A new approach to design of directional couplers

The paper introduces a fundamentally new approach to a classical problem of design of coupled line directional couplers. It is applicable mainly to short couplers of weak coupling. Unlike the classical approach, the new one does not require the coupler outputs to be well matched and the even and odd mode velocities in the coupled line segment to be equal. Instead, high directivity is obtained by a proper choice of impedances terminating the output ports and shunt impedance between those ports. Examples show exceptionally good results for waveguide and microstrip couplers.