Performance of microstrip couplers on an anisotropic substrate with an isotropic superstrate

It is noted that the directivity of inherently poor couplers on such anisotropic substrates as sapphire can be improved by adding an isotropic layer above the microstrips. It may be possible to equalize phase velocity for certain isotropic layer thicknesses and dielectric constants; and in most cases it is found possible to achieve even- and odd-mode phase velocities which, while unequal, are not dominant in the determination of directivity. These improvements are obtainable with available materials.