A Compact Ultra-Wideband Multimode Bandpass Filter With Sharp-Rejection Using Stepped Impedance Open Stub and Series Transformers

This brief presents the analysis and design of an ultra-wideband bandpass filter (UWBPF) with four transmission zeros and seven poles. The UWBPF model consists of series transformers, coupled lines, and a shunt-stepped impedance open stub. The even-odd mode analysis method and scattering parameters theory are used to extract the closed-form equations for transmission zeros and poles of the proposed UWBPF. For validation, a prototype with a center frequency of 2.49 GHz is designed, simulated, and tested. The measured 3 dB fractional bandwidth is 103.2%, and the fabricated prototype occupies an area of
$0.165\lambda _{g}^{2}$
. The circuit, full-wave simulated, and experimental results are in a good match.