Branchline-couplers with improved design flexibility and broad bandwidth

Analytical formulas that make it possible to fully exploit the design flexibility inherent in two-arm branchline-couplers with open stubs at the symmetry-planes in order to avoid low-impedance lines are derived. A broadbanding procedure that improves the operating bandwidth of a two-arm coupler up to 40% while maintaining very flat coupling characteristics is given. To verify the theoretical work, various 3-dB couplers were built. Two examples are given for two-arm couplers: (1) a hybrid coupler with two series lambda /2-length branches in cascade and (2) a hybrid coupler with one series lambda /2-length branch and one lambda /2-length open stub in parallel.<>