Transmission line model of substrate effects on capacitive mesh couplers

To use a transmission line model to calculate the optical properties of a thin metal mesh on a dielectric substrate, account must be taken not only of the different propagation conditions within the substrate and of Fabry-Perot resonances due to reflections at the second surface, but also of the effect of the dielectric on the capacitive component of the equivalent reactance of the mesh. Only when this effect is accounted for, which can be done using a simple formula based on Babinet's principle, is good agreement obtained with experimental measurements.