Sharp Roll-Off Lowpass Filter With Wide Stopband Using Stub-Loaded Coupled-Line Hairpin Unit

A novel application of shunt open-stubs at the feed points of a center fed coupled-line hairpin resonator is presented for the design of compact lowpass filter (LPF) with very wide stopband. The transmission zeros of the basic unit are minimally affected by its association with the stubs. The stopband bandwidth and filter roll-off rates achieved here are larger than those achieved earlier. A prototype microstrip LPF having a very wide stopband up to nine times the filter cutoff frequency is fabricated.