A New Design for Improving Amplitude Imbalance of Branch-Guide Coupler Utilizing Branch Structure With Dual Transmission Zeros

This article presents a new design for improving the amplitude imbalance of the branch-guide coupler (BGC) using a novel branch structure. This basic structure consists of a main TE301 mode cavity loaded with an $E$ -plane stub, which is located in the middle of the main cavity and can be considered as a TM110 mode cavity. Through analysis, it is found that this new topology will provide two transmission zeros (TZs), and the positions of the TZs can be independently controlled. Further, the amplitude imbalance of the BGC is greatly improved because of the existence of high- and low-frequency TZs. To verify the design concept for loose- and tight-coupling applications, 10 dB 4-branch and 3 dB 7-branch prototypes are designed, processed, and measured. The measurements show that the 4-branch BGC achieves a coupling of approximately 10.3 dB with ±0.2 dB variation over the full $D$ -band, and the return loss (RL) and isolation are greater than 20 dB. Furthermore, the measurements of the 3 dB 7-branch BGC show that, over the full $D$ -band, the RL and isolation are greater than 20 dB, and the amplitude and phase imbalance are less than 0.3 dB and ±2.2°, respectively. The measurements and simulations are in excellent agreement. To the best of the authors’ knowledge, these are the full-band BGCs with the lowest amplitude imbalance proposed to date. Therefore, the proposed BGCs have significant potential for application in ultrawideband millimeter-wave and submillimeter-wave waveguide systems.