Bandwidth Enhancement of Microstrip Patch Antenna Using Metasurface

— In this article, bandwidth enhancement of a microstrip patch antenna using a new kind of metasurface is discussed. This new geometry is used to generate new resonances in antenna and with an optimal position of the metasurface in relation to antenna we can overlap the resonances and obtain a large bandwidth. The proposed antenna showed a bandwidth from 5.1 GHz to 8.0 GHz what can it able to be applied in WiFi 5 and 6. Numerical results were obtained with Ansys HFSS software. A prototype was built and measurements for S 11 , Smith chart and gain were performed. Numerical and experimental results are in good agreement.

of microstrip antennas [7]. In recent years, patch antennas have been incorporated with EBG structures to improve surface wave suppression antenna performance, and are used as a ground plane [8], [9].
As an alternative to some limitations of the mentioned methods, the use of metasurfaces can be highlighted. Metasurfaces are typically two-dimensional arrays of small scatters or apertures in order to achieve some desirable electromagnetic behavior [10], [11]. The metasurface desired property is usually one that is not found in nature (negative permittivity and permeability, near-zero refractive index).
This work proposes a microstrip antenna coupled to a metasurface capable of operating in the WiFi 5 (5. 15 -5.725 GHz) and 6 (5.925 -7.125 GHz) bands. To make the antenna more compact and low profile, the patch and metasurface are put in direct contact, thus eliminating the air gap between them.
The aim is to achieve an antenna that operates from 5.1 GHz to 7.2 GHz with high gain in the entire band. The proposed structure is easy to build, inexpensive and compact compared to most previous configurations [6], [11] and has a higher bandwidth compared to [13], [15]. The results of the studies show that the metasurface characteristics and the unit cell pattern change the antenna impedance resulting in the overlapping of the new resonant frequencies, which allow a measured fractional bandwidth of 45.4 % (5.1 -8.0 GHz).

II. PREVIOUS WORKS
In [11], a broadband antenna coupled with a double-layered meta-surface is proposed. The metasurface consists of two metallic layers printed on two dielectric layers and each metallic layer contains an array of 4 × 4 square cells. The antenna is fed by coupling. This design makes the antenna work in two different operating modes, with resonant frequencies close to each other, increasing the bandwidth. The impedance bandwidth of -10 dB is about 43 % (from 4.08 to 6.38 GHz) and with a maximum realized gain of 11.6 dBi. The total size of the antenna is 65 × 65 mm 2 with a thickness of 5.03 mm.
In [12] an antenna combined with a fractal metasurface was proposed to increase bandwidth. The antenna patch is of the circular type with the insertion of four rectangular slots and six double rings on a FR4 substrate with 1.6 mm of thickness. The gain of the antenna is about 3.6 dB and 90 % efficiency with a size of 40 × 40 mm 2 . The final bandwidth was 2.3 -4.0 GHz.
In [13] the authors proposed a hybrid metasurface (HMS) to form an array of broadband antennas.
The antenna consists of a 4 × 4 matrix of square and metallic patches fed by a 50 Ω line through an Hshaped coupling slot on the ground plane. The total size of the array is 95 × 95 mm 2 with F4BTM substrate (r = 3.38, loss tangent of 0.0027) and 4.08 mm of thickness. The measurement shows an impedance bandwidth of 28 % (4.41 -5.85 GHz). The gain is 8.4 dBi in the operational band.
In [14], a low-profile microstrip antenna coupled with a metasurface as a ground plane was proposed. The metasurface is periodic with diamond-shaped unit cells. The antenna has a total size of 55 × 55 mm 2 and 0.5 mm of thickness. The antenna has an impedance bandwidth of 1.83 %, radiation efficiency of 83 % in the measurement and a maximum gain of 4.6 dBi.
In [15], an antenna with potential application in satellite communications was developed by coupling a metasurface to a microstrip antenna. The metasurface basically consists of four unit cells with parasitic square crossed gaps in an array of 2 × 2 elements. The bandwidth was 39.25 % (4.28 -6.37 GHz) and gain of 6.8 dBi. The total size of the antenna is 36 × 36 mm² on a FR4 substrate with a thickness of 3.5 mm.
In this work, we propose the use of a circular metasurface coupled to a patch antenna to increase bandwidth. The final volume of the antenna was approximately 3.82 cm 3 . We rotate the metasurface with respect to the antenna, to obtain the largest possible bandwidth. The antenna can be used for WiFi 5 and 6 applications. The final fractional bandwidth was 45.4 % and the peak gain was 3.8 dBi.
The fractional bandwidth is obtained for the -10 dB level of the S11 parameter. The performances of the work cited in this section are compared in Table I. The results are compared in terms of bandwidth, gain and antenna dimensions.

III. PROPOSED PATCH ANTENNA AND METASURFACE
This section demonstrates the proposed antenna design step by step. A traditional patch antenna is manufactured and optimized for a good impedance matching and low production cost. The substrate is FR4 and circular in shape with radius of R = 20 mm (εr = 4.4) and thickness of h = 1.524 mm. The antenna patch is a square with dimension 21.6 mm x 21.6 mm and was based on the one proposed in [20]. Four "L" -shaped slots of arm lengths equal to Ly = 8.55 mm and width Ws1 = 1.0 mm are incorporated into the patch as shown in Fig. 1.  The proposed metasurface is composed of an arrangement of unit cells periodically allocated along the XY-plane. It was designed on FR4 substrate, relative permittivity εr = 4.4, with single conductive side and circular shape radius of 20 mm and thickness h = 1.524 mm. The unit cell has the rectangular shape, inspired by the works [21][22][23], with insertion of a small circle with radius R = 1 mm. Fig. 4 illustrates the unit cell and the periodic arrangement of the proposed metasurface. The dimensions of the unit cell are: L1 = 8.0 mm, L2 = 1.0 mm, S1 = 11 mm, and S2 = 5 mm.
The proposed structure is designed by coupling the copper-free side of the metasurface to the top of the microstrip antenna and in direct contact with it, thus preventing air gap formation. As illustrated in Fig. 5. We used Teflon screws, which tightened the meta-surface and the antenna, without leaving an air layer between them.
It is important to show that the proposed cell behaves as a metamaterial in the frequency range of interest. Thus, we use the methodology proposed in [24], which uses an S-parameter retrieval method to characterize artificially structured metamaterials. In this method, the unit cell S-parameters are obtained with the ANSYS HFSS and, according to the equations provided in [24], are used to obtain the electrical permittivity, magnetic permeability and refractive index. The excitation used was the Wave Port type. It is equivalent to a semi-infinite waveguide that will excite the structure with an incident wave perpendicular to the surface. The retrieved material parameters are illustrated in Fig. 6.
The retrieved index is illustrated in Fig. 6(a) and it confirms the negative index band that lies between roughly 1 GHz and 9.5 GHz. Using the impedance and index we find functional forms for the permittivity and permeability, as shown in Fig. 6(b) and (c), respectively.    It is observed that the real part of the refractive index, permittivity and permeability are negative at every frequency of interest, which justifies that the unit surface meta-cell acts as a DNG material or left handed material.

IV. PARAMETRIC ANALYSIS
To analyze the effect of metasurface on the antenna resonant s, we plotted the S11 response to the antenna without the metasurface and with the metasurface, as shown in Fig. 7, considering this positioning as the metasurface at 0°, Fig. 4   Beginning the parametric analysis, clockwise meta-surface rotations were performed to obtain an optimal angle in which the resonant frequencies overlap, increasing the bandwidth in the range of interest. Fig. 8 illustrates the frequency response of |S11| for angles of 0°, 30°and 60°. We can see that as we increase the rotation angle the resonant frequencies overlap so that at 60°we get a bandwidth of approximately 3.5 GHz. Comparison of S11 for different metasurface rotation angles.
In [21] and [22] the authors state that the meta-surface (MS) can be considered as a special dielectric that has a variable relative permittivity when illuminated by a linearly polarized plane wave.
The permittivity value depends on the direction of the polarization of the plane wave. However, we understand that the MS has a variable impedance, whose real and imaginary parts depend on the direction of the polarization of the plane wave. Fig. 9(a)  To further optimize the response obtained, a parametric analysis of the physical dimensions of the meta-surface unit cell was performed. In this analysis, we vary only one physical parameter at a time.
The slot width in the unit cell (L2) together with the length (L1) were investigated to determine an optimal constructive value. Fig. 11 illustrates the results for L2 = 1.0 mm by varying the dimension of L1. As the length of the slot increases, there is a greater overlap of resonant frequencies and therefore a greater bandwidth. Thus, we have the optimal constructive value L1 = 8 mm.     dBi. The gain was measured using an Agilent E5071C two ports network analyzer and two calibrated SAS-571 double ridge guide horn antennas with a known gain, positioned at a distance of 50 cm. We measured the S21. After this, we changed one of the antennas by our proposed antenna and repeat the measurement. From the S21, the distance, and the gain known of the horn antenna, we calculated the gain of our antenna. We think that the deviation between the gains from 6.6 GHz to 6.8 GHz occurs 115 because the measurements were not performed in an anechoic chamber.  Another important parameter is the radiation pattern, as illustrated in Fig 17. It is important that the diagram undergoes few changes over the entire band. Thus, we plot the diagrams on the E and H planes, for the frequencies of 5.5 GHz and 6.5 GHz. We can notice that the radiation patterns undergo little change along the band, both on the XZ-plane, and in the YZ-plane. Also, we can have noticed that the antenna has no polarization purity, but it is not a circular polarized antenna.

VI. CONCLUSIONS
In this work, we propose the use of a metasurface coupled to a microstrip patch antenna for bandwidth enhancement. The final volume of the antenna was 3.82 cm 3 . The metasurface was rotated until the largest fractional bandwidth was obtained, whose best angle was 60°. The set can be used for