A compact 8×8 Butler matrix based on double-layer structure

In this paper, a novel 8×8 Butler matrix based on double-layer substrate structure is presented and realized. The double-layer structure is adopted to place components on the up and down layers without crossover. Two-section stepped coupling microstrip Schiffman phase shifters with λg/8 length are used to realize wideband phase shift. A three-branch line coupler is employed as wideband bridge. The 8×8 Butler matrix sample working in C band frequency is designed and fabricated, and the measured results indicate that the return loss of each port is better than -10dB in the frequency range over 4.2GHz-5.3GHz, while the isolation is better than 17dB, the distribution error is less than ±1dB, the phase error is less than ±12° and the average measured insert loss is about 2.5dB. Compared with conventional structure using crossover, the novel Butler matrix not only has lower loss and better adaptability to large scale multi-beam forming network, but also has more size reduction.