Investigation of Circular Patch Metasurface (MS) on Inverted Suspended Circular Polarized Antenna

In this paper, an analysis of different stage of circle patch metasurface (MS) on basic structure of inverted suspended circular patch with square slot antenna design is proposed to investigate its effect toward the basic antenna performance. The basic antenna was designed based on inverted suspended circular patch with air gap separation between the patch substrate and copper ground plane at a distance of 10 mm (Basic design). Then, different design of MS structure (Design A, B, C & D) consisting of 5 x 5 unit cell of homogeneous periodic element is place in between the air gap of the basic antenna. All of MS design is located at a fixed position to study the effect of different MS shape to the Basic antenna performance. The proposed antenna and MS structure had been designed and simulated by using Computer Simulation Technology (CST) Software. Target application for this antenna design is for Wireless Local Area Network (WLAN) at operating frequency of 2.4 GHz. Simulation performances in term of return loss, resonant frequency, bandwidth, realized gain, directivity, axial ratio, total efficiency and radiation pattern at the design frequency are investigated and discussed. From observation on simulation result, resonant frequency of Basic design is shifting to a lower frequency from 2.4 GHz to 2.19 GHz (Design C and D) with the integration of the MS structure. Design C obtained the widest axial ratio bandwidth among all of other design with 1380 MHz covered from 1.31 GHz to 2.69 GHz. Directivity for Basic designs at 2.4 GHz increases from 7.94 dBi to 8.17 dBi, 8.16 dBi, 8.15 dBi and 8.06 dBi (Design A, B, C and D) respectively.


Introduction
Antenna is one of the most important equipment in a wireless communication system which enables the transmission and reception of signals through air as medium. Nowadays, with the great requirement for compact wireless devices, a small effective antenna with common structures continues to be a subject of great attraction [1][2][3]. Other than that, antenna with circularly polarized (CP) [4][5][6] also getting more attention because they offer flexibility in the orientation angle between the transmitter and receiver. In recent year, a lot of efforts have been made by researchers to study and modified simple antenna design to be integrated with MS structure in order to represent as circular polarization antenna [7,8], compact [9,10] the structure and miniaturize [11,12] the size of planar antenna for wireless communication system application. Metasurface (MS) is a 2-D planar equivalent of metamaterial structure and has receiving researchers interest in the last few years for planar antenna performance enhancement such as wide bandwidth and high gain [13][14][15][16][17]. Other than that, as in  [18,19] it demonstrate that the performances of patch antenna can be enhanced simultaneously by the addition of MS at the top of the antenna design.
In this work, it is proposed to use different stage of circle patch unit cell of MS to study the effect of MS structure on Basic antenna behaviour in term of return loss, resonant frequency, bandwidth, return loss bandwidth (RLBW), realized gain, directivity, axial ratio, axial ratio bandwidth (ARBW), total efficiency and radiation pattern. A Basic design of inverted suspended circular patch with square slot (Basic) is proposed to be integrated with the MS structure. All of MS design consist 5 x 5 unit cell of homogeneous periodic element. Design A consist of unit cell of one circle patch, Design B and C consist two combination of circle patch and Design D consist of three combination of circle patch. The configuration of Basic design and design with integration of MS structure is demonstrated as in Figures 1.

Antenna Design
In this paper, the basic antenna and the MS structure was designed by using FR4 substrate with thickness, h = 1.6 mm, dielectric constant, ε r of the substrate = 4.4, tangent loss and tan δ = 0.019. Thickness of copper, t = 0.035 mm is used for the ground plane and as the conductive material printed antenna and MS. The basic circular polarized antenna is an inverted suspended circular patch with square slot as shown in Figure 1 (a). Where the antenna feedline and circular patch with square slot are printed at the bottom sided (back) of the antenna substrate as shown in Figure 1 (c) without any copper patch at other sided (front) of the substrate as shown in Figure 1 (b). The inverted suspended circular patch antenna and its ground are separated by 10 mm air gap as illustrated in Figure 1 (a) side view. The feedline was fed by using 50 Ohm SMA coaxial probe connector thus represent as an inverted suspended L-probe technique for the antenna input port.  Table 1.

Result and discussion
In this section, simulation antenna parameter which comprised of return loss (RL), resonant frequency (f r ), bandwidth (BW), return loss bandwidth (RLBW), realized gain, total efficiency, directivity, axial ratio, axial ratio bandwidth (ARBW) and radiation pattern are analysed and discussed at 2.  Table 2.

Return loss (RL), Resonant frequency (f r ), bandwidth (BW) and Realized gain
The comparison of simulation S11 and realized gain for Basic design, Design A, B, C and D are illustrated as in Figure 2

Axial ratio, Axial ratio Bandwidth (ARBW)
As demonstrated in Figure 4 is the simulation of axial ratio for Basic design and design with the addition of all different MS structure. Based on Figure 4, the minimum axial ratio for Basic design is at 2.1 GHz with 0.07 dB. While at 2.4 GHz, the axial ratio is 0.62 dB. With addition of all MS design, the axial ratio found to be more than 1 dB at 2.4 GHz. Where Design A has 3.13 dB (linear   Table 2. Based on Table 2, the result of return loss (RL), realized gain, axial ratio (AR), total efficiency and directivity are demonstrated. with 1380 MHz covered from 1.31 GHz to 2.69 GHz. At 2.4 GHz, only Design A axial ratio is more than 3 dB which is 3.13 dB. Radiation pattern of all design at 2.4 GHz are quite similar. With the integration of MS structure to the basic antenna, size reduction can be achieved by reducing the radius of circular patch. In order to assemble as more compact and smaller sized antenna, for the next step that may be taken is miniaturizing the antenna substrate size while maintaining a similar antenna performance. Antenna with compact in size, large return loss bandwidth (RLBW) and axial ratio bandwidth (ARBW) and can operate as circular polarization can represent as a good candidate for compact and broadband circular polarization antenna for WLAN application at targeted frequency of 2.4 GHz.