Characteristics of Microstrip Directional Couplers on Anisotropic Substrates

The properties of directional couplers fabricated on anisotropic substrates are examined on a quasistatic basis. Three substrate materials are considered sapphire, Epsilam (a ceramic-filled polytetrafluoroethylene dielectric), and boron nitride. VSWR, directivity, and coupling are presented for several representative 10-dB coupler designs, as well as variations in these parameters with crystal axis offsets. It is demonstrated that high directivity can he achieved by making use of the substrate anisotropy in conjunction with a top cover to equalize even- and odd-mode phase velocities.