Feed line optimization in pseudo-interdigital bandpass filters

To demonstrate the importance of feed line topology in the design of filter with coupled resonators, a pseudo interdigital bandpass filter is designed here. In the conventional approach the feedlines are of coupled type when the filter is designed over a substrate of RT/Duroid (εr = 10.8); however, when the filter is designed over FR4 (εr = 4.3) substrate, the coupled feed lines are unable to produce the desired filter response. To comply with the filter prototypes a tapped feedline structure is then implemented to produce the desired results. Therefore it may be deduced that feedline layout is not a matter of choice but an inevitable design requirement. An explicit design procedure and measured/simulated results are presented here.