A novel balanced‐to‐unbalanced branch‐line coupler with large power division ratio

Abstract

A novel balanced-to-unbalanced branch-line coupler with a large power division ratio (PDR) is proposed in this paper. The proposed coupler consists of six transmission-line sections, which can be easily synthesized with the prescribed PDR. By changing the electrical lengths of two transmission lines, a large PDR range can be obtained without the use of high-impedance transmission lines. The closed-form design equations are provided and discussed. To verify the proposed circuit, a coupler prototype with the PDR of 20 dB and the center frequency of f0 = 1.0 GHz is designed and fabricated. Simulation and measurement results are in good agreement. When the balanced port A is excited, the measured differential-mode to single-ended transmission coefficients at f0 for the unbalanced ports D and C are |SsdDA| = −0.28 dB and |SsdCA| = −20.02 dB, respectively, which meet the desired PDR. The measured |SddAA| at f0 is −33.6 dB with a −10-dB bandwidth of about 12.7% from 0.933 to 1.059 GHz. The |SscDA| and |SscCA| are less than −29.5 dB with a −18-dB bandwidth of 22.4% from 0.924 to 1.157 GHz.