Bandwidth Enhancement of a U-Slot Patch Antenna Using Embedded HIS Structure

This paper proposes a new generation of antenna that applies metamaterial as a base construction. With the use of dual band high impedance surface (HIS) structures, the bandwidth, return loss, and gain of U-slot patch antenna is improved at resonant frequencies 2.24 GHz and 5.8 GHz. The proposed new modified U-slot antenna has dual band impedance bandwidth from about 2.1886 to 2.27 GHz and 5.6149 to 7.2259 GHz. From the simulation result it was found that the upper frequency band of the proposed antenna lies in the band of 5.725∼5.825 GHz regulated by IEEE 802.11a (upper band) and can be used for bluetooth and WLAN applications. We perform this analysis on structures which composed of rectangular lattice patches periodic arrangements. All the dimensions and shapes of the unit cell geometry are optimized in order to get a broad bandwidth and high return loss. The lattice structure comprises of an array of 7×5 rectangular patches embedded in the substrate.


Introduction
R ecent developments in fast and rigorous full wave simula- tors and the concurrent availability of inexpensive manufacturing techniques for intricate shape and composite materials provide the opportunity to revolutionize the traditional design to new one.To provide multiple functions, it is practical to integrate systems of different frequencies into a single product.Developing a single unit with multi-frequency bands as well as wide band will certainly save considerable circuit area and is also required for accurately transmitting the voice, data, video, and multimedia, concurrently.Therefore, how to enhance the bandwidth and frequency bands [1][2][3][4][5][6][7] has became an important issue in the antenna design field.The FSS structure has a phenomenon with high impedance surface that reflects the plane wave in-phase and suppresses surface wave thus improving the radiation efficiency, bandwidth, and gain, and reducing the side and back lobe levels in its radiation pattern.For more than four decades the FSS has applications in the areas of filter [8][9][10][11][12][13], reflectors, absorbers [14][15][16], polarizers [17], planar metamaterials [18], and artificial magnetic conductors [19][20][21].
In this paper, a U-slot patch antenna is designed first to overcome the problem of narrowband and to achieve dual band operation at resonant frequencies of 2.24 and 5.8 Ghz.The dimen-sions of U-slot are varied and the feed location was changed until good results were observed.
In the later part, we propose a new U-slot patch antenna embedded with a lattice of an array of 7×5 rectangular patches without connection with the ground plane.Array of patches without via connection to the ground plane exhibit a high impedance with an exactly zero degree reflection phase at the resonance frequency.We have known that, there is a problem with most of proposed HIS structures because they present a shift of the resonant frequency versus the incidence angle [22].But in comparison with conventional antenna type placed above a metal ground plane, the antenna placed above the HIS has smoother radiation profile, less power wasted in the backward direction, better return loss, and higher gain and directivity [23,24].In simulations, the characteristics of U-slot patch antennas were obtained by using the Ansoft high-frequency structure simulator (HFSS).

The U-slot patch antenna with embedded HIS structure
The dimensions of a U-slot patch antenna are taken as 76×52 mm 2 .The thickness of substrate is taken as 3.4 mm .The material used for it is FR4 having a dielectric constant and electrical loss tangent of 4.4 and 0.02, respectively.The dimensions of U-slot radiator patch are 54×30 mm 2 .The length and width of  U-slot are 28.4 and 15 mm, respectively.The width of slot is kept as 2 mm.In our studies, a coaxial line with a characteristic impedance of 50 ohms is used as the feed of the U-slot patch antenna.The inner conductor of the coaxial line is attached on the top patch going through the dielectric substrate, and the outer conductor is shorted to the metallic plate on the other side of the patch antenna.The dimensions and layout of this U-slot radiator is shown in Fig. 1.For the proposed new antenna the FSS is constructed at a height of H=2.9 mm from ground plate.The FSS structure is constructed with the rectangular shaped patches array to improve the bandwidth and gain of the U-slot patch antenna.The detail dimensions of the FSS embedded U-slot patch antenna is shown in Fig. 2. The rectangular shaped patches FSS has dimensions of 3.5 mm×4 mm.The gaps between the array elements are also shown in Fig. 4.
In optimizing the onset of two resonant frequencies of 2.24 GHz and 5.8 GHz, the change of geometrical parameters W 1 and W 2 and H can be used to find the best bandwidth.Figs. 3  and 4 show the detail dimensions of unit cell and periodic array of rectangular shaped patch HIS structure.

Simulated results with and without HIS structure
The simulated result of the U-slot patch antenna is shown in Fig. 5 with and without HIS structure.Simulation results were investigated by checking the impedance matching with better than -10 dB return loss.
The result shows that the resonant frequency of the patch is

Conclusion
A technique for the gain and bandwidth enhancement of Uslot patch antenna using embedded HIS structure was presented.The parameters of the HIS structure (spacing, width, length, and number of the patches) and the thickness of the substrate were optimised using Ansoft HFSS to obtain the maximum gain and bandwidth.While the HIS structure reduced the surface wave, the bandwidth and gain enhancement was mainly because of coupling between the patch and the HIS structure.From simulation results, it is found that the bandwidths have been improved near the resonant frequencies of 2.24 and 5.8 GHz for the Uslot patch antenna implanted with a HIS consisting of regular rectangular patches elements array.

Figure 2 :
Figure 2: A modified U-slot patch antenna with embedded HIS at height of H=2.9 mm from ground plate.

Figure 3 :
Figure 3: Detail of a unit cell element of HIS structure.

Figure 4 :
Figure 4: Detail dimensions of HIS structure array embedded at a height H= 2.9 mm from ground plate.

Figure 5 :Figure 6 :
Figure 5: Comparison of return losses for the U-slot patch antenna with and using HIS structure.