Multi-Element Directional Couplers

It is shown that the backward wave in a directional coupler is related to the shape of the function describing the coupling between tansmission lines by the Fourier transform. This facilitates the design of directional couplers for arbitrary directivities over any prescribed frequency band. Tightly coupled directional couplers are analyzed in simple terms, and it is shown that any desired loss ratio, including complete power transfer between lines, may be achieved. The theories are verified using waveguide models operating at 4,000, 24,000, and 48,000 mc, and it is indicated that the work is applicable to many types of electrical and acoustic trnsmission lines.