A Study Of Compact Dual-Band Directional Couplers

With the rapid development of wireless communications, especially WLAN, single band communication systems cannot meet the requirements. Therefore, all components within the system capable of working in the dual-band or even in multiband are required. Meanwhile, in order to adapt to the development of modern communication systems, the design of compact, broadband and multiband passive components is a necessary requirement. The directional coupler, as the key device in the microwave and millimeter-wave systems, plays a critical role in the whole system, thus, the research on compact and multiband microstrip directional couplers also draw increasing attention.This paper carries out research on the miniaturization of dual-band directional couplers. The main work is the design of two common types of directional couplers, including the branch-line coupler and the rat-race coupler.First, a novel compact dual-band branch-line coupler is proposed in this paper. By employing a basic unit half elliptical-ring impedance stub line to branches, a dual-band branch-line coupler is obtained. The introduction of half elliptical-ring impedance stub increases the design freedom. By changing the impedance and axial ratio of the half elliptical-ring impedance stub, the longitudinal size of half elliptical-ring impedance stub is reduced effectively. So it can be folded inward the enclosed area of the coupler without overlap, and the miniaturization is realized. Based on the half elliptical-ring impedance stub, a single-section dual-band branch-line coupler operating at 2.4/5.8GHz is designed, which has high isolation and coupling. Meanwhile, by using the half elliptical-ring impedance stub, a two-section dual-band branch-line coupler working at 2.2/5.4GHz is also designed, which can not only realize the miniaturization, the bandwidth is also increased. Compared with the stepped-impedance-stub branch-line couplers, sizes of the proposed couplers are reduced by 43% and 30%, respectively. Couplers are tested and measured results agree well with simulated ones, proving the design concept.Next, a rat-race coupler operating at 2.4/5.8GHz is designed by using the same analysis method of branch-line coupler and employing six half elliptical-ring impedance stubs to branches. In order to further reduce the size, a novel rat-race coupler structure is proposed.By employing two half elliptical-ring impedance stubs to branches, the novel rat-race coupler operating at 2.4/5.2GHz has good performance. Compared with the rat-race coupler employing six half elliptical-ring impedance stubs, size of the novel coupler is reduced by 66%. At last, the novel rat-race coupler is fabricated and measured, the simulated and measured results show good agreements.