Small ultra-wideband microstrip bandpass filters with improved passband

Ultra-wideband (UWB) microstrip bandpass filters with wide passband and miniaturized size are presented. The proposed filters are designed to provide a wide passband, low insertion loss and return loss, linear phase over the passband, flat group delay and high fractional bandwidth. A modified multiple-mode resonator is formed by transversely attaching three pairs of non-uniform and folded stubs. Both sides of high-impedance sections are linked with two feed lines via direct coupled lines, leading to make up a UWB filter. The simulated filter1 results with passband of 7 GHz from 2-11 GHz, minimum insertion loss of approximately -0.5 dB, return loss of around -50 dB and -3 dB fractional bandwidth of 110 % while for the filter2, insertion and return losses are -1 dB and -35 dB. The computed group delay variation in the passband for both filters is 0.02 ns. The overall dimension of the filters is 3 × 10.7 × 1.6 mm³. They are simulated using electromagnetic (EM) simulator, IE3D. Simulated results show that the proposed filter can easily cover the band from 3.1 to 10.6 GHz.