Ultra-wideband bandpass filter with hybrid microstrip/CPW structure

A novel ultra-wideband (UWB) bandpass filter (BPF) is presented using the hybrid microstrip and coplanar waveguide (CPW) structure. A CPW nonuniform resonator or multiple-mode resonator (MMR) is constructed to produce its first three resonant modes occurring around the lower end, center, and higher end of the UWB band. Then, a microstrip/CPW surface-to-surface coupled line is formed and modeled to allocate the enhanced coupling peak around the center of this UWB band, i.e., 6.85 GHz. As such, a five-pole UWB BPF is built up and realized with the passband covering the entire UWB band (3.1-10.6 GHz). A predicted frequency response is finally verified by the experiment. In addition, the designed UWB filter, with a single resonator, only occupies one full-wavelength in length or 16.9 mm.