Wideband ring resonator bandpass filter with dual stepped impedance stubs

A wideband microstrip bandpass filter (BPF) using a ring resonator with dual stepped impedance stubs is presented. The two open-circuited stubs are placed on the symmetrical plane of the feed lines, the lengths and the characteristic impedances of which are calculated by the analysis of both resonance and transmission zero frequencies. Colinear feed lines, which are either direct connected lines or interdigital coupled lines, are used to connect the ring resonator. Measured results of the filter demonstrate a 3-dB fractional bandwidth of 87.3 % and a wide stopband bandwidth with sharp cutoff regions.