A Novel S-Band Bandpass Filter (BPF) with Extremely Broad Stopband

Abstract

This paper presents a new compact wideband bandpass filter (BPF) with an extremely broad stopband consisting of two parallel coupled-line sections and multiple open-ended stubs, for S-band applications. Due to symmetry of the design, even- and odd-mode analysis method is applied to analyze the proposed filter structure. To further enhance the upper stopband, additional TZ's are generated by attaching two open stubs at the input/output feed lines and a third one in the middle of the structure. The fabricated prototype filter covers the whole S-band frequency range, and has 3-dB fractional bandwidth (FBW) of 66%. The measured insertion loss within the passband is less than 0.8 dB and the measured return loss is greater than 15 dB within the entire passband. In addition, the proposed filter has an appealing compact size of ${0.09\lambda_{\mathrm{g}}}^{2}$ with the upper stopband extended up to $5.96f_{0}$ , for suppression level greater than 20 dB.