A compact planar ultra-wideband bandpass filter based on Multiple Resonant and defected ground Structure

A compact bandpass filter having an ultra wide pass band (UWB - 3.1 to 10.6 GHz) is proposed. It is based on hybrid microstrip coplanar waveguide (dual sided metal) structure. A Multiple Resonant Structure (MRS) is constructed using coplanar waveguide (CPW) planar transmission line. The MRS makes the resonance using quarter wavelength and half wavelength open ended CPW. The equispaced three resonances at lower edge (3.1 GHz), center (6.85 GHz) and higher edge (10.6 GHz) of the whole Ultra Wide Band are achieved using CPW MRS. To make the band as flat as possible, two more resonances are introduced using quarter wavelength microstrip patches on top of commonly shared substrate, so the proposed filter becomes a five pole bandpass filter. A dumb-bell shaped defected ground structure on either side of CPW MRS improves the return loss almost less than -20 dB over the whole UWB passband. The simulated results of proposed filter shows good transmission response within passband and good rejection in out of band. The measured results agree well with the simulated ones.