The effect of an air gap on the coupling between two planar microstrip lines

The finite difference (FD) technique is used to investigate the air gap effect on the coupling between two planar microstrip lines. The strip lines are embedded between a dielectric substrate and a dielectric overlay. The air gap between the substrate and the overlay is present due to the thickness of the strips and/or the roughness of the planar surfaces. A quasi-static FD technique based on the solution of Laplaceʹs equation is used to solve the potential distribution in the region around the strips. The FD mesh is truncated by an artificial boundary to allow for a numerical solution with the available computer resources. At this artificial boundary an approximate boundary condition is used to truncate the FD mesh. From the potential distribution, the charge and the capacitance and inductance matrices of the stripline system are evaluated. The coupling or cross-talk between the lines is then defined and computed. The effects of the conducting strips thickness and the properties of the materials between the strips on the coupling are illustrated through numerical examples.