Improved Side-Coupled Three-Line Wideband Bandpass Filter

This letter presents an improved side-coupled three-line filter, which is loaded by two short-ended microstrip stubs. The mechanism of improved filter is that the order of original three-line filter is increased from third order to fifth order. As a result, the fractional reflection bandwidth below -10 dB of improved filter is raised from 75% to 98%, and its selectivity (skirt factor) is enhanced from 64% to 75%. The proposed filter has been fabricated and verified on the substrate of 0.508 mm thickness. It is effective on this thin substrate.