A Compact Coupled-Line Marchand Balun with Symmetric Inductive Compensation for High-Impedance Transformation

This paper presents a compact coupled-line Marchand balun with symmetric inductive compensation for high-impedance transformation. The proposed circuit transforms a 50-Ω unbalanced input into a 300-Ω differential balanced output for S-band RF applications. The structure consists of two coupled-line sections, an input reactive matching network, and two identical compensation inductors placed symmetrically at the balanced output paths. The input matching network is used to support the impedance transformation at the unbalanced port, while the symmetric inductive compensation is introduced to adjust the effective modal response of the coupled-line paths and improve the output balance. The design is analyzed using even- and odd-mode circuit representation and optimized using ADS circuit-level simulation. A prototype was fabricated and measured, and its performance was compared with that of a conventional coupled-line Marchand balun under the same port-reference condition. At the design frequency of 2.25 GHz, the proposed balun achieved a measured input reflection coefficient of |S₁₁|=−28.05 dB. The measured transmission magnitudes were |S₂₁|=−3.11 dB and |S₃₁|=−3.05 dB, corresponding to an amplitude imbalance of 0.052 dB. The measured output phase difference was 180.52^∘, giving a phase error of 0.52^∘. Compared with the conventional prototype, the proposed balun provides improved input matching, smaller amplitude imbalance, and an output phase response closer to the ideal 180^∘ balanced condition. The measured results indicate that symmetric inductive compensation is a practical approach for improving compact high-impedance coupled-line Marchand baluns.