A Wideband Circularly Polarized Antenna with Characteristic Mode Analysis

A wideband circularly polarized (CP) antenna is presented to achieve enhanced impedance, axial ratio (AR), and gain bandwidths. *e 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-ringmicrostrip line, the operating bandwidth of the proposed antenna is increased. *e 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 1.486–2.236GHz (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.


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. erefore, 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][8][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°l oop 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]. e 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][16][17][18][19][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. e 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. e effects of the number of probes on the antenna performance are analyzed by the characteristic mode (CM) method. e 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 S 11 ≤ −18 dB.   e diameters of patch 1 and patch 2 are D 1 and D 2 , respectively. e distance between the two patches is h 3 . e inner and outer radii of the split-ring microstrip line are r 1 and r 2 , respectively. e beginning of the split-ring microstrip line contains a rectangular section with length l 1 . e rectangular section is introduced to enhance the impedance bandwidth. e distance between patch 1 and the split-ring microstrip line is h 2 . e split-ring 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. e 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. e probes are distributed along an arc line with radius rp.
e angle between the two adjacent probes is α. e number of probes and the angle α affect the CP performance and impedance bandwidth greatly. e diameter of the ground is rg. e distance between the ground and the splitring microstrip line is h 1 . e detailed dimensions of the proposed antenna are shown in Table 1.
ree 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. e proposed antenna contains six probes and a circular parasitic patch. e simulated S 11 and boresight AR of the three CP antennas are shown in Figure 3.
For antenna 1, by using the sequential feed structure with four probes, the RHCP wave is obtained. Antenna 1 achieves  International Journal of Antennas and Propagation an impedance bandwidth of 19.28% for S 11 ≤ −18 dB and 3-dB AR bandwidth of 9.36%. e 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 S 11 ≤ −18 dB. e 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 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 3-dB AR bandwidths.

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 S 11 ≤ −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.
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 3-dB AR bandwidth is only 18.3%. e impedance bandwidth of 41% for S 11 ≤ −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%.

Effects of Patch 1 and Patch 2.
Patch 1 is a main radiation patch, and patch 2 is a parasitic patch. e 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.  International Journal of Antennas and Propagation 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 high-frequency band.

Effects of the Sequential Feed Structure.
e parameters of the angle α, the length l 1 , and the radius rp are investigated to further illustrate the antenna design process. e 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 high-frequency band. us, α � 60°w as chosen. 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). e effects of the length l 1 of the rectangular section on the split-ring microstrip line are shown in Figure 9. e 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.   International Journal of Antennas and Propagation

Effects of the Height h 2 and h 3 .
e 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.
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 3-dB 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.

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.
us, the CP radiation  International Journal of Antennas and Propagation 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). is leads to the deterioration of AR in the high-frequency band. 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. us, 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. is indicates that patch 2 has a great influence on the CP radiation in the highfrequency band.    When seven probes with α � 51.429°are used, the AR in the low-frequency band has deteriorated. e 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 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. e simulated and measured S 11 of the proposed antenna are shown in Figure 17(a). e 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. e measured bandwidth for AR ≤ 3 dB is 31.6%, covering 1.6 to 2.2 GHz. Figure 17(    International Journal of Antennas and Propagation Figure 18. e 3-dB beamwidth is more than 45°for both planes. Table 2 summarizes some key indicators of the proposed antenna and other wideband CP antennas. e antenna proposed in [14] is fed by a single feed point. e 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. e 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. e size of the proposed antenna is also competitive.

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 S 11 ≤ −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
e data used to support the findings of this study are included within the article.

Conflicts of Interest
e authors declare that they have no conflicts of interest.