 5 at 1.96 GHz. Thus, when the six probes are used, the antenna can obtain a wide AR bandwidth. Furthermore, mode 5 has intense current at patch 2 at 1.96 GHz. This indicates that patch 2 has a great influence on the CP radiation in the highfrequency band.
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When seven probes with α = 51.429° are used, the AR in the lowfrequency band has deteriorated. The mode current distribution at 1.72 GHz is shown in Figure 15. It is shown that mode 2 is orthogonal to modes 3 and 5. However, the current in modes 3 and 5 flows in opposite directions, which leads to the amplitude imbalance between the orthogonal modes, resulting in the deterioration of the AR.
3. Measured Results and Discussion
Figure 16 shows the fabricated prototype. S_{11} was measured by using the Rohde & Schwarz ZVT 8 vector network analyzer. Radiation patterns, gain, and AR were measured in an anechoic chamber.
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The simulated and measured S_{11} of the proposed antenna are shown in Figure 17(a). The measured impedance bandwidth for S_{11} ≤ −18 dB is 40.3% from 1.486 to 2.236 GHz. Figure 17(b) shows the simulated and measured boresight AR. The measured bandwidth for AR ≤ 3 dB is 31.6%, covering 1.6 to 2.2 GHz. Figure 17(c) shows the measured boresight gain compared with the simulated result. The proposed antenna shows a stable measured gain of 8.89 ± 0.87 dBic within the 3dB AR bandwidth.
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The simulated and measured radiation patterns of two principal planes at 1.65, 1.9, and 2.15 GHz are shown in Figure 18. The 3dB beamwidth is more than 45° for both planes.
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Table 2 summarizes some key indicators of the proposed antenna and other wideband CP antennas. The antenna proposed in [14] is fed by a single feed point. The antenna proposed in [10] is fed equivalently by two feed points. To improve the AR of the antenna in [14], a reactive impedance surface is used. To improve the AR of the antenna in [10], an Lshaped parasitic patch and four parasitic patches are used. In [11], a wide 3dB AR bandwidth of 31% is achieved by generating an equivalent fourpoint feeding. It is observed that the more the feeding points, the easier it is to obtain a wider 3dB AR bandwidth (27.5% in [14], 28.1% in [10], and 31% in [11]) and a wider impedance bandwidth (44.5%% in [14], 38% in [10], and 60.5% in [11]). In this paper, an equivalent sixpoint feeding and a parasitic patch are used to extend the impedance and 3dB AR bandwidths. Compared with the CP antennas in [5, 9–16, 19, 20], the proposed antenna has a wider 3dB AR bandwidth and better impedance matching. The peak gain of the proposed antenna is 9.76 dBic, which is a good result. Although the antenna in [20] has a greater peak gain, the size is larger than that of the proposed antenna. The size of the proposed antenna is also competitive.
 
λ_{ARd} is the freespace wavelength at the starting frequency of the 3dB AR bandwidth. λ_{ARd} is the freespace wavelength at 2.15 GHz. λ_{ARd} is the freespace wavelength at 4.64 GHz. 
4. Conclusion
In this paper, a wideband CP antenna is studied and its advantages in impedance matching and CP radiation performance are discussed in detail. By introducing six probes, a parasitic patch, and a splitring microstrip line containing a rectangular section, the impedance bandwidth of 40.3% for S_{11} ≤ −18 dB and the 3dB AR bandwidth of 31.6% are achieved, which shows a great improvement in the operating bandwidth. A peak gain of 9.76 dBic and unidirectional radiation patterns are also achieved.
Data Availability
The data used to support the findings of this study are included within the article.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
This work was supported by the Young Elite Scientists Sponsorship Program by CAST, under Grant 2017QNRC001.
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Copyright
Copyright © 2020 Lei Chang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.