Design and Implementation of Broadband Bandstop Filter With Enhanced Bandwidth and In‐Band Rejection

In this paper, a miniaturized broadband bandstop filter is designed and implemented. The developed bandstop filter is based on a modified structure of the traditional bandstop filter with open‐circuited stubs, which is further realized by asymmetrically loading an additional quarter‐wavelength open stub. With this stub loaded, two additional transmission zeros are generated, thereby raising the total number to five across the stopband. Consequently, bandwidth and in‐band rejection for the developed bandstop filter can be adjusted by changing characteristic impedance of the asymmetrically loaded quarter‐wavelength open stub. Moreover, closed‐form analytical expressions that explicitly locate every generated transmission zero are derived, furnishing a ready‐to‐use roadmap for practical broadband bandstop filter design. To validate the proposed concept and synthesis approach, a miniaturized broadband bandstop filter centered at 3 GHz with a 40 dB rejection bandwidth of about 75% is designed, fabricated, and characterized. Remarkably, its core footprint is 31 mm × 10.6 mm, corresponding to only 0.5 λ g × 0.17 λ g .