Design and Investigation of Bandpass Filter with Enhanced Stopband

An innovative, exceptionally discriminating, and small-sized bandpass filter has been introduced. The skirt factors of the bandpass filter are independently improved by creating transmission zeros at the lower and upper cutoff frequencies. The skirt factors at the lower and upper cutoff frequencies are enhanced through coupling between SIRs (Stepped Impedance Resonators) and adjustments to the length of the coupled line. The bandpass filter operates at a center frequency of 3.59 GHz and has a fractional bandwidth of 53.5%, ranging from 2.63 to 4.55 GHz. A high skirt factor of over 0.92 is achieved at the upper cutoff frequency, while maintaining a compact physical size and normalized circuit size of 11.6mm × 12.82mm and 0.072, respectively.