Parallel Coupled-Line Bandpass Filters With Floating Plate Overlay for Enhanced Coupling and Spurious Suppression

A new type of parallel-coupled microstrip filter is proposed to have wide-band performance and suppress the second harmonic response. The proposed filters are constructed with using floating plate overlays to enhance the coupling between microstrip lines, and compensate the difference in phase velocity under even- and odd-mode excitations. The synthesis method is well developed and the design curves are given. Two Chebyshev filters with fractional bandwidths of 50% and 70%, respectively, were fabricated and measured to validate the design concept. All the measured results agree well with the simulation.