Broadband Circularly Polarized Microstrip Patch Antenna Using Circular Artificial Ground Structure and Meandering Probe

To enhance axial ratio (AR) bandwidth, this research proposes a circularly polarized (CP) single-fed microstrip patch antenna using a circular artificial ground structure (AGS) and meandering probe. To achieve broader AR bandwidth, the circular AGS is populated with rectangular unit cells and partially cut unit cells along the circular contour, while the meandering probe is used to improve the impedance bandwidth. Simulations are performed and results compared with that of conventional rectangular-AGS antenna. The simulation results show that the circular-AGS antenna, given 62 mm circular ground plane, achieves broader impedance (5.12 - 9.00 GHz and 54%), AR (5.21 - 8.27 GHz and 45%) and gain bandwidths (3.85 - 7.00 GHz and 58.06%), in comparison with the rectangular-AGS antenna (4.50 - 7.45 GHz and 49%; 4.52 - 7.42 GHz and 21%; and 4.00 - 6.80 GHz and 51.58% for impedance, AR and gain bandwidths). The circular-AGS antenna is capable of converting linear polarization in the off-axial ratio band into circular polarization. To verify, a circular-AGS antenna prototype is fabricated and experiments undertaken. The experimental impedance, AR and gain bandwidths of the circular-AGS antenna are 47.82% (5.17 - 8.42 GHz), 43.81% (5.24 - 8.17 GHz) and 60.74% (3.75 - 7.00 GHz). The proposed circular-AGS antenna can achieve broader AR bandwidth and is thus ideal for broadband CP applications. The novelty of this research lies in the use of circular AGS to effectively enhance AR bandwidth, as opposed to rectangular AGS which is conventionally used in CP polarizers.


I. INTRODUCTION
Circularly polarized (CP) antennas are utilized in wireless devices and systems to mitigate polarization loss, such as wireless local area network (WLAN), radio frequency identification (RFID), and satellite telecommunications [1].
As a result, artificial magnetic conductors (AMC) are used to realize wide impedance bandwidth with low-profile structure because the radiating patch could be positioned in close proximity to the AMC with a distance less than a quarterwavelength. In practice, AMC could be employed to achieve circular polarization [15], [16]. Specifically, the impedance and AR bandwidths of 36% and 33% are achieved with AMC as backed reflector [17]. In [18]- [20], AMC is positioned above the radiating patch to realize CP radiation with AR bandwidth of 23% and 28%, respectively. Dipoles and bowtie elements are positioned above AMC to achieve dual-band CP with the lower and higher bandwidths of 19% and 34%, but the antenna height is more than a quarter-wavelength [21].
In addition, complementary split-ring resonator (CSR) with AMC ground plane [22] and complementary crossbar fractal tree (CCFT) and three-turn complementary spiral resonators (TCSRs) with reactive impedance surface (RIS) ground plane [23] are utilized to tackle the antenna bulkiness. However, the techniques suffer from narrow AR bandwidths. As a result, a single-fed rectangular CP patch antenna with truncated corners and rectangular artificial ground structure (AGS) populated with rectangular-shaped unit cells is proposed to enhance AR bandwidth [24]. In [25], the AMCbased AGS could convert linear polarization in the off-axial ratio band (AR>3dB) into circular polarization and achieve broader AR bandwidth in higher frequency. Besides, the AR bandwidth is enhanced by varying the ground plane diameter, achieving an AR bandwidth of 30% [26].
To address the antenna bulkiness and enhance the AR bandwidth, this research proposes a CP single-fed microstrip patch antenna using a circular AGS structure and meandering probe on the circular ground plane. The circular AGS is deployed to enhance the AR bandwidth and the meandering probe to improve the impedance bandwidth. In [27]- [29], the meandering probe technique is used to improve impedance matching. The radiating patch of the proposed CP microstrip patch antenna is of square shape with diagonally truncated corners, and the circular AGS is populated with rectangular unit cells and partially-cut rectangular unit cells along the circular contour. Simulations are carried out, and an antenna prototype is fabricated and experimented. In addition, the performance of the circular-AGS antenna is compared with that of a rectangular-AGS antenna. Fig. 1 illustrates the geometry of the proposed broadband CP microstrip patch antenna, consisting of corners-truncated square radiating patch, circular AGS, meandering probe, and circular ground plane. The square radiating patch with diagonally truncated corners could independently achieve right-hand circular polarization (RHCP) at 6 GHz. The radiating patch is rotated 45 • to the x-axis to generate in-phase electric fields in the x-and y-directions. The radiating patch sits on Rogers RT/Duroid 5880 substrate of 1.6 mm in thickness and 62 mm in diameter (upper layer). Figure 2 illustrates the cross-sectional view of the meandering probe, consisting of upper and lower arms and shorter and taller vias. The SMA connector is driving the meandered line, which is exciting the antenna using a via. Two unit cells at the center of the circular AGS are removed to accommodate the upper arm of the meandering probe. The current on the upper and lower arms flow in the opposite direction, and so do the current on the vias. The phenomenon suppresses cross-polarization of the antenna [27]- [29]. A fan stub is incorporated for broadband impedance matching.  The width and length of the upper arm are 2.25 mm and 5.76 mm, and those of the lower arm are 2 mm and 8.55 mm. The fan stub with a radius of 4.2 mm and flare of 52 • is adjoined to one end of the upper arm, with a distance of 1.33 mm from the inner conductor center. The diameter of the vias is 1.27 mm. The meandering probe is underneath the center of the radiating patch and aligned 29.5 • to the y-axis for adjusting amplitude ratio of x-and y-components. The feeding point of the radiating patch (i.e., where the radiating patch is connected to the meandering probe) is 3.75 mm and 1 mm from the center of the radiating patch. The offset distance of 1 mm from the diagonal is slightly adjusted for matching impedance.

II. ANTENNA CONFIGURATION
In addition, there is an air gap of 0.5 mm between the radiating patch and AGS patch to enhance impedance matching. The air gap itself has little effect on characteristics [24]- [25]. In this research, simulations are carried out with two groundplane diameters: 51 and 62 mm, and the optimal circular ground plane dimension is determined.

III. THE AGS UNIT CELLS AND GROUND PLANE
A. RECTANGULAR AND PARTIALLY-CUT RECTANGULAR UNIT CELLS Fig. 3 illustrates the geometry of a rectangular unit cell of the circular AGS [24]- [26]. The structure could achieve circular VOLUME 8, 2020 polarization with +90 • and −90 • reflection phases for xand y-polarization. The width (U x ) and length (U y ) of the area encompassing the copper patch and substrate are 6.5 and 10 mm, and the width (P x ) and length (P y ) of the rectangular copper patch (i.e., unit cell) are 4.20 and 9.25 mm [24]- [26].
In [25]- [30], broad CP radiation is achieved by rotating the radiating patch 45 • to the x-axis, in conjunction with the rectangular AGS. Specifically, the RHCP is a synthesis of the wave from the radiating patch and the reflected wave from the AGS. Fig. 4 illustrates the proposed AGS populated with rectangular unit cells (type C) and partially-cut unit cells (types A and B) along the leftmost and rightmost of the circular AGS.  and y-polarization (E y ) of types-A, B, and C unit cells and the phase difference, using Ansoft HFSS version 16.0. The reflection phase difference of 180 • is achieved at 7.00 -7.50 GHz for type-A unit cell, 7.50 -9.00 GHz for type-B unit cell, and 6.00 -6.50 GHz for type-C unit cell. As a result, the concurrent use of types-A, B, and C unit cells enhances the AR bandwidth, covering the 6.00 -9.00 GHz frequency band. E x is less influenced by the unit cell types, compared to E y which is noticeably subject to the unit cell types. To achieve the CP (AR≤3dB), a phase-difference error between E x and E y of 20 • is acceptable, given 180 • -phase difference [24]- [26].
Figs. 6(a)-(f) respectively illustrate six circular AGS configurations containing types-A, B, and/or C unit cells along  Figs. 7 and 8 respectively illustrate the simulated AR along the +z direction, amplitude ratio (E y /E x ), and phase difference ( E y − E x ) of Antennas #1-#6. In Fig. 7, the minimum AR point occurred at the lowest frequency was caused by the driven patch. The other minimum AR points at higher frequencies were generated by the surface waves from a number of unit cells. The minimum AR points of Antennas #1-#4 (each antenna contains 34 unit cells) occurred at the similar frequencies, but the levels of minimum AR points are different due to the reflection phase of various types of unit cells from Fig.5. When some unit cells were removed as Antennas #5-#6, the minimum AR point caused by the driven patch slightly moved down, whereas the subsequent AR points were shifted to the higher frequencies. This phenomenon is similar to the antenna studied in [18]. At 7 GHz, AR of Antennas #2-#6 are greater than 3dB (AR>3dB), with E y /E x and E y − E x deviating from 0 dB and 90 • as shown in Fig.8. The simulation results indicate that, at 7 GHz, the concurrent implementation of types-A and B unit cells along the left and right contour of the circular AGS effectively reduces AR (≤3dB), vis-à-vis the rectangular-AGS antenna whose AR is > 3dB [24], [25]. Figs. 9-11 respectively depict the current distribution, at 7 GHz, of Antenna #1 (the proposed structure), Antenna #4 (containing type-C), and Antenna #5 (types-A and B unit cells removed from the leftmost and rightmost columns). The dominant direction of current flow with respect to time (t) is represented by the dark bold arrow.
In Fig. 9 (Antenna #1), the current flow is counterclockwise, generating RHCP along the AGS contour. The amplitude of the current along the contour of Antenna #1 (Fig. 9) is stronger than that of Antenna #4 (Fig. 10). The finding  is attributable to the truncated corners of types-A and B unit cells in the AGS of Antenna #1. The weak amplitude of Antenna #4 results in its amplitude ratio deviating from 0 dB, resulting in large AR (>3dB). In Fig. 11 (Antenna #5), the current distribution is of y-direction linear polarization due to the absence of the leftmost and rightmost columns of unit cells. This suggests that the circular polarization could be realized by incorporating types-A and B unit cells into the AGS. Fig. 12 illustrates the proposed AGS with types-D and E (trapezoid-shaped) unit cells, located at the uppermost and lowermost of the circular AGS. In addition, the shape and dimensions of types-D and E unit cells (Fig. 12) are altered from trapezoid to stunted rectangular shape (i.e., types-D and E unit cells) to compare the AR performance. The dimensions of the stunted rectangular-shaped types-D and E unit cells are 4.20 × 2.20 and 4.20 × 4.55 mm, respectively.      Figs. 14 and 15 respectively compares the simulated AR, amplitude ratio (E y /E x ), and phase difference ( E y − E x ) of Antennas #1, #7, #8, and #9 relative to frequency. At 7 GHz, AR of Antennas #1, #7, and #8 are less than 3 dB, indicating right-hand circular polarization. Types-D, D , E, and E unit cells balance the x-and y-direction current amplitude. Meanwhile, in the absence of types-D, D , E, and E unit cells, the y-direction current amplitude becomes stronger than that of the x-direction. Antenna #9 exhibits an AR greater than 3 dB because the phase difference is drastically deviated from 90 • as shown in Fig.15. The finding suggests that the absence of types-D, D , E, and E unit cells contributes to non-circular polarization, as shown in Fig. 16. The broadband AR characteristics can be achieved with the combination of minimum AR points from the driven patch and circular AGS with the concurrent use of partially cut unit cells along the circular contour.

B. ENLARGED CIRCULAR GROUND PLANE DIAMETER
Simulations are carried out using Antenna #1 with two ground plane diameters (D): 51 and 62 mm, given the circular AGS diameter of 51 mm. Figs. 17 and 18 respectively illustrate the simulated |S 11 | and AR in the +z direction, given the two ground plane diameters (D) using Antenna #1.   At 7 GHz and D = 51 mm, AR of the antenna is greater than 3 dB (>3dB) due to the diffractive effects causing the electric field at the edges to spread in both x-and y-directions and subsequently inducing electrical coupling with the adjoining patches. Nevertheless, in [26], at 7 GHz, enlarging the ground plane could improve the AR (≤3dB). After expanding the ground plane (D = 62 mm), the diffraction at the edges of each element and the interference between diffracted rays from neighbouring elements can be mitigated, and this results in AR improvement.
In light of AR in excess of 3 dB (AR>3dB) at 7 GHz for D = 51 mm, an optimal circular ground plane diameter (i.e., achieving AR≤3 dB at 7 GHz) is determined using eigenvalue analysis and the periodic boundary condition in the high frequency structure simulator (HFSS) with one single type-C unit cell (Fig. 3). Figs. 19(a)-(b) respectively depict the dispersion diagrams of the AGS in the x-and y-directions given the ground plane of 51 mm, where TE and TM denote transverse electric and transverse magnetic modes. The resonant frequency is calculated by where n is an integer, β is a propagation constant (deg/m), β' is a propagation constant in the enlarged substrate and ground plane (deg/m), D 0 is the initial AGS diameter (51 mm).  Table 1 tabulates the resonant frequencies based on Fig. 19 and eq. (1) for D = 51 and 62 mm, using the Antenna #1 structure. With D = 51 mm, a resonance occurs at 6.91 GHz (TM), consistent with Fig. 18 in which AR peaks around 7 GHz. This is attributable to the current on the circular AGS predominantly flowing in the y-direction. The phenomenon is also observed in Antenna #4 (Fig. 10), Antenna #5 (Fig. 11), and Antenna #9 (Fig. 16).
In Fig. 19(a), given the circular ground plane (D) of 51 mm, TE and TM in the x-direction diverge at higher frequency, as opposed to parallel to each other. The divergence of TE and TM currents induces strong resonance at 7 GHz, as shown in Fig. 18. The enlargement of the circular ground plane dimension (D) from 51 to 62 mm suppresses the resonance, improving the AR (≤3dB) (Fig. 18). The optimal circular ground plane diameter is thus 62 mm. Fig. 20 compares the simulated AR of the non-AGS [25], rectangular-AGS [24]- [25], and circular-AGS antennas,   given the corners-truncated radiating patch in Fig. 1. The AR bandwidth of the rectangular-AGS antenna is 21%, vis-à-vis 13% of the non-AGS. Meanwhile, the circular-AGS antenna (given 62 mm circular ground plane) the phenomenon of phase-difference in Fig. 5 can suppresses the resonance, resulting in an AR bandwidth of 43.81% and covering the 3-dB AR frequency band of 5.42 -8.17 GHz.   Unlike the rectangular AGS, the amplitude ratio and phase difference of the circular AGS, given 62 mm circular ground plane, exhibit no resonance at 7 GHz, which is advantageous to wideband CP antenna design. The suppressed resonance is attributable to partially cut unit cells along the circular contour (Fig. 4), and the circular AGS results in a wide AR bandwidth of 6 -9 GHz (Fig. 5). Fig. 23 compares the simulated gains at the +z direction of the rectangular-and circular-AGS antennas. The maximum gains of the rectangular-and circular-AGS antennas are 6.72 and 9.25 dBic. The bandwidth of 3-dB gain variation of the rectangular-and circular-AGS antennas are 4.00 -6.80 GHz (51.58%) and 3.85 -7.00 GHz (58.06%), respectively.  Fig. 24 compares the simulated co-and cross-polarization radiation patterns in the xz-and yz-planes of the rectangularand circular-AGS antennas at 6 GHz, given the fact that the rectangular-AGS antenna failed to achieve RHCP at 7 GHz. The difference between the co-and cross-polarization is 15 dB due to high cross-polarization which is attributable to the meandering probe [27], [28]. The front-to-back (F/B) ratios of the rectangular-and circular-AGS antennas are 18.76 and 16.03 dB. Despite the lower F/B ratio, the circular-AGS antenna could achieve a wider AR bandwidth (42%), compared to 21% of the rectangular-AGS antenna.

V. RESULTS AND DISCUSSION
To verify, a prototype of Antenna #1 is fabricated, and experiments undertaken. Fig. 25 illustrates the prototype of the broadband circularly polarized microstrip patch antenna with the circular AGS and meandering probe. The fabrication of the antenna and the AGS was independently carried out using RT/Duroid 5880 dielectric substrates.  Figs. 27(a)-(b) respectively illustrate the simulated and measured amplitude ratio and phase difference and AR in the +z direction of the circular-AGS antenna. The amplitude ratio and phase difference of the circular-AGS antenna is described above as in Fig. 5, for 5.5 -8.0 GHz, vacillate around 0 dB and 90 • (Fig. 27(a)). Thus, circular polarization (AR≤3dB) could be achieved in a broadband frequency range of 5.24 -8.17 GHz (43.81%) in the +z direction.
Figs. 28(a)-(e) respectively depict the simulated and measured co-and cross-polarization radiation patterns in the  xz-and yz-planes of the circular-AGS antenna at 5.5, 6.0, 7.0, 7.2, and 8.0 GHz, given 62 mm circular ground plane. The xz-and yz-plane main beam of the circular-AGS antenna are slightly titled due to the asymmetrical feeding position (Fig. 1). The simulated and measured radiation patterns are in good agreement. Fig. 29 illustrates the simulated and measured gains in the +z direction of the circular-AGS antenna. The simulation and measured results are in good agreement, with the respective maximum gains of 9.25 and 9.74 dBic. The simulated and measured bandwidth of 3-dB gain variation of the CP antenna with metasurface are 3.85 -7.00 GHz  (58.06%) and 3.75 -7.00 GHz (60.47%). The rapid fall in the broadside gain after 6.5 GHz is observed. This is due to the tilted beam direction and large side lobes caused by the higher order resonances of surface wave at high frequency. This antenna behavior is similar to that in the previous work [18]. Table 2 summarizes the simulated and measured radiation characteristics of the proposed CP microstrip patch antenna with circular AGS and meandering probe. Table 3 compares the performance of the proposed CP antenna with existing CP metasurface-based single-fed microstrip antennas in terms of  fractional bandwidth of |S 11 | ≤-10 dB, bandwidth of AR ≤ 3dB, 3-dB gain bandwidth, maximum gain and antenna dimension. By comparison, the fractional bandwidths of |S 11 | ≤-10 dB (47.82%), AR≤3dB (43.81%) and 3-dB gain variation (60.47%) of the proposed CP antenna are wider. The proposed antenna also achieves the highest maximum gain (9.74 dBic) with relatively low-profile dimensions.

VI. CONCLUSION
To achieve broader AR bandwidth, this research proposes a CP single-fed microstrip patch antenna using a circular AGS and meandering probe on the circular ground plane. The circular AGS, which is populated with rectangular unit cells and partially cut unit cells along the circular contour, is used to enhance the AR bandwidth; and the meandering probe to improve the impedance. Simulations are carried out, and an antenna prototype is fabricated and experimented. The performance of the circular-AGS antenna is also compared with that of rectangular-AGS antenna. The simulation results show that the circular-AGS antenna, given 62 mm circular ground plane, achieves broader impedance (5.12 -9.00 GHz and 54%), AR (5. 21 -8.27 GHz and 45%) and gain bandwidths (3.85 -7.00 GHz and 58.06%), vis-à-vis the rectangular-AGS antenna (4.50 -7.45 GHz and 49%; 4.52 -7.42 GHz and 21%; 4.00 -6.80 GHz and 51.85 % for impedance, AR and gain bandwidths). The circular-AGS antenna can convert linear polarization in the off-axial ratio band into right-hand circular polarization. The measured impedance, AR and gain bandwidths of the proposed circular-AGS antenna are 47.82% (5.17 -8.42 GHz), 43.81% (5.42 -8.17 GHz) and 60.47% (3.75 -7.00 GHz). In essence, the CP single-fed microstrip patch antenna with circular AGS and meandering probe can achieve broader bandwidth and thus is ideal for broadband CP applications.