Design of a flexible ultra-wideband bandpass filter

In this paper, a flexible ultra-wideband bandpass filter (UWB BPF) fabricated on a polyimide film substrate is presented. The passband of the filter is obtained by cascading the stepped impedance resonator and the half-wavelength short circuit resonator. The interdigital coupling structure and the defective ground ring structure constitute a parallel resonant circuit to generate a transmission zero outside the passband, which improves the out of band suppression effect significantly. Measured results show that the passband frequency of this filter is 3.13-11GHz. The insertion loss of the filter is less than 1.46dB in the passband and larger than 20dB at 14GHz outside the passband. Simulated and measurement results of the filter in bent states are all basically consistent with those in flat state.