The Synthesis of a Class of Branch-Line Directional Couplers

The design of a class of branch-line couplers comprising coupled-line branches is presented. The method employs classical network synthesis to derive a distributed network prototype from which the modal sub-networks of the coupler are found. The branch-line couplers overcome realization difficulties that are often encountered with conventional couplers. The synthesis of a 3-dB branch-line coupler is presented in detail and further implemented on a microstrip board. The measured performance of the fabricated device shows close agreement with the underlying theory.