Wideband Circularly Polarized Antenna Based on a Non-Uniform Metasurface

: This paper presents a non-uniform metasurface (MS)-based circularly polarized (CP) antenna that is able to perform with a wideband operation characteristic. A squared patch with truncated corners was chosen as a radiating CP source. Then, unlike the conventional CP MS antennas with a uniform MS, the proposed design employed a non-uniform MS placed above the driven patch. Apart from increasing the impedance bandwidth, the non-uniform MS was also capable of generating two additional CP bands in the high-frequency region, which contributed to signiﬁcantly increasing the antenna’s overall performance. For demonstration, an antenna prototype was fabricated and experimentally tested. The measured operating bandwidth of the fabricated antenna was about 30% and the broadside gain within this band was around 6.6 dBic. Compared to the other reported CP MS antennas in the open literature, the proposed design has the advantages of very wideband operation with a comparable size and gain.


Introduction
In recent years, many researchers have focused on designing antennas with circularly polarized (CP) radiation [1,2] rather than linear polarization [3]. The benefits of adopting CP radiation in antennas have been illustrated for multipath interference reduction and polarization mismatch mitigation. Besides, as electronic devices are becoming increasingly small and high-speed communication is also highly desirable, antennas with a simple configuration and low profile as well as wideband operation features have received a great deal of attention from researchers.
Single-fed patch antennas are considered to be a potential candidate to fulfil the abovementioned requirements. However, an inherently narrow operating bandwidth (BW) caused by a high quality factor is the critical drawback of this antenna type [4]. To date, the task of the BW enhancement of single-fed CP patch antenna while maintaining the low profile feature continues to be very challenging. In the open literature, one of the most common methods is the use of a low dielectric constant substrate in which an air gap is introduced between the ground plane and the radiating element [5][6][7][8][9][10][11][12]. In fact, this method can significantly increase the BW up to approximately 24% [12]; however, the antenna's profile is typically higher than 0.1λ o at the center operating frequency. Alternatively, wideband performance is also attained by using a meandering probe feed technique [13,14]. Nonetheless, multiple stacked substrates are required in such designs, leading to a high profile configuration as well. To overcome this deficiency, another effective solution is the combination of the fundamental mode and high-order mode of the square patch presented in [15]. Although the resulting antenna has a very low profile of 0.01λ o , its operating BW is limited by 15.8%.
In recent years, the metasurface (MS) has been widely applied to obtain BW enhancement for microstrip patch or slot antennas [16][17][18][19][20][21][22][23][24][25]. A MS layer is commonly constructed with an array of periodical uniform unit cells of various shapes, such as square, rectangular, H-shape, etc. Then, the MS is arranged surrounding, above or below the primary radiating element. In general, this antenna type is able to exhibit a wide operation BW, which is typically less than 25%, with a low profile configuration of lower than 0.1λ o . To the best of our knowledge, the best performance with more than 30% BW was obtained in [19] by using a modified cross-slot coupled with a uniform MS.
In this paper, a MS-based low profile CP antenna with more than 30% operating BW is proposed. It is noted that, unlike conventional CP MS antennas, in which the MS layer is comprised of multiple uniform unit cells, our antenna employs a different approach; here, the non-uniform MS is utilized and coupled with a truncated corner square patch to achieve a wide operating BW. To make the design procedure straightforward, the BW enhancement using uniform MS is considered first. Next, the antenna is designed with a non-uniform MS.

Antenna Geometry
The geometrical configuration of the wideband CP antenna with uniform MS is illustrated in Figure 1. The antenna is constructed with a driven microstrip patch, an MS and a ground plane. The antenna is fabricated on two substrates with a dielectric constant of ε r = 3.5 and loss tangent of tan δ = 0.0018. The driven patch with truncated corners, acting as a primary radiating CP source for the proposed design, is located between the MS layer and the ground layer. The MS is formed by different sizes of unit cells, in which the four center cells are larger than the others. The antenna is excited by the 50 Ω SMA (Sub Miniature version A) connector, whose outer and inner conductors are respectively connected to the ground plane and the driven patch. The high-frequency structure simulator (HFSS) is used to characterize the antenna and then validated by measurement.
Appl. Sci. 2020, 10, x FOR PEER REVIEW 2 of 11 Alternatively, wideband performance is also attained by using a meandering probe feed technique [13,14]. Nonetheless, multiple stacked substrates are required in such designs, leading to a high profile configuration as well. To overcome this deficiency, another effective solution is the combination of the fundamental mode and high-order mode of the square patch presented in [15]. Although the resulting antenna has a very low profile of 0.01λo, its operating BW is limited by 15.8%. In recent years, the metasurface (MS) has been widely applied to obtain BW enhancement for microstrip patch or slot antennas [16][17][18][19][20][21][22][23][24][25]. A MS layer is commonly constructed with an array of periodical uniform unit cells of various shapes, such as square, rectangular, H-shape, etc. Then, the MS is arranged surrounding, above or below the primary radiating element. In general, this antenna type is able to exhibit a wide operation BW, which is typically less than 25%, with a low profile configuration of lower than 0.1λo. To the best of our knowledge, the best performance with more than 30% BW was obtained in [19] by using a modified cross-slot coupled with a uniform MS.
In this paper, a MS-based low profile CP antenna with more than 30% operating BW is proposed. It is noted that, unlike conventional CP MS antennas, in which the MS layer is comprised of multiple uniform unit cells, our antenna employs a different approach; here, the non-uniform MS is utilized and coupled with a truncated corner square patch to achieve a wide operating BW. To make the design procedure straightforward, the BW enhancement using uniform MS is considered first. Next, the antenna is designed with a non-uniform MS.

Antenna Geometry
The geometrical configuration of the wideband CP antenna with uniform MS is illustrated in Figure 1. The antenna is constructed with a driven microstrip patch, an MS and a ground plane. The antenna is fabricated on two substrates with a dielectric constant of εr = 3.5 and loss tangent of tan δ = 0.0018. The driven patch with truncated corners, acting as a primary radiating CP source for the proposed design, is located between the MS layer and the ground layer. The MS is formed by different sizes of unit cells, in which the four center cells are larger than the others. The antenna is excited by the 50 Ω SMA (Sub Miniature version A) connector, whose outer and inner conductors are respectively connected to the ground plane and the driven patch. The high-frequency structure simulator (HFSS) is used to characterize the antenna and then validated by measurement.

Top view
Side view

Antenna with Uniform MS
In order to achieve wideband operation for both reflection coefficient (S 11 ) and axial ratio (AR) BWs, the driven patch and the MS are designed to operate at adjacent frequency bands. Here, the driven patch is designed so that it can radiate CP waves. First, a square patch with dimensions of W p × W p is chosen to work at the fundamental TM 01 mode (f r ). W p can be approximately calculated based on the following equation: where c is the speed of light and ε eff is the effective dielectric constant of the substrate. Then, two opposite corners are truncated to produce two orthogonal modes. By tuning d, the magnitudes and the phases of these orthogonal modes can be adjusted so that they have an equal magnitude and are 90 • out of phase for CP radiation. W p is optimized around the predicted value according to Equation (1), while d only has a significant effect on the AR value. The optimal values of W p and d can be obtained through the simulation.
To broaden the operating BW of the square patch antenna, the MS is designed to allow operation in a higher frequency band. This phenomenon has been extensively investigated in [17,26]. According to these studies, the resonances of the surface waves propagating on the MS can be qualitatively calculated by where β sw is the propagation constant and N and P are the unit cell number and its periodicity, respectively. Figure 2 presents the dispersion diagram, which shows the phase shift of the single unit cell at a given frequency at the first two eigenmodes, TM (transverse magnetic) and TE (transverse electric). Here, when the phase shift is 90 • , the surface wave resonant frequencies of the MS, which are defined by the intersection between the vertical lines (β SW P = 90 • ) and the dispersion curves, are around 5.7 and 6.2 GHz. According to Equation (2), the number of unit cells is 2 × 2 with m = 1 and 4 × 4 with m = 2. Since the MS is the radiating aperture of the antenna, a larger MS will have a larger aperture, leading to higher gain radiation. Thus, we choose m = 2 to obtain high gain radiation for the proposed design. It is noted that the number of unit cells can be increased; however, the drawback is large antenna dimensions.

Antenna with Uniform MS
In order to achieve wideband operation for both reflection coefficient (S11) and axial ratio (AR) BWs, the driven patch and the MS are designed to operate at adjacent frequency bands. Here, the driven patch is designed so that it can radiate CP waves. First, a square patch with dimensions of Wp × Wp is chosen to work at the fundamental TM01 mode (fr). Wp can be approximately calculated based on the following equation: where c is the speed of light and εeff is the effective dielectric constant of the substrate. Then, two opposite corners are truncated to produce two orthogonal modes. By tuning d, the magnitudes and the phases of these orthogonal modes can be adjusted so that they have an equal magnitude and are 90° out of phase for CP radiation. Wp is optimized around the predicted value according to Equation (1), while d only has a significant effect on the AR value. The optimal values of Wp and d can be obtained through the simulation.
To broaden the operating BW of the square patch antenna, the MS is designed to allow operation in a higher frequency band. This phenomenon has been extensively investigated in [17,26]. According to these studies, the resonances of the surface waves propagating on the MS can be qualitatively calculated by where βsw is the propagation constant and N and P are the unit cell number and its periodicity, respectively. Figure 2 presents the dispersion diagram, which shows the phase shift of the single unit cell at a given frequency at the first two eigenmodes, TM (transverse magnetic) and TE (transverse electric). Here, when the phase shift is 90°, the surface wave resonant frequencies of the MS, which are defined by the intersection between the vertical lines (βSWP = 90°) and the dispersion curves, are around 5.7 and 6.2 GHz. According to Equation (2), the number of unit cells is 2 × 2 with m = 1 and 4 × 4 with m = 2. Since the MS is the radiating aperture of the antenna, a larger MS will have a larger aperture, leading to higher gain radiation. Thus, we choose m = 2 to obtain high gain radiation for the proposed design. It is noted that the number of unit cells can be increased; however, the drawback is large antenna dimensions.  The simulated performance of the antenna without an MS and with a uniform MS (Wi = Wo) is compared and depicted in Figure 3. As observed, significant improvements in both −10 dB |S11| and The simulated performance of the antenna without an MS and with a uniform MS (W i = W o ) is compared and depicted in Figure 3. As observed, significant improvements in both −10 dB |S 11 | and 3 dB AR BWs can be obtained with the presence of MS. In the |S 11 | profile, besides the lower resonances at 4.7 and 5.1 GHz produced by the driven patch, there are also two additional resonances at 5.8 and 6.3 GHz. These resonances are generated by the MS and are quite well matched with the prediction based on the dispersion diagram in Figure 2. This phenomenon has been thoroughly investigated in [26]. In terms of AR, there are two CP bands at around 5.1 and 5.7 GHz, which are respectively produced by the driven patch and the MS. For better demonstration, Figure 4 presents the simulated AR of the antenna for different sizes of the patch (W p ) and the unit cell (W i ). It can be observed that tuning W p has a strong impact on the lower CP resonance, which shifts to a higher band as W p decreases. Meanwhile, the higher CP resonance is almost stable with the variation of W p . The data confirm that the patch is critical for the lower band operation. A similar behavior with higher CP resonance when tuning W i can be observed in Figure 4b. However, as the MS is an effective radiating aperture of the proposed antenna, the variation of W i also has a significant effect on the lower frequency, which shifts downwards as W i increases.
3 dB AR BWs can be obtained with the presence of MS. In the |S11| profile, besides the lower resonances at 4.7 and 5.1 GHz produced by the driven patch, there are also two additional resonances at 5.8 and 6.3 GHz. These resonances are generated by the MS and are quite well matched with the prediction based on the dispersion diagram in Figure 2. This phenomenon has been thoroughly investigated in [26]. In terms of AR, there are two CP bands at around 5.1 and 5.7 GHz, which are respectively produced by the driven patch and the MS. For better demonstration, Figure 4 presents the simulated AR of the antenna for different sizes of the patch (Wp) and the unit cell (Wi). It can be observed that tuning Wp has a strong impact on the lower CP resonance, which shifts to a higher band as Wp decreases. Meanwhile, the higher CP resonance is almost stable with the variation of Wp. The data confirm that the patch is critical for the lower band operation. A similar behavior with higher CP resonance when tuning Wi can be observed in Figure 4b. However, as the MS is an effective radiating aperture of the proposed antenna, the variation of Wi also has a significant effect on the lower frequency, which shifts downwards as Wi increases.    resonances at 4.7 and 5.1 GHz produced by the driven patch, there are also two additional resonances at 5.8 and 6.3 GHz. These resonances are generated by the MS and are quite well matched with the prediction based on the dispersion diagram in Figure 2. This phenomenon has been thoroughly investigated in [26]. In terms of AR, there are two CP bands at around 5.1 and 5.7 GHz, which are respectively produced by the driven patch and the MS. For better demonstration, Figure 4 presents the simulated AR of the antenna for different sizes of the patch (Wp) and the unit cell (Wi). It can be observed that tuning Wp has a strong impact on the lower CP resonance, which shifts to a higher band as Wp decreases. Meanwhile, the higher CP resonance is almost stable with the variation of Wp. The data confirm that the patch is critical for the lower band operation. A similar behavior with higher CP resonance when tuning Wi can be observed in Figure 4b. However, as the MS is an effective radiating aperture of the proposed antenna, the variation of Wi also has a significant effect on the lower frequency, which shifts downwards as Wi increases.

Antenna with Non-Uniform MS
Next, the MS is modified with different sizes of unit cells (W i W o ) to achieve better performance. The simulated performances of the antenna with a uniform MS and non-uniform MS are illustrated in Figure 5. It can be seen that, by using a non-uniform MS, an additional CP band around 6.8 GHz is produced, which contributes to significantly increasing the antenna's overall 3 dB AR BW to 33.3% (5.0-7.0 GHz). The additional CP resonance can be adjusted by tuning the size of the outer MS unit cell, W o . Figure 6 shows the antenna's AR characteristics for different values of W o . Obviously, tuning W o has a significant influence on the CP band at around 6.8 GHz, which increases as W o is decreased. and (b) unit cell size (Wi = Wo).

Antenna with Non-Uniform MS
Next, the MS is modified with different sizes of unit cells (Wi ≠ Wo) to achieve better performance. The simulated performances of the antenna with a uniform MS and non-uniform MS are illustrated in Figure 5. It can be seen that, by using a non-uniform MS, an additional CP band around 6.8 GHz is produced, which contributes to significantly increasing the antenna's overall 3 dB AR BW to 33.3% (5.0-7.0 GHz). The additional CP resonance can be adjusted by tuning the size of the outer MS unit cell, Wo. Figure 6 shows the antenna's AR characteristics for different values of Wo. Obviously, tuning Wo has a significant influence on the CP band at around 6.8 GHz, which increases as Wo is decreased.  Since the MS is a radiating aperture of the proposed antenna, the surface currents (Js) on the MS are considered to demonstrate the CP radiation of the proposed antenna. Figure 7 shows the simulated Js at the three different minimum AR points of 5.2, 6.2 and 6.8 GHz. It can be seen that, in these cases, when the phase changes from 0° to 90°, the vector Js is rotated in a clockwise direction. Thus, the antenna will generate left-hand CP (LHCP) radiation in the broadside direction. At 5.2 GHz, this resonance is created by the driven patch, and thus the Js is highly concentrated at the four center unit cells of the MS. At 6.2 and 6.8 GHz, more currents are occupied in the outer unit cells because these resonances are generated by the surface waves propagating on the MS. However, it can be seen

Antenna with Non-Uniform MS
Next, the MS is modified with different sizes of unit cells (Wi ≠ Wo) to achieve better performance. The simulated performances of the antenna with a uniform MS and non-uniform MS are illustrated in Figure 5. It can be seen that, by using a non-uniform MS, an additional CP band around 6.8 GHz is produced, which contributes to significantly increasing the antenna's overall 3 dB AR BW to 33.3% (5.0-7.0 GHz). The additional CP resonance can be adjusted by tuning the size of the outer MS unit cell, Wo. Figure 6 shows the antenna's AR characteristics for different values of Wo. Obviously, tuning Wo has a significant influence on the CP band at around 6.8 GHz, which increases as Wo is decreased. Since the MS is a radiating aperture of the proposed antenna, the surface currents (Js) on the MS are considered to demonstrate the CP radiation of the proposed antenna. Figure 7 shows the simulated Js at the three different minimum AR points of 5.2, 6.2 and 6.8 GHz. It can be seen that, in these cases, when the phase changes from 0° to 90°, the vector Js is rotated in a clockwise direction. Thus, the antenna will generate left-hand CP (LHCP) radiation in the broadside direction. At 5.2 GHz, this resonance is created by the driven patch, and thus the Js is highly concentrated at the four center unit cells of the MS. At 6.2 and 6.8 GHz, more currents are occupied in the outer unit cells because these resonances are generated by the surface waves propagating on the MS. However, it can be seen Since the MS is a radiating aperture of the proposed antenna, the surface currents (J s ) on the MS are considered to demonstrate the CP radiation of the proposed antenna. Figure 7 shows the simulated J s at the three different minimum AR points of 5.2, 6.2 and 6.8 GHz. It can be seen that, in these cases, when the phase changes from 0 • to 90 • , the vector J s is rotated in a clockwise direction. Thus, the antenna will generate left-hand CP (LHCP) radiation in the broadside direction. At 5.2 GHz, this resonance is created by the driven patch, and thus the J s is highly concentrated at the four center unit cells of the MS. At 6.2 and 6.8 GHz, more currents are occupied in the outer unit cells because these resonances are generated by the surface waves propagating on the MS. However, it can be seen obviously that at 6.9 GHz, the J s on the outer unit cells is very strong. This explains why an additional band can be obtained by modifying the uniform MS to a non-uniform MS.
Finally, the impedance matching of the proposed design is considered. The parameter studies indicate that the feeding position is a critical parameter to determine the antenna's matching performance. Figure 8 shows the simulated reflection coefficients and ARs against the variation of feeding position, L f . Here, increasing L f results in a significant improvement in the matching performance. Regarding the AR response, tuning the feeding position changes the length of the feeding line. This variation will lead to different coupling scenarios between the driven patch and the MS. Thus, the AR characteristic, particularly in the high-frequency band, is affected.
obviously that at 6.9 GHz, the Js on the outer unit cells is very strong. This explains why an additional band can be obtained by modifying the uniform MS to a non-uniform MS.
Finally, the impedance matching of the proposed design is considered. The parameter studies indicate that the feeding position is a critical parameter to determine the antenna's matching performance. Figure 8 shows the simulated reflection coefficients and ARs against the variation of feeding position, Lf. Here, increasing Lf results in a significant improvement in the matching performance. Regarding the AR response, tuning the feeding position changes the length of the feeding line. This variation will lead to different coupling scenarios between the driven patch and the MS. Thus, the AR characteristic, particularly in the high-frequency band, is affected.

Design Procedure
Based on the abovementioned investigation, the antenna design procedure can be summarized as follows:


Step 1: Design the truncated corner square patch with operation in the low-frequency band. The

Design Procedure
Based on the abovementioned investigation, the antenna design procedure can be summarized as follows: • Step 1: Design the truncated corner square patch with operation in the low-frequency band. The size W p will have a strong effect on the operating frequency, and it is initially defined by Equation (1). Meanwhile, the AR value is tuned by the corner truncation parameter, d.

•
Step 2: Design the unit cell with operation in the high-frequency band. The operating frequency can be predicted based on the dispersion diagram. The number of unit cells (N) is chosen as a trade-off between the antenna's overall size and the gain. A higher gain can be achieved with a greater number of cells, but the antenna's size is consequently increased.

•
Step 3: Design the antenna with a uniform MS, tuning the patch size and unit cell size to allow operation at the desired band. The AR value is mainly determined by the truncation parameter d.

•
Step 4: Design the antenna with a non-uniform MS. In this step, the size of the four center unit cells is slightly changed in comparison with the optimal value achieved in Step 3. Meanwhile, the size of the outer unit cells is significantly tuned for a higher CP operating band. The matching performance and the AR value are optimized by tuning the dimensions of the stub and the corner truncation of the patch.

Measurement Results
The proposed antenna was fabricated and experimentally checked. The PNA Network Analyzer N5224A (Keysight, Santa Rosa, CA, USA) was used to measure the antenna's reflection coefficient, and the far-field characteristics were implemented by the Microwave Technologies Group [27]. Generally, the measured data matched well with the simulated data. There were still several discrepancies, which were caused by the tolerance in fabrication and the imperfection of the measurement setup.
The simulated and measured |S 11 | results of the proposed MS-based antenna are illustrated in Figure 9. The antenna showed good matching performance in a wide frequency range from 4.7 to 7.2 GHz, equivalent to 42.1%. Figure 10 depicts the far-field radiation characteristics in terms of AR and gain in the boresight direction. As observed, the measured 3 dB AR BW was 30% (5.1-6.9 GHz); furthermore, the measured gain was from 5.6 to 7.6 dBic.
Appl. Sci. 2020, 10, x FOR PEER REVIEW 8 of 11 by the difference between the right-hand CP (RHCP) level and LHCP level is higher than 16 dB. Besides, the front-to-back ratio is always greater than 12 dB across the operating band.    Figure 11 plots the antenna gain radiation patterns in the x-z and y-z planes at three different frequencies of 5.2, 6.2 and 6.8 GHz. It can be seen that LHCP radiation is the dominant operating mode of the proposed antenna. In the broadside direction (θ = 0 • ), the polarization isolation defined by the difference between the right-hand CP (RHCP) level and LHCP level is higher than 16 dB. Besides, the front-to-back ratio is always greater than 12 dB across the operating band.     Finally, the comparisons between the low-profile, wideband CP antennas are summarized and presented in Table 1. The data indicate that, except for [19], the proposed antenna shows the best operating BW with an acceptable gain and overall size. Although the designs in [19,20] have a wide Finally, the comparisons between the low-profile, wideband CP antennas are summarized and presented in Table 1. The data indicate that, except for [19], the proposed antenna shows the best operating BW with an acceptable gain and overall size. Although the designs in [19,20] have a wide operating BW of around 30%, their lateral dimensions are larger and the maximum gains are also lower than those of the proposed antenna. The structures in [5,8,9,11,12] exhibit high gain radiation; however, a high profile is their critical drawback. Meanwhile, the antennas in [15,16] have the advantage of a lower profile, but their operating BW is significantly smaller than that of the presented design.

Conclusions
A non-uniform MS-based wideband CP antenna has been presented and investigated in this paper. This paper utilizes a non-uniform MS, which has the potential to not only increase the impedance BW but also to produce two additional CP bands to significantly widen the overall 3 dB AR BW of the proposed design. The measurements implemented on a fabricated antenna demonstrate that a wide operating BW of 30% (5.1-6.9 GHz) can be attained. Additionally, the antenna also exhibits good radiation characteristics and a gain in the broadside direction better than 5.6 dBic. The proposed antenna can be a potential candidate for communication systems in the C-band, such as wireless local area networks (WLANs), satellites, cordless telephones and so on.

Conflicts of Interest:
The authors declare no conflict of interest.