Isolation Enhancement between Microstrip Patch Antennas using Dual-band EBG Structure without

Common Ground Plane

Wonsang Choi Department of Electronic Engineering

Kyonggi University Suwon, Korea

cws2k2k@gmail.com

Hongmin Lee Department of Electronic Engineering

Kyonggi University Suwon, Korea

hmlee@kyonggi.ac.kr

Abstract—This paper presents an efficient method to enhance the isolation level between two dual-band E-shape microstrip patch antennas using dual-band EBG unit cells which operates in UMTS Tx (1.92-1.98GHz) and Rx (2.11-2.17GHz) band, respectively. The proposed EBG structure made with a periodic array of two different geometric dimensions of unit cell which has a modified mushroom-type. Overall size of the fabricated antenna is 210.5 mm × 117 mm. The fabricated two different EBG unit cell sizes are 15.6 mm × 4 mm and 17.4 mm × 4 mm, respectively. The experimental results show that the isolation levels between antennas in Tx/Rx band are under -35 dB.

I. INTRODUCTION The increased demand for higher data rate and high quality

services in wireless communication is driving the researcher to investigate new ways to operate multiple antennas. If multiple antennas use in wireless device, the mutual coupling between two antennas cause serious problem in antenna performance. The general method for improving isolation between antennas is to build slit in antenna ground plane [1]-[2] or neutralization line connecting each antenna radiator [3]-[4]. However, the building slit in antenna ground plane increase the backward radiation and it is not easy to select optimum connecting point in the neutralization line method. In a closely spaced two microstrip antennas design, a mushroom-type electromagnetic band gap (EBG) structure is used in order to improve these problems in the conventional studies. However, in the design of a microstrip antenna using EBG structure on same ground plane of antenna large array of unit cells are needed [5]. In this paper, the isolation enhancement method between two dual-band E-shape microstrip patch antennas is proposed using dual band EBG structure. The proposed EBG unit cell does not have a common ground plane. In the design of the proposed antenna with EBG cells, the 3-D field simulation tool, CST MWS (Micro Wave Studio) was used [6].

II. DESIGN OF EBG UNIT CELL The geometry of the proposed EBG unit cell and parallel-

plate waveguide model to examine its transmission characteristic responses as functions of the frequency is shown

in Fig. 1. The mushroom type EBG unit cell consists of two parallel rectangular plates and via is located on the edge of plate to obtain a size reduction of the unit cell. According to via position moved from the center to the edge, the electrical path is increased and this can be used to obtain a size reduction of the unit cell [7]. The proposed EBG unit cell acts as spatial filter because it does not share with the common ground plane of the antenna. The total size of unit cell is L = 15.6 mm, W = 4 mm at 1.95 GHz (UMTS Tx-band) and L = 17.4 mm, W = 4 mm at 2.14 GHz (UMTS Rx-band). Inside two parallel rectangular plates, dielectric material (ɛr = 4.6) is filled and the height of substrate is 3.93mm. Fig. 2 shows the simulated transmission characteristics response of the proposed unit cell as function of the frequency.

Figure 1. Parallel-plate wave guide model and unit cell.

Figure 2. Transmission characteristics of the proposed EBG structure by wave guide model

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When the proposed unit cell is not used the transmission coefficient S21 between two parallel plate waveguide shows near -30 dB. When the unit cell is inserted, it shows a dual stop-band characteristic at the aimed design frequency of 1.96 GHz and 2.15 GHz, respectively.

III. DESIGN TO PROPOSED ANTENNA The geometry of the proposed antenna is shown in Fig. 3.

In the design of the proposed antenna, FR4 (ɛr = 4.6, thickness = 3.93 mm) substrate is used. The proposed EBG structure without common ground plane is placed between E-shaped microstrip patch antennas. The sizes of two different EBG unit cells are 15.6mm × 4mm and 17.4mm × 4mm, respectively. The simulated results of the proposed antenna with/without EBG structure are shown in Fig. 4. The simulated impedance bandwidth (VSWR ≤ 2) of the proposed antenna with EBG structure is 290 MHz (1.89 - 2.18GHz). The simulated results show that the isolation level between two antennas in UMTS Tx/Rx-band is improved by about 9 dB and 12 dB, respectively. The measured results are shown in Fig. 5. The measured bandwidth (VSWR ≤ 2) is 260 MHz and isolation level between two antenna ports in UMTS Tx/Rx-band are under -35 dB.

(a) Top view (b) Bottom view

Figure 3. Geometry of the proposed antenna structure.

Figure 4. Simulated reflection coefficient and isolation level of the proposed antenna with/without cells.

Figure 5. Measured reflection coefficient and isolation level.

IV. CONCLUSION In this paper, a new dual-band EBG structure which

operates in UMTS Tx and Rx band is proposed. The proposed EBG structure made with a periodic array of two different size unit cells which has modified mushroom-type for isolation improvement between two antennas. The simulation results show that the isolation levels between antennas in Tx/Rx band are improved by about 9 dB and 12 dB, respectively. Thus it can be applied for isolation enhancement in the microstrip patch antenna design.

ACKNOWLEDGEMENT This research was supported by the Basic Science Research

Program through the National Research Foundation of Korea (NRF) and funded by the Ministry of Education, Science and Technology (No. 2010-0011646).

REFERENCES

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[6] CST STUDIO SUITE TM 2008 MWS (Micro Wave Studio) manual, CST Corporation

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