C-Band MIMO Antenna with Isolation and Gain

Enhancement for MIMO Radar Applications

Po-Ting Liu, Tzu-Chun Tang, and Ken-Huang Lin

Department of Electrical Engineering National Sun Yat-sen University

Kaohsiung, Taiwan louis79050S@pcs. ee.nsysu.edu.tw,a565656kimo@pcs.ee.nsysu.edu.tw and khlin@mail. nsysu.edu.tw

Abstract-A miniaturized C-band multiple-in put-multiple-output

(MIMO) antenna with good isolation and gain enhancement is

investigated. Based on the reaction theorem, an equivalent

magnetic current is introduced to decrease the effect caused by

the electric current source, contributing to wideband isolation

enhancement. Such a miniaturized antenna usually suffers from gain deterioration. Therefore, a three stacked superstrates with

the metal strips are proposed for wideband gain enhancement.

Good SII « -10dB) , S2t « -ISdB) and gain enhancement (4.3 dB at the most) for 4-8 GHz are achieved.

I. INTRODUCfION

Recently, the concept of MIMO communication has created considerable attention both in mobile communications and advanced radar technology. In MIMO radar technology, the antenna should be capable of operating in wideband and high gain in order to maintain good target detectable performance and angular estimation accuracy[I]-[3]. In addition, the need of non-contact vital sign sensor surged in recent years; it can be expected that the portable medical device equipped with MIMO radar technology for high-accuracy data acquisition will be appealing. In the portable device, the volume of the antennas needs to be small and the antennas are very close, resulting in high coupling. To alleviate the problems, the reaction theorem is adopted for isolation enhancement by introducing an equivalent magnetic current to decrease the effect caused by the electric current source, and the separated substrates design is developed for wideband gain enhancement.

II. MIMO ANTENNA DESIGN

Fig. 1 shows the proposed structure of MIMO antenna. In this structure, the elements are etched on a dielectric substrate

having a relative dielectric constant of Er =4. 4 and a thickness of O.S mm. The antenna element is driven by CPW feeding.

First we design one half of the antennas. We choose CPW feeding because it provides the advantage of wideband impedance matching. Main antenna is resonating at 4 GHz and S GHz by the effect of harmonic, and then we add a branch resonating at 6 GHz so that Sit is lower than -10 dB over 4-S GHz. Next, a MIMO antenna is formed by the main antenna and its mirror image, and a common ground structure is connected to the two antennas. This common ground structure

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is similar to protruding structure [4]-[5] which is used for isolation enhancement.

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4.5 5.5 6.5 7.5 Frequency (GHz)

Fig. 2. S-parameters of MIMO antenna

From Fig. 2, it can be seen that even though the distance between two antennas is only 5 mm, S21 < -15 dB still covers 4-7.2 GHz. However, this design technique faces the challenges in wideband MIMO antenna design. In order to further improve the isolation over the desired frequency band, the reaction theorem [5]-[6] is applied. Isolation is relevant to mutual impedance (Z21). In (1), it can be seen that the Z:!I is related to the magnetic current source (K2), and its magnitude can be reduced by introducing an equivalent K2

(1)

AP-S 2014

Fig. 3. Stucture of three stacked supestrates

The way to create equivalent magnetic current is introducing slot on the protruding structure, as shown in Fig. 1. Fig. 2 shows a comparison of S21 of protruding structure with (wi) or without (w/o) the slot.

III. GAIN ENHANCEMENT USING THREE STACKED

SUPERSTRATE

In order to enhance the gain over 4--8 GHz, three stacked superstrates are developed to achieve this purpose. Conventionally, superstrate structure should be placed at a distance of O.5A away from antenna, and the operating bandwidth is usually narrow. The papers in the open literatures show the studies of using multi-layer superstrate to achieve gain enhancement. However, the operating bandwidth is narrow [8]-[10]. The proposed structure in this paper is inspired by [11]. Nevertheless, it is used for narrow band operation. We extended the approach to wideband operation.

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Fig. 4. S-parameters of antenna

Frequency (GHz)

Fig. 5. Gain enhancement of the antenna

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By analyzing the electric field on the xz, yz plane of the antenna, finding out the height that the electric field would be parallel to the superstrate. We then put the superstrate at that height and create a metal strip on the corresponding location on the superstrate. The metal strip serves as an electric field phase shifter, contributing to the constructive interference at the far field of the antenna. Therefore, the antenna gain can be improved. Furthermore, the distance between the antenna and the superstrate can be close to O.25A. As shown in Fig. 3, each superstrste is designed corresponding to the desired frequency. Superstrate 1 is operating at nearly 4 GHz, superstrate 2 at nearly 6 GHz, and superstrate 3 at nearly 8 GHz. Fig. 4 shows the influence of S-parameters by adding superstrates, and Fig. 5 shows that gain enhancement is achieved over 4--8 GHz.

IV. CONCLUSION

A miniaturized MIMO antenna operating over 4--8 GHz with isolation and gain enhancement is proposed. The reaction theorem is adopted for further improving the isolation and the isolation bandwidth. In addition, the stacked superstrates with metal strip are developed to enhance the gain over wideband operation. These results can be used for C-band radar applications, especially for those portable medical devices.

REFERENCES

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[2] H. He, P. Stoica and 1. Li, "Designing unimodular sequence sets with good correlations - including an application to MlMO radar," IEEE Trans. Signal Process .• vol. 57, no. 11, pp. 4391-4405, Nov., 2009.

[3] 1. Li and P. Stoica, MIMO Radar Signal Processing. New Jersey: John Wiley & Sons, 2009.

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[5] T. S. P. See, and Z. N. Chen, "An ultrawideband diversity antenna," IEEE Trans. Antennas Propag., vol. 57, no. 6, pp.1597-1605, Jun., 2009.

[6] Y. H. Rumsey, "Reaction concept in electromagnetic theory," Phys. Rev., vol. 17, pp.I483-I491, June 15, 1954.

[7] 1. H. Richnok, "A reaction tlleorem and its application to antenna impedance calculations", IRE Trans. Antennas and Propag., voL9, no. 6, pp. 515-520, Nov., 1961.

[8] M. A. AT-Tarifi, D. E. Anagnostou, A. K. Amer!, K. W. Whites, "Bandwidth enhancement of the resonant cavity antenna by using two dielectric superstrates", IEEE Trans. Antennas Propag., vol. 61, no. 4, Apr., 2013.

[9] H. Attia, and O. M. Ramahi, "EBG superstrate for gain and bandwidth enhancement of microstrip array antennas", in Proc. IEEE Antennas Propag. Soc. Int. Symp. (AP-S 2008), July 5-11, 2008.

[10] D. Li, Z. Szab6, X. Qing, E. P. Li, and Z. N. Chen, "A high gain antenna with an optimized metamaterial inspired superstrate", IEEE Trans. Antennas Propag., vol. 60, no. 12, pp. 6018-6023, Dec., 2012

[11] T. N. Chang, and J. M. Lin, "Enhanced retum-Ioss and flat-gain bandwidths for microstrip patch antenna," IEEE Trans. Antennas Propag., vol. 59, no. 11, pp. 4322-4325, Nov., 2011.