Design of Fully Integrated 4x4 and 8x8 Butler Matrices in Microstrip/Slot Technology for Ultra Wideband Smart Antennas Marek E. Bialkowski 1 , Feng-Chi E. Tsai 2 , Yu-Chuan Su, 2 Kai-Hong Cheng 2 1 School of ITEE, University of Queensland St Lucia, Brisbane, QLD 4072, Australia, meb@itee.uq.edu.au 2 Antenna Design Department, Wistron NeWeb Corporation No. 10-1, Li-Hsin Road 1, Hsinchu Science Park, Hsinchu 300, Taiwan R.O.C., { eddie_tsai, vincent_su, kh_cheng}@wneweb.com.tw Abstract: The paper presents the design of ultra wideband (UWB) 4x4 and 8x8 Butler matrices in multi-layer microstrip-slot technology that are aimed for inclusion in UWB switched-beam smart antennas. The design exploits the multi-layer microstrip/slot technology to accomplish UWB 3dB couplers and differential phase shifters that form these two devices. The choice of this technology helps to overcome the problem of cross- overs, which is the main challenge in practical realizations of Butler matrices. Introduction Recent years have seen a lot of research activities concerning smart antennas for use in wireless communication systems. The reason for this activity is that these intelligent antennas can considerably improve capacity of wireless systems by efficiently combating interference. In general, they are divided into two categories: switched-beam and fully adaptive ones. Out of the two categories, switched-beam antennas are simple and straight forward to implement in the already existing radio systems. One of the main challenges to realize them concerns the design of an antenna switching sub-system which would operate over a specified frequency band. In order to make it cost-effective such a sub- system should preferably be accomplished in a fully integrated planar technology. With respect to linear array antennas, the Butler matrix approach to realizing a switched-beam antenna system seems to be an attractive choice. The reason is that such a switching device (with N input ports and N output ports) can be assembled using 3dB couplers and differential phase shifters following many designs available in the microwave literature. An important feature of this device is that when one of the input ports is fed, the signals appearing at the output ports have equal magnitudes and progressive phase shift between adjacent ports. By the proper choice of phase shifts, the feeding of different ports leads to orthogonal beams pointing at different directions. Unfortunately, the design of Butler matrix introduces an extra challenge which concerns cross-overs in the case of planar microstrip technology. They are required for connecting some of the couplers and phase shifters that form this device. A number of strategies for the Butler matrix implementation have been described in [1]. Following this, the designs of 4x4 and 8x8 Butler matrices with wideband performance have been demonstrated. For the 4x4 matrix case, full planar integration in microstrip technology has been achieved while the 8x8 design used the hybrid approach with the 4x4 matrix being the fundamental building module. In order to achieve the operation over the frequency band of 1.8-2.2 GHz, elliptical disc couplers and phase shifters, being the combination of section microstrip lines and Lange couplers, have been applied. The use of Lange couplers served the purpose of overcoming the problem of cross-overs. 978-1-4244-2042-1/08/$25.00 ©2008 IEEE.