In this letter, a novel microstrip line balun with transparent port impedance and unconnected flexible output arms is presented. The proposed balun is designed from a symmetric four-port device with an open end at its isolation port. To achieve balun function, a shunt stub is attached between the unbalanced port and the virtual open-ended port to generate a transmission zero under the even-mode circuit. Its odd-mode circuit is treated as an impedance transformer, where the transformer is independent of the terminal impedance. In addition, the proposed balun has flexible open arm branches and does not require a closed loop structure, which provides design flexibility to integrate with other devices compared to conventional closed-loop branch line baluns. The theoretical design equations are derived first to prove the design concept. Then, two prototypes are designed with 35 $\Omega $ system impedance and loaded with 50 and 75 $\Omega $ port impedance, and the measured results match well with the simulation, which further validates the proposed design theory.

中文翻译：

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一种具有透明端口阻抗和柔性开臂结构的新型微带线巴伦

在这封信中，介绍了一种具有透明端口阻抗和未连接的柔性输出臂的新型微带线巴伦。建议的巴伦设计为对称四端口设备，其隔离端口有一个开放端。为了实现巴伦功能，在不平衡端口和虚拟开端端口之间连接了一个分流短截线，以在偶模电路下产生传输零。其奇模电路被视为阻抗变压器，其中变压器与终端阻抗无关。此外，所提出的巴伦具有灵活的开放臂分支，不需要闭环结构，与传统的闭环支线巴伦相比，这提供了与其他设备集成的设计灵活性。首先推导出理论设计方程来证明设计概念。然后， $\欧米茄 $ 系统阻抗和负载 50 和 75 $\欧米茄 $ 端口阻抗，测量结果与仿真结果吻合良好，进一步验证了所提出的设计理论。