Compact wideband microstrip bandpass filter with wide upper stopband based on coupled-stub loaded resonator
A novel wideband microstrip bandpass filter (BPF) based on a coupled-stub loaded resonator (CSLR) is presented in this article. The CSLR is constructed by attaching one short-circuited parallel coupled microstrip line (PCML) in shunt to a high impedance microstrip line. The filter bandwidth can be conveniently controlled via reasonable adjusting of the impedance of PCML. Moreover, new defected microstrip structures (DMSs) introduced in the PCML functions as a means of adjusting the positions of transmission zeros, created by the PCML. The resonant mode and transmission zero chart are given, indicating that the higher modes could be suppressed by the transmission zeros. Finally, to validate the proposed method, two wideband BPF filters with and without DMSs centered at 3 GHz with 3 dB fractional bandwidth of 87% are designed and fabricated. The measured results show that both the return losses are better than 15.8 dB, while the BPF with DMSs has a ?19.4 dB isolation wideband from 1.57 to 4.23 . The measured results are in excellent agreement with full-wave electromagnetic simulation results.  2014 Wiley Periodicals, Inc. Int J RF and Microwave CAE 25:122128, 2015.
coupled-stub loaded resonator defected microstrip structure wideband bandpass filter wide stopband