Strip-type directional coupler having elongated aperture in ground plane opposite coupling region

Claim 1

In a directional coupler of the type comprising: a dielectric substrate, a pair of narrow strip-like conductors on one surface of said dielectric substrate, a single ground planar conductor on the surface of said substrate opposite said one surface so as to be spaced directly opposite said striplike conductors, said pair of narrow strip-like conductors being closely spaced to each other and to said ground planar conductor so as to provide an electromagnetic coupling region between said strip-like conductors so that electromagnetic signals applied to said coupler are propagated in odd and even modes along said pair of conductors at said region, said substrate having a dielectric constant of at least six causing a marked difference between the phase velocity of signals propagating in the odd mode and in the even mode, said ground planar conductor having an elongated aperture which extends along said surface opposite said one surface and directly opposite and parallel to said region for perturbing the odd mode sufficiently so that said odd mode phase velocity equal said even mode phase velocity.