Compact wide-band branch-line coupler with meander line, cross, and two-step stubs

Abstract

Despite being widely used, branch-line couplers (BLC) still suffer from large size. This makes it difficult to incorporate BLC into miniature microwave devices. The work presented here is the design and implementation of a 3-dB reduced-size wide-band 3-branch-line coupler (3-BLC). By combining meander line, cross, and two-step stubs techniques, size reduction and bandwidth improvement is successfully achieved on 3-BLC design. The 3-BLC length was reduced along the horizontal branches using two-step reduction technique while the center vertical branch was reduced using cross stubs. A simple two-turn meander line is utilized at the two ends vertical branches of the coupler to obtain balanced length with the middle vertical branch. Hence, its width was optimized to maximize the coupler bandwidth. The 3-BLC was designed to operate at 2.45 GHz wireless local area network frequency band (IEEE 802.11 b/g) and fabricated on a 1.6 mm thick FR-4 board. The simulated and measured results are in very good agreement and show a fractional bandwidth of 50% and a sized reduction of 61.64% at the center of the frequency band. The proposed coupler design provides BLC that occupies less space, simple to implement, and wide-band BLC architecture with generic 3-BLC wide-band characteristics retained.