A Novel Dual-Band Transmission Line With Equal-Phase Responses

This article presents a novel dual-band transmission line (TL), consisting of a multibranch stub, which can provide equal unwrapped phases at two different frequencies. The frequency ratio can be close to one without any significant degradation in the device performance. Besides, the bandwidth of the proposed line can be manipulated at both frequencies by adjusting the design parameters. The behavior of the structure over the frequency bands is investigated theoretically, and the design procedure and relations are derived. The simulation and measurement results of a quarter-wavelength open-ended stub and a branch-line coupler (BLC) are presented and compared to verify the performance of the proposed dual-band TL. It is shown that the first structure has a simulated and measured insertion phase of 90° at 2 and 3 GHz. The BLC is designed for 4G and 5G applications with two center frequencies of 3.52 and 4.78 GHz, resulting in a frequency ratio of 1.36. The measured insertion loss values at these frequencies are 3.8 and 3.2 dB, the coupled port loss values are 3.5 and 3.1 dB, and the output phase differences are 91.7° and 92.8°, respectively.