A Balanced Branch-Line Coupler With Arbitrary Power Division Ratio

In this paper, a balanced branch-line coupler with an arbitrary power division ratio is proposed. Configured by four quarter- and half-wavelength microstrip lines, the proposed design achieved desired functions of branch-line coupler for differential-mode signal and common-mode suppression with the merits of wide bandwidth, a simple configuration and a design method, low insertion loss, and low cost. The even-and odd-mode equivalent circuits combining with the standard and mixed-mode S-parameters are adopted to derive the analytical equations. Three prototypes with the power division ratios of 1:1, 2:1, and 4:1 are designed to verify the theoretical prediction. The prototypes with the size of 0.52 λg X 0.5 λg (λ
_g
is the guided wavelength at 1.86 GHz) exhibits the operating bandwidths (IS
_ddAA
/DA|, |S
_ccDA
| <; -15 dB) of 4.3%, 7.3%, and 10.2% and the minimum insertion losses of 0.45, 0.25, and 0.1 dB, respectively, while the isolation and common-mode suppression are higher than 20 dB within the whole passband.