Parallel Coupled Bandstop Filter Using Double Negative Coupled Transmission Line

In this letter, a novel parallel coupled bandstop filter (BSF) using a double negative (DNG) coupled transmission line is presented. Between microstrip as a double positive (DPS) transmission line and mushroom structure as a DNG transmission line, strong coupling broadly arises in the cross range of dispersion curves of isolated microstrip and mushroom structure because of a complex propagation constant in the cross range. Strong coupling inhibits wave propagation, so that this kind of structure can be utilized as a BSF. This BSF utilizes coupled transmission line instead of coupled resonator, resulting in broad bandwidth (>30%), sharp-rejection, and high rejection level. The strong coupling between the DPS and DNG transmission line makes it possible to shorten the coupling length, resulting in compact size