Broadband Eight-Port Forward-Wave Directional Couplers and Four-Way Differential Phase Shifter

We propose a broadband eight-port forward-wave directional coupler (FWDC) with the dually symmetric structure. The novel coupler approximately achieves broadband impedance matching of the four modes. In addition, it can also provide almost constant phase differences among the four modes over a wide frequency range. Therefore, an eight-port FWDC with a broad bandwidth can be expected. Based on the Bloch-Floquet theorem, we build the equivalent models to predict the propagation properties. Also, a complete design procedure for the arbitrary power division is provided. Two samples are fabricated on Duroid substrate. Both measured and simulation results are shown for comparison and have good consistency. The achieved FBW is 47% for 0.5-dB variation. We also introduce a novel four-way phase shifter, composed of the proposed broadband eight-port FWDC with 0 dB through level and four delay transmission lines. It can support four independent pairs of the phase-shifting routes at the same time. The unique property can be applied to the three-layer broadband 8 × 8 Butler Matrix, as shown in this paper. It not only avoids the design of the complex crossovers, but also reduces the number of the needed phase shifters and the total size. A test sample is fabricated and measured. The obtained FBW is 36%. By properly choosing other broadband components, a compact and broadband 8 × 8 Butler matrix can be expected.