Parallel coupled microstrip filters with floating ground-plane conductor for spurious-band suppression

Floating strip conductors printed on the backside of the substrate are shown to be useful to suppress the spurious transmission band appearing at twice the central frequency of coupled-line microstrip filters. It is shown that proper adjustment of the dimensions of the floating conductors yields equal even and odd electrical lengths. An attractive feature of this design is its flexibility because the equalization of the modal electrical lengths can be achieved with various geometries. Additionally, the floating conductor provides an extra coupling mechanism that relaxes tolerances of strip width and spacing in those cases where tightly coupled sections are required. A fast quasi-TEM analysis is used to find the structure yielding equal mode phase velocities. Fine tuning to equalize the modal electrical lengths for each coupled stage is based on the use of a commercial electromagnetic simulator. Experimental verification is finally provided.