Broadband Multi-Shaped Metasurface Circularly Polarized Antenna With Suppressed Non-CP Radiation Modes

In this research, a multi-shaped metasurface broadband circularly polarized (CP) patch antenna with parasitic elements is proposed for 5G new radio (NR) applications. The proposed metasurface CP patch antenna comprises triple-layered substrates without air gap. The upper layer sits with multi-shaped metasurface elements and parasitic patches. The middle layer consists of an L-shaped slot functioning as the ground plane, and the lower layer contains a microstrip and a fan-shaped stub functioning as the feed line. The proposed metasurface CP patch antenna with parasitic elements is evaluated using characteristic mode analysis (CMA). The CMA results indicate that the modal significance of Modes 1 and 2 of the multi-shaped metasurface CP antenna are orthogonal, giving rise to circular polarization. The non-CP radiation of Modes 3 and 4 are suppressed by using the multi-shaped metasurface elements and parasitic patches. The measured impedance bandwidth and axial ratio bandwidth are 42.85% (3.4 – 4.9 GHz) and 38% (3.27 – 4.6 GHz), achieving the maximum gain of 7.23 dBic at 3.7 GHz. The experiments demonstrate that the multi-shaped metasurface CP patch antenna with parasitic elements is suitable for 5G NR wireless applications. The novelty of this study is attributed to its utilization of multi-shaped metasurface elements and parasitic patches, which effectively suppress non-circularly polarized radiation modes.

Circular polarization is a type of electromagnetic wave polarization in which the electric field vectors rotate in a circular motion as it propagates through space [7].The rotation is either clockwise or counterclockwise.Circularly polarized (CP) waves are less susceptible to multipath interference and losses, making the CP waves suitable for wireless communication applications [8].
Characteristic mode analysis (CMA) is a methodology employed to assess the current distribution and radiation patterns of antennas [9].In addition, CMA is used to determine the antenna configuration and dimension, based on the total surface current on a perfect electric conductor.
In modern antennas, metasurface CP patch antennas are deployed in a variety of applications [10], [11], and [12].The metasurface CP antenna structure typically consists of periodic elements of thin metal sheet in subwavelength and external sources (e.g., microstrip feed line or aperture-coupled feed).The metasurface CP antennas are compact, lightweight, and easy to fabricate.Besides, various shapes of periodic metasurface elements are proposed to convert linearly polarized waves into CP waves [10], [11], [12].
In [10], CP patch antenna with an S-shaped metasurface structure was proposed for C-band uplink applications.The patch antenna achieved an impedance bandwidth (IBW) of 43.22%, covering 4.05 -6.6 GHz; and axial ratio bandwidth (ARBW) of 22%, covering 5.3 -6.6 GHz.In [11], a fourcluster metasurface CP array antenna was proposed for Cband small satellites.The array antenna achieved an IBW of 84.74%, covering 4 -9 GHz, and ARBW of 57.6% for the frequency bands that fall within the range of 4.2 -7.6 GHz.In [12], an asymmetric metasurface CP microstrip antenna could achieve an IBW of 15.7% (1.58 -1.85 GHz) and ARBW of 13% (1.58 -1.8 GHz).However, none of these research works use the CMA technique to realize the CP radiation.
In [13], a four-cluster CP patch antenna array with leafshaped metasurface structure characterized by CMA could achieve an IBW of 62.5% (from 3.4 GHz to 5.9 GHz) and ARBW of 21% (from 3.8 GHz to 4.54 GHz).In [14], a CMA-characterized H-shaped metasurface CP patch antenna was proposed for C-band communication.The patch antenna achieved an IBW of 38.8% within the frequency range of 4.42 -6.55 GHz, and ARBW of 14.3% occurring the spectrum range of 5.2 -6 GHz.In [15], a CMA-characterized irregular-shaped metasurface CP antenna array was proposed for C-and X-band applications.The antenna array achieved an IBW of 49.6%, occurring the frequency between 6.23 -10.37 GHz, and ARBW of 33.13% between the spectrum range from 7 GHz to 9.78 GHz.However, no attempts have been made to suppress the non-CP radiation modes, resulting in narrow ARBW.
This research thus proposes a broadband multi-shaped CP patch antenna with parasitic elements for 5G NR wireless technology.The proposed multi-shaped metasurface patch antenna is defined by CMA.The non-CP radiation modes are suppressed by using multi-shaped metasurface elements and parasitic patches.The feed structure is made up of an L-shaped slot ground plane and a microstrip feed line, exciting the CP modes.Simulations are performed using CST Studio Suite and experimented carried out with an antenna prototype.The performance The upper layer contains multi-shaped metasurface elements and parasitic elements (i.e., corner-truncated squareshaped patches).The multi-shaped metasurface includes four square-shaped metasurface elements, eight octagonal-shaped metasurface elements, and four corner-truncated squareshaped metasurface elements.The middle layer contains an L-shaped slot that functions as the ground plane.On the lower layer sits a microstrip and a fan-shaped stub functioning as the feed line.The stub is utilized to enable impedance matching.

B. THEORY OF CHARACTERISTIC MODES
The theory of characteristic mode analysis (CMA) is used to define the configuration and dimension of the antenna parameters, characterizing the resonant frequency [16].The CMA utilizes the entire surface current on the perfect electric conductor (PEC) structure.The entire surface current (J(ω)) on the PEC structure is evaluated by equation (1) [17].
where α(ω) is the modal weighting coefficient (MWC), J(ω) is the characteristic current, and N is the number of modes.The MWC is calculated by equation ( 2) [18].
where λ n (ω) is the characteristic eigenvalue and v i n (ω) is the modal excitation coefficient (MEC), which is calculated by equation (3) [19].
where E i is the surface area excitation on the PEC structure.The modal significance (MS ≥ 0.707) is well-known to represent the characteristic current (J(ω)) and antenna radiation modes.The phases difference between two orthogonal modes are investigated by characteristic angle (CA).The MS of two orthogonal modes must be identical and their CA is approximately ±90 • to realize the CP radiation.Equations ( 4) and ( 5) are used to evaluate the MS and CA [20].

C. DEVELOPMENT PROCEDURES OF METASURFACE SCHEMES
The antenna design was simulated using  elements (11 mm × 11 mm).In Model-III, the lateral metasurface elements are transformed into the metasurface elements of octagonal shape (15 mm × 15 mm).In Model-IV, the Model-III scheme is surrounded by corner-truncated square-shaped elements (the parasitic elements; 8 mm × 8 mm).The surface currents are related to the characteristic current J(ω) in equation ( 1), which are represented by MS in equation ( 4).Besides, the characteristic eigenvalue (λ n (ω)) indicates each mode status: λ n (ω) > 0 is inductance mode; λ n (ω) = 0 is resonant mode; and λ n (ω) < 0 is capacitance mode [21].To suppress the resonant mode of Modes 3 and 4 (non-CP radiation modes), the characteristic eigenvalue must not equal 0 between specific frequency ranges.Thus, the surrounding parasitic patches are used to reduce the surface currents on the multi-shaped metasurface elements of Modes 3 and 4.
The MS of non-CP radiation modes (Modes 3 and 4) can be suppressed by manipulating the metasurface elements, with no effect on the CP radiation modes (Modes 1 and 2).First, the metasurface elements at the four corners are truncated into corner-truncated metasurface elements (Model-II).Next, the lateral metasurface elements are transformed into the metasurface elements of octagonal shape (Model-III).Then, the multi-shaped metasurface elements are surrounded by corner-truncated square-shaped elements or parasitic elements (Model-IV).In Figure 5(c), The eigenvalues of Modes 1 -4 in Model-II exhibit values close to zero, indicating resonance occurring between the frequency range of 3 -4.3GHz.In Mode 1, the eigenvalue is decreased (almost 0) between 4.3 -4.6 GHz while in Mode 2 is negligible increased.As a result, Modes 1 and 2 could generate CP radiation between 3 -4.6GHz.On the other hand, Mode 4 is almost 0 between 4.3 -4.6 GHz, giving rise to non-CP resonant mode interference.Thus, Mode 4 must be suppressed by reducing surface currents.In Figure 5(d), the phase differences between Modes 1 and 2 of simulated CA are 0 • .Meanwhile, the surface currents around the metasurface elements at the four corners of Model-II at 3.5 GHz (the center frequency) are substantially decreased, as shown in Figure 5(e).
Figure 6(a) shows the geometry of Model-III.The MS of Mode 4 is suppressed by transforming four pairs of the lateral metasurface elements into the octagonal-shaped metasurface elements.The simulated MS of Modes 1 -4 are all equal to 1 at frequencies of 3.45 GHz, 3.44 GHz, 3.74 GHz, and 3.76 GHz, as shown in Figure 6(b).This means that the antenna resonant frequency is between 3.44 and 3.76 GHz.The simulated MS of Modes 3 and 4 shift to higher frequency (from originally 3.59 GHz and 3.6 GHz for Model-II to 3.74 GHz and 3.76 GHz for Model-III).
In Figure 6(c), the eigenvalues of Modes 1 -4 in Model-III are nearly 0 which indicates resonance frequency between 3.3 -4.4 GHz.The eigenvalues of Mode 4 are sharply increased between 4.4 -5 GHz for resonant mode suppression.Meanwhile, the eigenvalues of Mode 3 declined rapidly.Thus, Mode 3 should be further suppressed.The phase differences between Modes 1 and 2 of simulated CA are 0 • , as shown in Figure 6(d).In Figure 6(e), the surface currents around the lateral octagonal-shaped metasurface elements of Model-III at 3.5 GHz are significantly decreased.
Figure 7(a) shows the geometry of Model-IV.The surrounding parasitic elements are used to further suppress the surface current of Modes 3 and 4. The simulated MS of Modes 1 -4 are all equal to 1 at 3.45 GHz, 3.44 GHz, 3.68 GHz, and 3.75 GHz, as shown in Figure 7(b).The antenna resonant frequency is between 3.44 and 3.75 GHz, as indicated by the simulated MS (Modes 1 -4).The simulated MS (Modes 3 and 4) of Model-IV slightly shifts to higher frequency (from originally 3.74 GHz and 3.68 GHz for Model-III to 3.76 GHz and 3.75 GHz for Model-IV).
In Figure 7(c), the eigenvalues of Modes 1 -4 of Model-IV are rather steady (nearly 0) between 3.5 -4.3 GHz for resonant modes.The eigenvalues of Modes 3 and 4 are sharply increased between 4.4 -5 GHz because of the surrounding parasitic elements.As a result, the CP radiation modes (Modes 1 and 2) could be attained within 3 -4.6GHz of the frequency ranges.The phase difference between Modes 1 and 2 of simulated CA is 0 • , as shown in Figure 7(d).
In Figure 7(e), the parasitic elements reduce the surface current around the metasurface elements at the four corners (Mode 3) and along the edges of the lateral metasurface elements (Mode 4).To achieve CP radiation, an external source (i.e., L-shaped slot ground plane) must excite Modes 1 and 2 of the multi-shaped metasurface elements and parasitic patches.Due to the right-hand circular polarization (RHCP), the surface current distribution must be traveled in an orthogonal direction between two modes (Modes 1 and 2) while rotating in a clockwise direction (all-time phase variation).In all scheme models, the current distribution is demonstrated at 0 • of the time phase variation.
In Model-IV, the parasitic elements around the metasurface scheme are used to reduce the non-CP modes coupling between the metasurface elements in all directions, while maintaining the two orthogonal modes.The flowing currents from the metasurface elements to parasitic elements generated a high resonant frequency.In comparison, the ARBW difference between Model-III and Model-IV is between 4.12 -4.61 GHz (Figure 15(b)).
The non-CP radiation modes (Modes 3 and 4) are suppressed by transforming the peripheral metasurface elements and surrounding it with the parasitic elements (i.e., Model-IV).The transformed metasurface elements and the parasitic    must be located underneath the intensive magnetic current of Model-IV.
The magnetic current of Model-IV can be determined by the modal magnetic field.In Figure 9, the intensive modal magnetic field at 3.5 GHz of Modes 1 and 2 of Model-IV.The L-shaped slot excitation is calculated by equation ( 6) [22].
where H n (ω) is the modal magnetic field of the number of n mode and M(ω) is the magnetic current of the L-shaped slot ground plane.The parametric study is performed to determine the results of different feed structures on IBW and ARBW of the CMAcharacterized multi-shaped metasurface CP patch antenna with parasitic elements.In this research, the feed structure refers to the L-shaped slot ground plane on the middle layer and the microstrip feed line on the lower layer.The parameters being studied include the microstrip feed line length (L f ), the microstrip feed line width (W f ), the width of the L-shaped slot ground plane (L a ), the horizontal length of the L-shaped ground plane (L b1 ), and the vertical length of the L-shaped ground plane (L b2 ).The simulated IBW and ARBW under variable W f (i.e., 1, 3, and 5 mm) are illustrated in Figures 11(a) and (b).In Figure 11(a), given W f = 1 mm and 5 mm, an impedance mismatch occurs between 3.68 -4.25 GHz and between 4.08 -4.5 GHz, respectively.With W f = 3 mm, IBW is between 3.4 -4.92 GHz.The optimal W f is 3 mm.In Figure 11(b), the results also show that W f has a negligible effect on ARBW of the proposed metasurface CP patch antenna.
The simulated IBW and ARBW under variable L a (i.e., 1, 2, and 3 mm) are depicted in Figures 12(a) and (b), respectively.As shown in Figure 12(a), IBW are between 3.19 -3.36 GHz for L a = 1, between 3.4 -4.92 GHz for L a = 2, and between 4.3 -5 GHz for L a = 3.In Figure 12(b), AR > 3 dB between 3.94 -4.12 GHz for L a = 1 mm.ARBW are between 3.29 -4.61 GHz for L a = 2 mm and between 3.25 -4.62 GHz for L a = 3 mm.However, L a = 3 mm results in narrow IBW.The optimal L a is thus 2 mm.
The simulated IBW and ARBW under variable L b2 (13,16, and 19 mm) are illustrated in Figures 14(a  37.71% (3.29 -4.61 GHz), respectively (Figure 15(b)).In Model-II, ARBW becomes wider than that of Model-I.In Model-III, ARBW slightly shifts to higher frequency and becomes wider, compared to Model-II.In Model-IV, ARBW becomes wider than that of Model-III, resulting in broader ARBW.
In comparison, the corner-truncated square-shaped metasurface elements (Model-II) and the octagonal-shaped metasurface elements (Model-III) have an effect on high frequency due to the size reduction of the metasurface elements, giving rise to the S 11 and ARBW changed.However, IBW (|S 11 | ≤ -10 dB) and ARBW (AR ≤ 3 dB) have become wider at the high-frequency range.
In Figure 15(c), the simulated gains of Model-I, Model-II, Model-III, and Model-IV are 6.71, 7.05, 7.15, and 7.2 dBic at 3.4, 3.5, 3.7, and 3.7 GHz, respectively.The transformed metasurface elements and the surrounding parasitic elements cause the maximum gain of Model-II, Model-III, and Model-IV to shift to high frequency.In Model-IV, the surrounding parasitic elements have minimal effect on the maximum gain.Table 1 organizes the dimension parameters of the proposed CMA-characterized multi-shaped metasurface CP patch antenna with parasitic elements.
The simulated IBW, ARBW, gain, and electric field of Model-I, Model-II, Model-III, and Model-IV with the feed structure are shown in Figures 15(a The current distribution (center frequency) of the proposed metasurface CP patch antenna with parasitic elements is shown at four different phases: 0 • , 90 • , 180 • , and 270 • (Figure 16).Specifically, the electric field vectors on the multi-shaped metasurface elements and the parasitic elements traverse clockwise (right-hand circular polarization) in the +z direction.
Figures 17 show the prototype antenna of the proposed multi-shaped metasurface CP patch antenna with parasitic elements.Due to the fabrication limitations, the three outermost parasitic elements (upper layer) are replaced with ground plane to connect with an SMA connector (Figure 17(a)).Polytetrafluoroethylene (PTFE) Teflon spacers are used to secure the triple-layered substrates without air gap.

III. EXPERIMENTAL RESULTS
The prototype antenna performances were measured in an anechoic chamber, which used a vector network analyzer (ZNLE6 Rohde & Schwarz).A pair of ETS-Lindgren spiral antenna (3102 model series) and a prototype antenna functioned as transmitting and receiving antennas, respectively.The distance between the antennas was 4 meters [24].The spiral antennas were used to measure the polarization characteristics (RHCP and LHCP) of the prototype antenna (Figure 18).
The axial ratio (AR) of the prototype antenna was determined by measuring the co-polarized (RHCP) and cross-polarized (LHCP) electric fields between the (prototype antenna), corresponding to |E RHCP | and |E LHCP |, respectively.The AR is calculated by equation ( 7) [25].
where |E RHCP | and |E LHCP | are the electric field magnitudes of the RHCP and LHCP between the spiral and prototype antennas.
Figure 19(a) shows the comparison between simulated and measured IBW of the proposed CMA-characterized multi-shaped metasurface CP patch antenna with parasitic elements.In spite of the similar IBW between simulated and measured results of the proposed CMA-characterized multi-shaped metasurface CP patch antenna with parasitic elements, the S 11 between simulation and measurement is different within frequency ranges of 3.8 -4.5 GHz since the replacement of three outermost parasitic elements with a ground plane for the SMA connector.
Figure 19(b) illustrates the simulated and measured ARBW and gain.The simulated and measured results are agreeable.Specifically, the comparison between simulated and measured IBW is 43.4% (3.4 -4.92 GHz) and 42.85% (3.4 -4.9 GHz).The simulated and measured ARBW is 37.71% (3.29 -4.61 GHz) and 38% (3.27 -4.6 GHz).Moreover, the maximum gain at 3.7 GHz between simulated and measured results is 7.2 dBic and 7.23 dBic.Despite the unstable gain of the antenna, the proposed multishaped metasurface CP patch antenna with parasitic elements achieves a 3-dB boresight gain of 22.85% (3.2 -4 GHz) which corresponded to ARBW of 38% (3.27 -4.6 GHz) for CP radiation, resulting in the proposed antenna covered the entire the global 5G NR spectrum (3.3 -4 GHz) [26].To achieve a stable gain of the metasurface-based CP antenna, the metasurface structure could be symmetrically shaped and periodically arranged.Besides, slot plane with a feed could be optimized for achieving CP radiation [27].
In Figures the simulated and measured of the RHCP and LHCP radiation patterns were compared at frequency of 3.5, 4, and 4.5 GHz, respectively.The simulated and measured RHCP and LHCP are in good agreement.The proposed CP patch antenna with parasitic elements is RHCP characteristic (Figure 16).Thus, the co-polarization between transmitting antenna and receiving antenna is RHCP while the cross-polarization is LHCP.In other words, co-polarization (RHCP) refers to the radiation of an antenna in the intended direction (i.e., front of antenna), while cross-polarization is the antenna radiation in undesired directions.
At lower frequencies, the RHCP demonstrates negligible loss in transmission (far-field region), whereas higher frequencies result in substantial transmission loss.As a result, the power radiation difference between RHCP and LHCP is extremely high at low frequencies.On the other hand, the power difference between RHCP and LHCP is decreased at high frequencies.
Table 2 compares the previous metasurface CP patch antennas and the proposed CMA-characterized multi-shaped metasurface CP patch antenna with parasitic elements.In [13], a four-cluster CP patch antenna array with leafshaped metasurface structure achieves wide IBW but suffers from narrow ARBW.In [14], CP patch antenna with Hshaped metasurface structure has high antenna gain but its ARBW is narrow.In [15], CP patch antenna array with irregular rectangular-shaped metasurface structure achieves wide IBW, ARBW, and high gain.However, it also suffers from  a complicated feed network and large dimensions.In [28], a single-layered metasurface CP patch antenna achieves narrow IBW and ARBW.
In [29], a rectangular-ring-shaped metasurface CP patch antenna has very narrow IBW and ARBW.In [30], a square-shaped metasurface CP patch antenna with microstrip meander feed line achieves narrow IBW and ARBW while high gain.In [31], a low-profile irregular rectangular-shaped metasurface CP patch antenna has a very narrow IBW and ARBW.In [32], a low-profile multi-shaped metasurface CP sequentially rotated patch antenna achieves narrow IBW and ARBW.In [33], a low-complexity CP patch antenna with corner-truncated metasurface structure achieves a very narrow ARBW.
In [34], a low-profile uniformly metasurface CP patch antenna with dual microstrip feed line and circular-ring slot ground plane achieves wide IBW.However, the antenna requires high-complexity feed network.Unlike the previous works, this research suppresses the non-CP radiation modes by using the multi-shaped metasurface elements (i.e., cornertruncated square-shaped metasurface elements and the metasurface elements of octagonal shape) and the parasitic patches.The multi-shaped metasurface elements and the parasitic patches also improve the IBW and ARBW.As a result, the proposed multi-shaped metasurface CP patch antenna with parasitic elements is suitable for sub-6 GHz 5G NR applications.

IV. CONCLUSION
In this research, a multi-shaped metasurface broadband CP patch antenna with parasitic elements is proposed for 5G NR (sub-6 GHz) applications.The proposed metasurface CP patch antenna involves using triple FR-4 substrate layers stacked together without any air gap.The upper layer contains multi-shaped metasurface elements and cornertruncated square-shaped patches, and the middle layer consists of an L-shaped slot ground plane.The lower layer comprises a microstrip and a fan-shaped stub functioning as the feed line.The design of the proposed metasurface antenna involves four evolutionary stages: Model-I, Model-II, Model-III, and Model-IV, and the surface current distribution and radiation patterns of the four metasurface schemes are analyzed and characterized using CMA.
The CMA results of Model-IV (i.e., the CMAcharacterized multi-shaped metasurface CP patch antenna with parasitic elements) show that the modal significance of Modes 1 and 2 are orthogonal, resulting in circular polarization.The non-CP radiation of Modes 3 and 4 of the Model-IV scheme are suppressed by using the multi-shaped metasurface elements (i.e., corner-truncated square-shaped metasurface elements and octagonal-shaped metasurface elements) and the parasitic patches, resulting in wider ARBW.To excite the orthogonal modes, the L-shaped slot (middle layer) is located underneath the intensive magnetic current of the Model-IV scheme.The measured IBW and ARBW are 42.85% and 38%, over the frequency range of 3.4 -4.9 GHz and 3.27 -4.6 GHz.The antenna maximum gain is 7.23 dBic at 3.7 GHz.In addition, the comparison of the simulated and measured radiation patterns for RHCP and LHCP are agreeable.The proposed multi-shaped metasurface CP patch antenna with parasitic elements is therefore functional for 5G NR wireless communication.

FIGURE 1 .
FIGURE 1. Configuration of the proposed CMA-characterized multi-shaped metasurface CP patch antenna with parasitic elements: (a) front view, (b) rear view, (c) side view.

Figure 3 (
a) shows the geometry of Model-I, Figure 3(b) presents the simulated modal significance (MS) of Model-I, Figure 3(c) presents the simulated eigenvalue of Model-I, and Figure 3(d) presents the simulated characteristic angle (CA) of Model-I.The simulated MS of Modes 1 -4 of Model-I are equal to 1 at 3.31 GHz, 3.31 GHz, 3.47 GHz, 3.51 GHz, achieving the resonant frequency between 3.3 -3.51 GHz, as shown in Figure 3(b).In Figure 3(c), the eigenvalues of Modes 1 -4 of Model-I are almost 0 for resonate mode between 3 -4.3GHz.In Modes 2 and 4, the eigenvalues between 4.3 -5 GHz are slightly increased while Modes 1 and 3 sharply increased.As a result, Modes 1 -4 could generate the resonant frequency between 3 -4.3GHz.Thus, the eigenvalues of Mode 1 between 4.3 -5 GHz could be 0 for resonant frequency bandwidth enhancement and CP radiation.The phase differences of Modes 1&2 and 3&4 of simulated CA are almost 0 • , as shown in Figure 3(d).The simulated CA of Model-1 failed to achieve the 90 • phase difference.To further validate, the magnitude of the modal 3D radiation patterns and surface current distribution between Modes 1 and 2 of Model-I must be identical with orthogonal phase to realize the CP radiation.Figures 4(a)-(b) show the exemplary surface current distribution and the modal 3D radiation patterns of Model-I at 3.3 GHz.The surface current of Modes 1 and 2 of Model-I flow along the metasurface edges at the center of the metasurface scheme in respectively vertical and horizontal directions, as shown in Figure 4(a).The surface current of Mode 3 is concentrated around the four corners of Model-I,

Figure 5 (
a) illustrates the geometry of Model-II, where the metasurface elements at the four corners are truncated into the corner-truncated metasurface elements.The simulated MS of Modes 1 -4 are all equal to 1 at 3.36 GHz, 3.35 GHz, 3.59 GHz, and 3.6 GHz (Figure 5(b)).This indicates that the antenna resonant frequency is between 3.35 -3.6 GHz.The simulated MS of Modes 3 and 4 of Model-II slightly shift to higher frequency (from originally 3.47 GHz and 3.51 GHz for Model-I to 3.59 GHz and 3.6 GHz for Model-II).

Figure 8 (
a) illustrates the L-shaped slot ground plane, and Figure 8 (b) depicts the microstrip feed line.The L-shaped slot sits at the center of the ground plane (middle layer).The dimensions of the horizontal slot (L a × L b1 ) and vertical slot (L a × L b2 ) are 2 mm × 13.5 mm and 2 mm × 16 mm.The microstrip feed line consists of a feed line (W f × L f : 3 mm × 49 mm) and a fan-shaped stub (R: 4 mm).The linearly polarized waves along the X-and Y-axes are generated from the coupling between the microstrip feed line and the Lshaped slot ground plane.To excite the orthogonal modes (Modes 1 and 2), the L-shaped slot on the middle layer

FIGURE 8 .
FIGURE 8. Geometry of the feed structure: (a) L-shaped slot ground plane, (b) microstrip feed line.

FIGURE 9 .
FIGURE 9. Modal magnetic fields of Modes 1 and 2 of Model-IV at the center frequency (3.5 GHz).

Figures 10 (
a)-(b) show the simulated IBW and ARBW under different L f : 46, 49, and 52 mm.In Figure 10(a), IBW fails to cover the sub-6 GHz spectrum for L f = 46 mm.IBW are between 3.4 -4.92 GHz for L f = 49 mm and between 3.2 -3.48 GHz for L f = 52 mm.The AR is greater than 3 dB between 4.3 -4.43 GHz for L f = 46 mm (Figure 10(b)).ARBW is between 3.29 -4.61 GHz for L f = 49 mm and between 3.29 -4.6 GHz for L f = 52 mm.The optimal L f is 49 mm.

FIGURE 16 .FIGURE 17 .
FIGURE 16.Surface current distribution of the proposed CP patch antenna with parasitic elements at 3.5 GHz.

FIGURE 18 .
FIGURE 18.The measurement equipment in an anechoic chamber.

FIGURE 19 .
FIGURE 19.Comparison between simulated and measured IBW, ARBW and gain of the CMA-characterized metasurface CP patch antenna with parasitic elements: (a) IBW, (b) ARBW and gain.

FIGURE 20 .
FIGURE 20.Simulated and measured RHCP and LHCP radiation patterns of the proposed multi-shaped metasurface CP patch antenna with parasitic elements at: (a) 3.5 GHz, (b) 4 GHz, (c) 4.5 GHz.