Design of compact single-section directional coupler for butler matrix beam-forming MIMO

This paper demonstrates a new design of 3 dB coupler operating in UWB frequency band between 3.1 GHz and 10.6 GHz. Together with UWB phase shifter, this coupler is claimed to be one of the key factors in designing a Butler Matrix that can operate in very wide frequency range. Butler Matrix has been seen to be the most preferable technique to realize the beam-forming network besides provide compact in size and more cost-effective. The arrangement between beam-forming and multiple-input and multiple-output (MIMO) can broaden the coverage and capacity of users for wireless communication system. The proposed coupler design was accomplished in multilayer microstrip-slot technology by employing broadside coupling technique. The initial dimension of the coupler was obtained using a simple mathematical formula and finalized using CST Microwave Studio simulator. The simulation results show the coupler is having a very tight coupling of 3 dB ± 2 dB, the return loss and isolation are better than 21 dB and the phase difference performance is in the range of 90° ± 2° over the designated band. The designed device occupies an area of 25 mm × 15 mm.