Improved performance of microstrip couplers through multi-layer substrates

Edge-coupled microstrip directional couplers are widely used in microwave circuits, however practical limits on center conductor spacings limit the level of coupling that can be attained. In addition, differences in the even and odd mode phase velocities degrade the performance of broad-side coupled microstrip directional couplers. A new technique for equalizing the phase velocities is presented which enhances the coupling and directivity, but uses wider center conductor spacing than traditional designs. This new technique uses two substrate layers where the upper substrate dielectric constant is much larger than the dielectric constant of the lower substrate. This allows the design of directional couplers with tighter couplings and better performance.