Linear Tapered Slot Antenna with Defected Side for Improved Gain

Dae-Myoung In, Seongmin Pyo, Jung-Woo Baik, Won-Sang Yoon, and #Young-Sik Kim

Department of Computer and Radio Communications Engineering, Korea University

Seoul 136-713, Korea Email: yskim@korea.ac.kr

1. Introduction Linear tapered slot antennas (LTSAs) have been widely investigated as typical examples of coplanar antennas because the LTSAs offer the advantages of wide impedance bandwidth, high gain, and easy fabrication [1], [2]. However, they have high back lobes in the E-plane [3]. In general, the LTSAs are very sensitive to the thickness and the width of their supporting substrate. It is shown that radiation patterns of a tapered slot antenna (TSA) is affected on magnetic currents induced in the edge of the finite width of a substrate [4]. This degradation of the radiation pattern, which is caused by the small width of an antenna aperture, is a significant problem on a compactness of TSA [5]. A corrugation structure at edges of TSA for decrement of degradation has reported in [6]. However, the effect of a corrugation structure proposed in [6] is not enough to improve an antenna gain.

In this paper, a new LTSA with defected sides are proposed for gain improvement. The defected sides are similar to a conventional dumbbell-shaped defected ground structure (DGS) which has been used for microwave circuit application such as a filter implementation. The proposed defected side-LTSA (DS-LTSA) is designed to suppress unwanted magnetic currents which result in a radiation to a backward direction of the antenna. The results of simulation and experiment show that the proposed antenna has a gain improvement due to preventing of opposite-directed magnetic currents. 2. Defected Side-LTSA Design Fig. 1 shows that the proposed DS-LTSA with periodic arrangement of a defected ground unit cell. The lengths of antenna and aperture are 60 and 20.8 mm, respectively. And the distance between the inner edge of an aperture and the outer edge of an antenna is 4.6 mm. Both of the proposed DS-LTSA and the conventional LTSA are fabricated on the RT/Duroid 5880 substrate with a dielectric constant of 2.2 and a thickness of 0.254 mm. In the side of a conventional LTSA, the defected structures are employed for preventing unwanted magnetic currents. The measured gain of the proposed antenna is 14 dBi at the center frequency of 15 GHz when the DGS-edge has a of 1.5, b of 2, d of 0.5 and c of 1 mm, respectively. The optimized dimensions of the proposed antennas are obtained by using the Ansoft HFSS commercial software [7]. For broad impedance matching between the radiating element and a feed structure a microstrip-to-slotline transition is utilized. In this design, the impedance and mode matching between microstrip line and slotline can be achieved by the virtual short- and open-circuit caused from an open circuited microstrip line with λg(μ-stripline)/4 and short circuited slotline with λg(slotline)/4 [7]. The slotline (s = 0.15 mm) with the characteristic impedance of 120 Ω is connected with microstrip tapered line with 120 Ω. Fig. 2 shows the normalized magnetic current distributions at 15 GHz when the defected sides are loaded. As expected from the magnetic current simulation, the unwanted current in the side is lower and weaker than the conventional one.

The 2009 International Symposium on Antennas and Propagation (ISAP 2009)October 20-23, 2009, Bangkok, THAILAND

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3. Experiment results The photographs of the proposed antenna are shown in Fig. 3. Fig. 4 shows the simulated

and measured reflection coefficients of the proposed antenna. From the results, the measured reflection coefficient has a good agreement with the simulation one. The DS-LTSA yields an impedance bandwidth of Ku-band (12.5 ~ 18.0 GHz) below -10 dB. Fig. 5 illustrates the measured radiation patterns of the proposed DS-LTSA at 15GHz. The 3-dB beamwidth of the proposed antenna is narrower than that of a conventional one. From the discussion on the effect of defected side, the conventional antenna obtained a gain of 10 dB at 15 GHz. The proposed DS-LTSA antenna gain is measured by lager 4.35 dB than the conventional LTSA one. It is reason that the defected sides are available for preventing magnetic currents. The measured maximum gain of the proposed antenna shows in Fig. 6. 4. Conclusion In this paper, a new LTSA with a defected side is presented for gain improvement. This technique shows that radiation patterns can be adjusted via the dimensions and the periodicity of the DGS in the sides of an antenna. Comparing with a conventional LTSA, the gain of the proposed DS-LTSA has been improved by up to 4.35 dBi at 15 GHz. This technique can be suitable for millimeter wave applications. 5. Figures and Tables

Fig. 1: Geometry photograph of the proposed DS-LTSA.

The 2009 International Symposium on Antennas and Propagation (ISAP 2009)October 20-23, 2009, Bangkok, THAILAND

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magnetic current

(a) (b)

Fig. 2. Comparison of E-field distributions and magnetic current densities. (a) Conventional LTSA. (b) Proposed DS-LTSA.

(a) (b)

Fig. 3. Photographs of the proposed DS-LTSA. (a) Top view. (b) Bottom view.

10 12 14 16 18 20Frequency (GHz)

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Fig. 4. Measured and simulated reflection coefficients of the proposed DS-LTSA.

The 2009 International Symposium on Antennas and Propagation (ISAP 2009)October 20-23, 2009, Bangkok, THAILAND

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-180 -135 -90 -45 0 45 90 135 180Angle (degree)

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Fig. 5. Measured radiation patterns of conventional and proposed DS-LTSA at 15 GHz. (a) E-plane. (b) H plane.

11 12 13 14 15 16 17 18 19Frequency (GHz)

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Fig. 6. Measured gains of conventional and the proposed LTSA. References [1] P. R. Acharya, J, Johansson, and E. L. Kollberg, “Slotline antennas for millimeter and

submillimeter wavelength,” Proc. 20th Eur. Microwave Conf., Budapest, Hungary, pp. 353-358, Sep. 1990.

[2] P. R. Acharya, H. Ekstrom, S. S. Gearhart, S. Jacobsson, J. F. Johansson, E. L. Kollberg, and G. M. Rebeiz, “Tapered slotline antenna at 802 GHz,” IEEE Trans. Microw. Theory Tech., vol. 41, no. 10, pp. 1715-1719, Oct. 1993.

[3] P. Acharya, S. S. Gearhart, S. Jacobsson, J, F, Kollberg, and G. M. Rebeiz “Tapered slot antennas at 802 GHz,” IEEE Trans. Microw. Theory Tech., vol. 41, pp. 1715-1719, Oct. 1993.

[4] K. S. Yngvesson, “Endfire tapered slot antennas on dielectric substrates,” IEEE Trans. Antennas Propag., vol. 33, pp. 1392-1400, Dec 1985.

[5] R. Janaawamy and D. H. Schaubert “Analysis of the tapered slot antenna,” IEEE Trans. Antennas Propag., vol. 35, pp. 1058-1065, Sep. 1987.

[6] S. Sugawara, Y. Maita, K. Mori, and K. Mizuno “A MM-wave Tapered slot antenna with improved radiation pattern,” IEEE MTT-S Int. Microw. Symp. Dig., pp. 959-962, Jun. 1997.

[7] “HFSS User Manual,” Ansoft Corp., Pittsburgh, PA, 2005.

The 2009 International Symposium on Antennas and Propagation (ISAP 2009)October 20-23, 2009, Bangkok, THAILAND

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