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 <inline-formula> <tex-math notation="LaTeX">$0.52~\lambda _{g} \times 0.5~\lambda _{g}$ </tex-math></inline-formula> (<inline-formula> <tex-math notation="LaTeX">$\lambda _{g}$ </tex-math></inline-formula> is the guided wavelength at 1.86 GHz) exhibits the operating bandwidths (<inline-formula> <tex-math notation="LaTeX">$\vert S_{\mathrm {dd}\mathrm {AA/DA}}\vert $ </tex-math></inline-formula>, <inline-formula> <tex-math notation="LaTeX">$\vert S_{\mathrm {cc}\mathrm {DA}}\vert < -15$ </tex-math></inline-formula> 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.