A Dual-Band Beam-Switched Slot Array for GSM 900/1800MHz

Proceedings of Asia-Pacific Microwave Conference 2006 A Dual-Band Beam-Switched Slot Array for GSM 900/1800MHz Yijun Liu, Zhongxiang Shen, Boyu Zheng and Weihua Tan School of Electrical and Electronic Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798 Tel: +65-67905397, Fax: +65-67920415, E-mail: ezxshen@ntu.edu.sg Abstract A dual- and wide-band beamswitched slot antenna array for GSM 900/1800 is presented in this paper. The cavity-backed slot element is employed to achieve the wide bandwidth with good isolation and high gain. A prototype of the phased slot array is fabricated and tested in an anechoic chamber. The beam can be steered in the ±45 azimuth range by a beam-forming network operating at two bands. The Butler matrix employing four hybrids is used to form four beams to be selected by a single pole 4-throw (SP4T) switch under different states. The measured 10 db return loss bandwidth is larger than 20% for 900 MHz band and 23% for 1800 MHz band. The measured peak gain is 16.5 dbi at 920 MHz, 15.2i db at 1800 MHz, respectively. Index Terms Cavity-backed slot antenna, dualband, wide-band, phased array, Butler matrix. I. INTRODUCTION Phased antenna arrays with large bandwidth for two or more frequency bands, compact size and high gain are highly desirable in mobile communications. In the open literature, there are several designs reported for dual-band arrays [1]- [3]. However, all the previous designs are for fixed-beam dual-band antenna arrays. Some of them are either for narrow-band operation over two frequencies or for wide-band operation over a single band. Furthermore, these available designs are complicated in structure, which increases the fabrication cost. For the reported design in [1], a two-channel microstrip patch array is comprised of four nearly square microstrip patches; all of them are stacked in the vertical direction. It shows only 30 MHz bandwidth of 3 db axial ratio at the L band. In [2], an L/X dual-band and dualpolarized planar array for synthetic aperture radar was described. The X-band elements and L-band elements are on separate layers with the L band element on the top. The bandwidths are 6%, 3% at L and X bands, respectively. A dual-band omnidirectional microstrip patch antenna array presented in [3] was designed to operate at 2 GHz. This array consists of four stacked patches mounted on the side walls of a truncated pyramid. Its non-planar structure needs large space and also results in high manufacturing cost. Recently, an inverted configuration was proposed to design microstrip-fed cavity-backed slot antennas [4-6]. This configuration is employed in this paper to design a planar beamsteerable dual- and wide-band slot antenna array printed on the same dielectric substrate. The designed phased array can be operated over two frequency bands: 900MHz and 1800MHz, and is capable of steering the radiation beam in the ±45 azimuth range for different Butler matrix [7] states. The measured gains are larger than 14 dbi at 920 MHz and 13.3 dbi at 1800 MHz for different beam directions. When the array is connected to an equal-phase power combiner, the measured gains in the broadside are 16.5 dbi and 15.2 dbi at 920 MHz and 1800 MHz, respectively. The developed antenna array can be detachable from the controlling circuit so that it is portable and easily deployable. II. ANTENNA CONFIGURATION Figure 1 shows the configuration of the beamswitched array, which consists of 32 slots backed by 18 metallic cavities. The larger slots are for 900 MHz band, and the smaller slots are for 1800 MHz band. The final size of the totally assembled phased array is 800 mm 800 mm 40 mm. Two printed circuit boards are fabricated: one for the array and the other for beam-forming networks. The slots and feeding networks are fabricated on a RT5880 substrate from Rogers. In order to avoid the overlap between the feeding lines for 900MHz slot elements and 1800MHz slot elements, a 4- layered structure is employed. Some of the signal lines are printed on the back layer and are connected to the lines on the top layer through via holes. This also applies to the beam-forming network using Butler matrix [7]. The beam-forming network, realized by a 4-way planar Butler matrix, is etched on a RO4003 substrate also from Rogers because RO4003 is much easier to deal with and readily available. SMA connectors are to be soldered to the feeding Copyright 2006 IEICE

(a) Layout of the slots printed on the lower side of the RT 5880 substrate ports of the antenna array as well as the input and output ports of the Butler matrix. Coaxial cables with the same length are used to connect the antenna array and the Butler matrix. A switch, covering DC-3.5 GHz with high isolation of 36 db and a low insertion loss of 0.5 db at 2 GHz, is used to select one input port of the Butler matrix to obtain the corresponding scanning beam. The switching circuit is realized using a non-reflective SP4T switch controlled by two control inputs with four signal path states: 00, 01, 10, 11. In the back of the slot antenna array, there are eighteen metallic cavities that are formed using an aluminum plate. These cavities are indented in a common aluminum plate, on which the fabricated substrate is placed with the slots directly on the lower side of the substrate for simple assembly. Fig. 2 Layout of the Butler matrix at 1800MHz. (b) Layout of the 900MHz feeding network printed on the upper side o the same RT5880 substrate Fig. 3 Photograph of the finally assembled dual-band slot array. (c) Layout of the cavities to back the substrate Fig. 1 Geometry of the designed dual-band beamswitched slot array. As shown in Fig. 1(b), three-port Y-branch power combiners are realized as the feeding network on the top layer of the substrate to match the output ports when the phased slot array is used as a receiving antenna. Four linear arrays at each

frequency band are then formed; they are then connected to the beam-forming network for steering the radiation beam in the azimuth plane. Fig. 2 provides the layout of the Butler matrix at 1800MHz on the top layer. Fig.3 shows the final slot antenna array in an anechoic chamber. III. MEASURED RESULTS Figure 4 presents the measured return loss results of the developed beam-switched slot array at two feeding ports for different states of the Butler matrix. It is seen that the phased slot array can operate over two frequency bands. The 10 db return loss bandwidth is larger than 20% at the center frequency 920 MHz and 23% at the center frequency 1795 MHz, respectively. Only a small difference in S 11 curves can be seen between the different states. Fig. 5 shows the measured isolation results between the two feeding ports for state 00 of the Butler matrix, and little difference between the S 21 curves for different states is observed. The measured isolation S 21 between the two feeding ports is less than 30 db at 900 MHz band and less than -22 db at 1800 MHz band. Isolation S21(dB) -20-30 -40-50 -60-70 -80 0.5 1.0 1.5 2.0 2.5 Frequency (GHz) Fig. 5 Measured isolation between two feeding ports of the slot array (Butler matrix state 00). Figs. 6 and 7 provide the co-polarized radiation patterns of the designed slot array measured at 920 MHz and 1800 MHz for both E- and H-planes, respectively. For all the states of Butler matrix, the cross-polarization level is below 20 db for H- plane patterns at 920 MHz and below 22 db for H-plane patterns at 1800 MHz. 0 Return Loss S11(dB) -10-20 -30-40 0.5 1.0 1.5 2.0 2.5 Frequency (GHz) 0 (a) S 11 Butler Matrix state 00 Butler Matrix state 01 Butler Matrix state 10 Butler Matrix state 11 (a) f=920 MHz Return Loss S22(dB) -10-20 -30 Butler Matrix state 00 Butler Matrix state 01 Butler Matrix state 10 Butler Matrix state 11-40 0.5 1.0 1.5 2.0 2.5 Frequency (GHz) (b) S 22 Fig. 4 Measured return loss results of the slot array at two feeding ports for different Butler matrix states. (b) f=1800 MHz Fig. 6 Measured co-polarized radiation patterns in the H plane for different Butler matrix states. The radiation beam of the slot array connected to a Wilkinson power combiner is along the broad-side of the planar array, which is defined as 0. As seen from the radiation pattern presented in

TABLE I MEASURED GAINS OF THE DESIGNED PHASED SLOT ARRAY. 920 MHz 1800 MHz Butler matrix state Beam direction ( ) Gain (db) Beam direction ( ) Gain (db) Boresight 0 16.5 0 15.2 00-14 16-12 14 01-34 14.1-30 13.3 10 34 14.1 30 14 11 14 16 12 13.3 Fig.6, the radiation beam direction will change when the state of the Butler matrix is varied, which leads to different progressive phase differences between elements. The 3 db beam width is 25º, and can be steered to cover the ±45 azimuth range for different Butler matrix states at two frequency bands, which demonstrates the beam steering capability of the designed slot array. The high side lobe in the E plane at 1800 MHz is due to the large vertical spacing between elements. The measured gains of the slot array are listed in Table I. The gains are maximal at the broadside when the array is connected to a Wilkinson power combiner. The larger the beam directions are, the smaller are the gains. It should be noted that the gains of the slot array will be larger for a longer horizontal spacing between slots if a larger total size of the array is allowed. (a) f=920 MHz (b) f=1800 MHz Fig. 7 Measured co-polarized and cross-polarized radiation patterns in the E plane. IV. CONCLUSION A dual- and wide-band cavity-backed beamswitched slot array with a 4-way Butler matrix has been presented in this paper. The array, consisting of 32 ridged slots, connected to a beam forming Butler matrix and fed by two separate ports for two frequency 900 and 1800MHz bands, can steer the beam to cover the ±45 azimuth range for different Butler matrix states at two frequency bands. The measured results of the designed phased array show that the 10dB return loss bandwidth is 20% for 900 MHz band and 23% for 1800 MHz band, respectively. The measured gains are maximal when the array is connected to a Wilkinson combiner, achieving 16.5 dbi at 920 MHz and 15.2 dbi at 1800 MHz, respectively. Because of its compact size, steered radiation patterns and good isolation between the two bands, this beam-switch slot array should be very useful in many dual-band wireless communication systems. REFERENCES [1] S. Chebolu, S. Dey, R. Mittra, and M. Itoh, A dual band stacked microstrip antenna array for mobile satellite applications, IEEE Antennas and Propagation Society International Symposium. 1995 Digest, vol.1, pp. 598-601, 1995. [2] D. M. Pozar and S. D. Targonski, A shared aperture dual band dual polarized microstrip array, IEEE Trans.Antennas Propagat., vol. 49, no.2, pp. 150-157, 2001. [3] T. Rutkowski and C. Peixeiro, Dual band omnidirectional microstrip patch array antenna for a mobile communication system, Proc. Of Asia Pacific Microwave Conference, pp. 429-432, 1997. [4] Q. Li and Z. Shen, Inverted Microstrip-fed cavitybacked slot antennas, IEEE Antennas and Wireless Propagation Letters, vol. 1, pp. 98-101, 2002. [5] Q. Li and Z. Shen, An inverted microstrip-fed cavity-backed slot antenna for circular polarization IEEE Antennas and Wireless Propagation Letters, vol. 1, pp. 190-192, 2002. [6] Y. Liu, Z. Shen, and C. L. Law, A compact dualband cavity-backed slot antenna, IEEE Antennas and Wireless Propagation Letters, vol. 5, pp. 4-6, 2006. [7] B. Pattan, The versatile Butler matrix, Microwave Journal, Nov. pp.126-138, 2004.

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