Miniaturized branch-line coupler with coupling-dependent dual-frequency operation

A miniaturized branch-line coupler that is able to operate at two arbitrary frequencies is presented in this paper. The dual-band frequency ratio depends on the coupling coefficient in the proposed transmission line element. It will be shown later that the dual-band separation decreases as the coupling strength increases. By replacing the traditional quarter-wavelength lines with the proposed artificial transmission lines of a prescribed coupling coefficient, a size-reduced branch-line coupler can be implemented at dual frequencies of interest. An example of the 0.9- and 1.8-GHz branch-line coupler, which corresponds to a band ratio of 2, is experimentally developed and characterized. The presented coupler occupies only 44% footprint of the conventional coupler operating at the lower frequency of the dual-band operation. Excellent agreement between the simulated and measured results is obtained.