Wideband bandstop filter using hybrid microstrip/CPW-DGS with via-hole connection

A novel wideband bandstop filter (BSF) with a minimum attenuation better than 28.5 dB and three transmission zeros within the stopband is proposed. The designed filter is composed of a top microstrip transmission line and a bottom coplanar waveguide open-circuited resonator loaded by a periodical L-shaped defected ground structure resonant unit cell. The top and bottom dual-metal-plane conductors are connected by the periodically metallic via-hole within the designed substrate. With the appropriate height of substrate and separation between the adjacent metallic via-holes, an excellent parallel-plate-metal synthesised transmission line with an improved coupling between the top and bottom resonators can be achieved by this via-hole-loaded technology. Finally, a microstrip BSF centred at 5.44 GHz with a measured 20 dB rejection bandwidth of more than 35.5% are designed, fabricated, and measured for this topology of validation.