International Journal of Antennas and Propagation
Volume 2020 (2020), Article ID 5379892, 13 pages
https://doi.org/10.1155/2020/5379892

Research Article

A Wideband Circularly Polarized Antenna with Characteristic Mode Analysis

Lei Chang,¹ Ling-Lu Chen,¹ Jian-Qiang Zhang,¹ and Dan Li²

¹No. 36 Research Institute of CETC, Jiaxing 314033, China
²Academician Workstation of Zhejiang Longyou Gongren Electronics Co., Ltd., Quzhou, Longyou 324014, China

Correspondence should be addressed to Lei Chang; yutian_1986@163.com

Received 14 March 2020; Revised 11 May 2020; Accepted 19 May 2020; Published 12 June 2020

Academic Editor: Hervé Aubert

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.

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 split-ring microstrip line with six probes, and a circular ground plane. By using these six probes which are placed in sequence on the split-ring 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 3-dB AR bandwidth. Measured results show that the proposed antenna obtains an impedance bandwidth of 1.486–2.236 GHz (40.3%) for S₁₁ ≤ −18 dB, a 3-dB 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 annular-ring microstrip antenna using an equal-split power divider is proposed in [1], which has a 3-dB axial ratio (AR) bandwidth of 6%. A CP antenna fed by an L-probe is presented with a 3-dB 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 3-dB 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 3-dB AR bandwidth of 16% [5]. By using an H-shaped patch and a probe in conjunction with a printed monopole, a 3-dB AR bandwidth of 19.4% is obtained [6]. Based on the meandering feed structures, patch antennas are studied to achieve wide 3-dB AR bandwidths [7–9]. 3-dB 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 L-shaped parasitic patch is used to excite four square parasitic patches to achieve a 3-dB AR bandwidth of 28.1%. By using a differentially fed method, a 3-dB AR bandwidth of 31% is obtained [11]. The CP antenna using an n-shaped proximity coupling probe configuration has the impedance and 3-dB AR bandwidths of 25% [12]. In [13], a quasi-magnetic-electric CP patch antenna with a single feed is studied, which can achieve a 3-dB AR bandwidth of 15.3%. An H-shaped microstrip patch and a reactive impedance surface are applied to improve the 3-dB 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 3-dB AR bandwidth of 16% is achieved. Four asymmetric cross slots and a microstrip ring with eight matching segments are applied to obtain an enhanced 3-dB AR bandwidth of 26% [16]. Two coin-shaped patches and a sequential feed structure using four probes are presented in [17], and a 3-dB AR bandwidth of 15.9% is achieved. The 3-dB AR bandwidth is enhanced by using a ring-shaped 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 3-dB AR bandwidth (16.4%). Two probes connected to a horizontal L-shaped strip are as a sequential feed structure, and a shorting pin is loaded to enhance the 3-dB AR bandwidth (17.9%) [19]. In [20], four slot elements fed by a 4-way sequential-phase feeding network is proposed to achieve a 3-dB AR bandwidth of 15.6%.

In this paper, a wideband right-hand 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 3-dB 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 3-dB 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 S₁₁ ≤ −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 D₁ and D₂, respectively. The distance between the two patches is h₃. The inner and outer radii of the split-ring microstrip line are r₁ and r₂, respectively. The beginning of the split-ring microstrip line contains a rectangular section with length l₁. The rectangular section is introduced to enhance the impedance bandwidth. The distance between patch 1 and the split-ring microstrip line is h₂. The split-ring microstrip line, patch 1, and patch 2 are printed on the substrate F₄BMX220 with εr = 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 split-ring 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 split-ring microstrip line is h₁. The detailed dimensions of the proposed antenna are shown in Table 1.

(a)

(b)

(c)

(d)

(e)

Figure 1: Geometry of the proposed antenna. (a) 3D view, (b) side view, (c) patch 1, (d) patch 2, and (e) microstrip line.

Table 1: Dimensions of the proposed antenna.

Parameters
Value

D1
85 mm

r1
20.5 mm

rp
27 mm

h1
2.4 mm

h3
12 mm

rg
160 mm

D2
58 mm

r2
28.5 mm

l1
8.5 mm

h2
19 mm

α
60°

rsub
145 mm

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 S₁₁ and boresight AR of the three CP antennas are shown in Figure 3.

(a)

(b)

(c)

Figure 2: Three CP antennas. (a) Antenna 1. (b) Antenna 2. (c) Proposed antenna.

(a)

(b)

Figure 3: Simulated results of the three CP antennas. (a) S₁₁ and (b) AR.

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 S₁₁ ≤ −18 dB and 3-dB AR bandwidth of 9.36%. The impedance and 3-dB AR bandwidths are still narrow.

By adding a parasitic patch, the impedance bandwidth of antenna 2 is increased to 21.41% for S₁₁ ≤ −18 dB. The 3-dB 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 3-dB AR bandwidths are improved to 41% (1.483–2.247 GHz) for S₁₁ ≤ −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 3-dB 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 3-dB AR bandwidth is only 7.8% and the impedance bandwidth for S₁₁ ≤ −18 dB is 21.2%. To extend the 3-dB AR bandwidth, we proposed a new method to achieve an enhanced AR bandwidth by increasing the number of probes.

(a)

(b)

Figure 4: Effects of the number of probes on (a) S₁₁ and (b) AR.

When five probes with α = 72° are used, the impedance bandwidth for S₁₁ ≤ −21 dB is 34.6% (1.547–2.194 GHz). However, the 3-dB AR bandwidth is only 18.3%. The impedance bandwidth of 41% for S₁₁ ≤ −18 dB and the 3-dB 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 3-dB 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 S₁₁ and AR by varying D₁ are shown in Figure 5. As D₁ increases, the operating frequency band of the antenna is shifted to a lower frequency.

(a)

(b)

Figure 5: Effects of the diameter D₁ of patch 1 on (a) S₁₁ and (b) AR.

Figure 6 shows the effects of patch 2 on the performance of S₁₁ and boresight AR. It is shown that patch 2 has a significant effect on the AR, especially in the high-frequency band.

(a)

(b)

Figure 6: Effects of patch 2 on (a) S₁₁ and (b) AR.

2.3. Effects of the Sequential Feed Structure

The parameters of the angle α, the length l₁, and the radius rp are investigated to further illustrate the antenna design process.

The effects of the angle α on the performance of S₁₁ and boresight AR are shown in Figure 7. It is shown that the maximum value of S₁₁ 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 high-frequency band. Thus, α = 60° was chosen.

(a)

(b)

Figure 7: Effects of the angle α on (a) S₁₁ and (b) AR.

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).

(a)

(b)

Figure 8: Effects of the radius rp on (a) S₁₁ and (b) AR.

The effects of the length l₁ of the rectangular section on the split-ring microstrip line are shown in Figure 9. The length l₁ has a slight effect on the AR. Compared with l₁ = 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 l₁ = 4.5 mm; while with l₁ = 12.5 mm, the impedance matching in the band of 1.7–2.2 GHz becomes worse.

(a)

(b)

Figure 9: Effects of the length l₁ on (a) S₁₁ and (b) AR.

2.4. Effects of the Height h₂ and h₃

The effects of varying height of patch 1 and patch 2 on the performance of S₁₁ and boresight AR are shown in Figures 10 and 11, respectively.

(a)

(b)

Figure 10: Effects of the height h₂ on (a) S₁₁ and (b) AR.

(a)

(b)

Figure 11: Effects of the height h₃ on (a) S₁₁ and (b) AR.

As the height h₂ decreases, the impedance matching and boresight AR in the operating band get worse. Increasing h₂ makes the impedance matching and boresight AR better, but narrows the operating bandwidth.

A slight effect of the height h₃ on the impedance matching has been observed. However, the height h₃ has a significant effect on the boresight AR. Increasing h₃ greatly degrades the 3-dB AR bandwidth. When h₃ 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.

(a)

(b)

Figure 12: The surface current distribution on (a) patch 1 and (b) patch 2 at 1.91 GHz.

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 high-frequency band.

(a)

(b)

Figure 13: Modal current distribution of the antenna with four probes at (a) 1.72 GHz and (b) 1.96 GHz.

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 high-frequency band.

(a)

(b)

Figure 14: Modal current distribution of the antenna with six probes at (a) 1.72 GHz and (b) 1.96 GHz.

When seven probes with α = 51.429° are used, the AR in the low-frequency 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.

Figure 15: Modal current distribution of the antenna with seven probes at 1.72 GHz.

3. Measured Results and Discussion

Figure 16 shows the fabricated prototype. S₁₁ was measured by using the Rohde & Schwarz ZVT 8 vector network analyzer. Radiation patterns, gain, and AR were measured in an anechoic chamber.

(a)

(b)

(c)

Figure 16: Photograph of the fabricated antenna.

The simulated and measured S₁₁ of the proposed antenna are shown in Figure 17(a). The measured impedance bandwidth for S₁₁ ≤ −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 3-dB AR bandwidth.

(a)

(b)

(c)

Figure 17: Measured and simulated results: (a) S₁₁, (b) AR, and (c) gain.

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 3-dB beamwidth is more than 45° for both planes.

(a)

(b)

(c)

Figure 18: Measured and simulated radiation patterns of the proposed antenna at (a) 1.65, (b) 1.9, and (c) 2.15 GHz.

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 L-shaped parasitic patch and four parasitic patches are used. In [11], a wide 3-dB AR bandwidth of 31% is achieved by generating an equivalent four-point feeding. It is observed that the more the feeding points, the easier it is to obtain a wider 3-dB 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 six-point feeding and a parasitic patch are used to extend the impedance and 3-dB AR bandwidths. Compared with the CP antennas in [5, 9–16, 19, 20], the proposed antenna has a wider 3-dB 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.

Table 2: Comparisons of the proposed and other CP antennas.

Ref. year
Dimension of patch × height of antenna (λARd)
Impedance matching
Impedance bandwidth
3-dB AR bandwidth
Gain (dBic)

[5] 2014
ϕ0.55 × 0.14
S₁₁ ≤ −14 dB
27% (1.45–1.9 GHz)
16% (1.51–1.77 GHz)
About 7.7–8

[9] 2014
0.24 × 0.24 × 0.13
S₁₁ ≤ −10 dB
42.3% (4.1–6.3 GHz)
16.8% (5.05–5.98 GHz)
Peak gain: 6.7

[10] 2017
About 0.27 × 0.27 × 0.07
S₁₁ ≤ −10 dB
38% (5–7.35 GHz)
28.1% (5.32–7.06 GHz)
Peak gain: 8.4

[11] 2018
0.32 × 0.32 × 0.21
S₁₁ ≤ −10 dB
60.5% (1.5–2.8 GHz)
31% (1.74–2.38 GHz)
6.5 ± 0.4

[12] 2017
0.45 × 0.44 × 0.11
S₁₁ ≤ −10 dB
25% (2.15–2.75 GHz)
25% (2.1–2.7 GHz)
Peak gain: 8.3

[13] 2019
0.67 × 0.45 × 0.21
S₁₁ ≤ −10 dB
31.7% (2.35–3.3 GHz)
15.3% (2.6–3.06 GHz)
6.46–7.64

[14] 2018
0.15 × 0.15 × 0.09
S₁₁ ≤ −10 dB
44.5% (4.64–7.3 GHz)
27.5% (4.55–6 GHz)
Peak gain: 7.2

[15] 2014
0.31 × 0.31 × 0.19
S₁₁ ≤ −10 dB
24% (3.2–4.1 GHz)
16% (3.5–4 GHz)
Peak gain: 9.6

[16] 2015
ϕ0.34 × 0.16
S₁₁ ≤ −10 dB
29.7% (3–4.1 GHz)
21.6% (3.3–4.1 GHz)
Peak gain: 8.6

[19] 2014
0.35 × 0.35 × 0.08
S₁₁ ≤ −10 dB
23.6% (2.09–2.65 GHz)
17.9% (2.18–2.61 GHz)
About 8.5

[20] 2018
1.7 × 1.7 × 0.22
S₁₁ ≤ −10 dB
53.1% (3.9–6.72 GHz)
15.6% (4.73–5.53 GHz)
12.4–13.4

Prop.
ϕ0.45 × 0.18
S₁₁ ≤ −18 dB
40.3% (1.486–2.236 GHz)
31.6% (1.6–2.2 GHz)
8.02–9.76

λARd is the free-space wavelength at the starting frequency of the 3-dB AR bandwidth. λARd is the free-space wavelength at 2.15 GHz. λARd is the free-space 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 split-ring microstrip line containing a rectangular section, the impedance bandwidth of 40.3% for S₁₁ ≤ −18 dB and the 3-dB 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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