Design and analysis of wideband nonuniform branch line coupler and its application in a wideband butler matrix

This paper presents a novel wideband nonuniform branch line coupler. An exponential impedance taper is inserted, at the series arms of the branch line coupler, to enhance the bandwidth. The behavior of the nonuniform coupler was mathematically analyzed, and its design of scattering matrix was derived. For a return loss better than 10dB, it achieved 61.1 bandwidth centered at 9GHz. Measured coupling magnitudes and phase exhibit good dispersive characteristic. For the 1dB magnitude difference and phase error within 3°, it achieved 22.2 bandwidth centered at 9GHz. Furthermore, the novel branch line coupler was implemented for a wideband crossover. Crossover was constructed by cascading two wideband nonuniform branch line couplers. These components were employed to design a wideband Butler Matrix working at 9.4GHz. The measurement results show that the reflection coefficient between the output ports is better than 18dB across 8.0GHz-9.6GHz, and the overall phase error is less than 7°.