A directional coupler of a vertically installed planar circuit structure

A 3-dB directional coupler is described which is fabricated by installing a vertical second printed circuit board on a main printed circuit board. The added vertical substrate significantly increases the freedom of design parameters. Coupling parameters are easily controlled by changing either the dielectric constants or the thicknesses of the substrates. This makes almost any coupling constant available along with good isolation and a suitable input impedance. The three-dimensional structure also makes a good use of space, resulting in a compact circuit structure. Experimental results agree well with a numerical calculation based on the boundary-element method and show the relaxed tolerances possible in fabrication. Effective permittivities and characteristic impedances for both even and odd modes were obtained and were substituted into the scattering matrix to obtain the parameters for a symmetrical directional coupler. A simplified design chart is also provided showing the dependence of characteristic impedances for odd and even modes on substrate thicknesses used