Wideband absorptive bandpass filter with multiple transmission zeros and flat group delay

Abstract

This letter proposes a compact and wideband absorptive bandpass filter (ABPF) with flat-in-band group delay and multiple out-of-band transmission zeros (TZs). The presented absorptive filter consists of a quarter-wavelength three-line coupled structure (TLCS) and four stubs that are basically a series connection of a lumped resistor and a short-circuited quarter-wavelength transmission line. The TZs are expressed by the adjacent and cross-coupling coefficients. It is revealed that the four stubs affect the filter's bandwidth and TZ frequencies. To verify the design concept, a wideband absorptive filter operating at 2.4 GHz is fabricated and measured, whose simulated and measured results are in good agreement.