Design of novel dual-band branch-line coupler with wide-range frequency ratio

As the conventional 3 dB dual-band branch-line coupler is of narrow operation frequency ratio, the method to design a novel dual-band branch-line coupler loaded with stepped impedance stub lines is presented. The adoption of stepped impedance stub lines increases the design degree of freedom and reduces the restriction from the physical realization of impedance range. Thus, a 3 dB dual-band branch-line coupler with wide-range frequency ratio (1.7~6.3) can be obtained. Moreover, a 3 dB microstrip branch-line coupler operating at 2.4/5.8 GHz WLAN bands is simulated and fabricated to verify the proposed design method. The results show that, as compared with the conventional Π-shaped dual-band branch-line coupler, the novel coupler achieves a stub-size reduction up to 25.1%. The simulated results accord well with the measured ones, which means that the proposed design method is feasible.