MIMO Antenna Design for WALN Applications

Tzu-Chun Tang and Ken-Huang Lin Department of Electrical Engineering

National Sun Yat-sen University Kaohsiung, Taiwan

a565656kimo@pcs.ee.nsysu.edu.tw and khlin@mail.nsysu.edu.tw

Abstract—A closely-packed coplanar waveguide (CPW)-fed dual band multi-input-multi-output (MIMO) antenna operating at WLAN (IEEE 802.11a/b/g) is presented. A metal strip is located between signal and ground of the CPW to excite the surface wave mode that contributes to not only two resonant modes at lower (2.4–2.48 GHz) and higher (5.13–5.825 GHz) bands, but also a good isolation (S21<−15 dB) over these bands. In this case, the spacing between two antennas can be reduced to 2 mm (0.016λ at 2.45 GHz) without any decoupling elements or network. The experimental results show good agreement with the simulation results, demonstrating the feasibility of the proposed method.

I. INTRODUCTION Wireless communication using MIMO technology has drawn a lot of attention in recent years. The IEEE standard 802.11n defines specifications for MIMO systems to use. To overcome the mutual coupling between antennas, several decoupling techniques have been proposed [1]–[7]. CPW technology has drawn a great attention due to its advantages of lower dispersion and easy integration of active devices. Works using CPW technology for MIMO antenna applications have been published in [6]–[7]. However, these methods only applied to single band applications. In this paper, a more compact and closely spaced dual band MIMO antenna using CPW technology is developed.

II. MIMO ANTENNA DESIGN Fig. 1 shows the geometry of the proposed MIMO antenna.

The CPW feed line and antenna elements are printed on the 0.8-mm thick FR4 substrate with relative dielectric constant of 4.4 and loss tangent of 0.02. The width of the feed line and the gap of the CPW are designed to match the 50 Ω . The width of the middle ground plane is designed to retain space for two coaxial connectors during the implementation. The space is used to prevent the coaxial connector from being touched to each other. Each antenna element is designed in two inverted-L structures. One of the inverted-L structures is extended from the feed line; the other structure is extended from the middle ground plane. The spacing ( g ) between two structures is chosen to provide the coupling path to excite the loop mode operation. The metal strip is placed between the feed line and the ground plane. The design values of the antenna parameters are as follows; L = 11 mm, LG = 10 mm, La = 12 mm, La1 = 8.2 mm, La2 = 7 mm, S = 2 mm, SP = 4 mm, H = 4 mm, Wa1 =

2 mm, Wa2 = 3.5 mm, WG = 10 mm, WG1 = 6 mm, Wg = 0.2 mm, Wp = 0.5 mm, Ws = 2 mm, g = 0.6 mm. The overall size of the antenna is chosen for the wearable devices. The simulation results of this paper are predicted by the Ansys HFSS.

Fig. 2 shows the comparison of the antenna with (W/) and without (W/O) the metal strip in S parameters. Referring to Fig. 2, it shows that only one mode is excited at 2.2 GHz without the metal strip. In contrast, for the antenna with the metal strip, two modes are excited to cover 2.4–2.48 GHz and 5–6 GHz under S11<−10 dB specification and the S21<−15 dB isolation band covers 2.38–2.49 GHz and 5–6 GHz, respectively. The antenna spacing (edge-to-edge) is only 2 mm (0.016λ at 2.45 GHz). The simulation result demonstrates the feasibility of our method to realize the dual band MIMO antenna used for WLAN under IEEE 802.11n standard at 2.4–2.48 GHz and 5.15–5.825 GHz.

Fig. 1. The configuration of the proposed MIMO antenna.

Fig. 2. The S-parameters of the proposed antenna.

492978-1-4799-3540-6/14/$31.00 ©2014 IEEE AP-S 2014

III. IMPLEMENTATION AND MEASUREMENT To verify the feasibility of the proposed idea, the antenna is

implemented and measured. The fabricated prototype is shown in Fig. 3. The 50 Ω load is connected to the other input port when measuring the return loss. Fig. 4 shows the comparison between measurement and simulation results. The measured impedance bandwidth S11<−10 dB covers 2.35–2.48 GHz and the measured isolation bandwidth S21<−15 dB covers 2.38–2.55 GHz. The measurement shows good agreement with the simulation.

For the higher band, the measured impedance bandwidth S11<−10 dB covers 5–5.9 GHz (antenna 1) and 5–5.87 GHz (antenna 2), and the measured isolation bandwidth S21<−15 dB covers 5–6 GHz. The slight difference between the simulation and measurement results at S11 is likely due to the coaxial connector which may extend the resonant current path; therefore, the measured frequency is slightly shifted toward the lower frequency. Nevertheless, the measurement result confirms that the fabricated antenna covers the design requirement of operating band (5.13–5.825GHz).

(a) (b)

Fig. 3. The fabrication of the proposed MIMO antenna.

Fig. 4. The comparison of 11S , 22S , and 21S between simulation and measurement results.

The measured gains over 2.4–2.484 GHz and 5.15–5.9 GHz

are shown in Fig. 5. For frequencies over 2.4–2.484 GHz, the antenna gain is about 0.6–2 dBi; on the other hand, the antenna gain is around 2.5–3.4 dBi from 5.15 to 5.9 GHz. The simulated radiation efficiency is about 73–80% over 2.4–2.484 GHz, while that over 5.15–5.825 GHz is about 94–95%.

Fig. 5. Measured antenna gain of the two closely spaced antennas over 2.4–2.484 GHz and 5.15–5.9 GHz.

IV. CONCLUSION A dual band MIMO antenna for WLAN (IEEE 802.11a/b/g)

is presented. The dual band isolation is achieved by exciting surface wave modes, resulting in two resonant modes at lower (2.4–2.48 GHz) and higher (5.13–5.825 GHz) bands, respectively. Without the decoupling element or matching network, the spacing between two antennas can be as close as 2 mm (0.016 λ at 2.45 GHz). Moreover, the technique simplifies the MIMO antenna design in CPW technology.

REFERENCES

[1] S.-C. Chen, Y.-S. Wang, and S.-J. Chung, “A decoupling technique for increasing the port isolation between two strongly coupled antennas,” IEEE Trans. Antennas Propag., vol. 56, no. 12, pp. 3650–3657, Dec. 2008.

[2] Z. Li, Z. Du, M. Takahashi, K. Saito, and K. Ito, “Reducing mutual coupling of MIMO antennas with parasitic elements for mobile terminals,” IEEE Trans. Antennas Propag., vol. 60, no. 2, pp. 473–481, Feb. 2012.

[3] S. Zhang, B. K. Lau, Y. Tan, Z. Ying, and S. He, “Mutual coupling reduction of two PIFAs with a T-shape slot impedance transformer for MIMO mobile terminals,” IEEE Trans. Antennas Propag., vol. 60, no. 3, pp. 1521–1531, Mar. 2012.

[4] C. H. See, R. A. Abd-Alhameed, Z. Z. Abidin, N. J. McEwan, and P. S. Excell, “Wideband printed MIMO/diversity monopole antenna for WiFi/WiMAX applications,” IEEE Trans. Antennas Propag., vol. 60, no. 4, pp. 2028–2035, Apr. 2012.

[5] D. A. Ketzaki and T. V. Yioultsis, “Metamaterial-based design of planar compact MIMO monopoles,” IEEE Trans. Antennas Propag., vol. 61, no. 5, pp. 2758–2766, May. 2013.

[6] Y. Li, Z. Zhang, W. Chen, Z. Feng, and M. F. Iskander, “A dual-polarization slot antenna using a compact CPW feeding structure,” IEEE Antennas Wireless Propag. Lett., vol. 9, pp. 191-194, 2010.

[7] J. Zhu and G.V. Eleftheriades, “A simple approach for reducing mutual coupling in two closely spaced metamaterial-inspired monopole antennas,” IEEE Antennas Wireless Propag. Lett., vol. 9, pp. 379-382, 2010.

493